Dave Irby, Kele Design Engineer

About Dave Irby, Kele Design Engineer

Relay Fundamentals

Open an HVAC control panel, and you’re likely to find at least one relay mounted inside. Sometimes you find lots of relays! Even though relay technology has been around since the 1800s, there is still a need for relays in control panels.

What components make up a relay? How are relays typically shown in wiring diagrams? What functions can a relay perform? Stay tuned for some answers…

What Components Make Up A Relay?

A relay is constructed from the following components:

  • Electromagnet (coil of wire with metal core)
  • Armature (arm that moves when attracted by the energized electromagnet)
  • Spring (keeps the armature retracted when the electromagnet is de-energized)
  • Contact(s) (one or more electrical contacts that open or close when the armature moves)
  • Electrical Terminals (so you can make connections to the electromagnet and contacts)

How Does It Work?

You might have built an electromagnet in high school science class and used it to pick up small metal objects.  An electromagnet is a coil of wire wound around a metal core.  When the coil of wire is hooked to an electrical power source and current flows through the coil, a magnetic field is produced which surrounds the coil and the core:

While the electromagnet is energized, any metal objects which come close will be physically pulled towards core.

When the switch is opened, the current flow stops and the magnetic field disappears.  When the magnetic field disappears, nearby metal objects are no longer attracted to the core:

We now place a pivoted metal arm (armature) above the electromagnet, and we connect a spring to keep the armature pulled away from the electromagnet under “normal” (electromagnet de-energized) conditions:

If we now energize the coil, the magnetic field will overcome the spring force and pull the armature down:

So… we have an armature which we can move up and down by de-energizing or energizing the electromagnet coil.  What do we do with it?

We use the moving armature to open and close electrical contacts which are separate from the coil circuit.  We can do it like this:

In this example, we’re using the metal armature itself as the “Common” or “Wiper” for a double-throw switch.  The tip of the armature makes contact with one of two possible points depending on the position of the armature.

When the relay is de-energized, the armature is up and the wiper makes connection with the “Normally Closed” (NC) contact.

When the relay is energized, the armature is down and the wiper makes connection with the “Normally Open” (NO) relay contact:

This example relay would be known as a “Single Pole, Double Throw” (SPDT) or “1 Pole, Double Throw” (1PDT) relay.  It has one set of contacts (1 Pole) with both Normally Open and Normally Closed (Double Throw) connections.

Relays can have more than one set of contacts (poles).  In the next example, the metal armature is not part of the electrical circuit but instead moves a plastic connector that activates two sets of contacts:

This would be known as a “Double Pole, Double Throw” (DPDT) or “2 Pole, Double Throw” (2PDT) relay.

The number of poles (contact sets) can be expanded (3PDT relays, 4PDT relays etc.).  Typically 4 poles is the largest relay we would see in an HVAC control panel.  If you need more than 4 poles in a relay, you would probably use two relays with the coils wired together in parallel.  Shown below are a DPDT relay and a 4PDT relay wired together to create 6PDT relay action:

 

Electrical Ratings of a Relay

The coil of a “classic” relay (no electronics) is typically rated for a specific voltage and either AC or DC operation but not both.

There are some modern relays (like Functional Devices “Relay In A Box”) that contain electronic circuits that allow the coil to operate over a range of voltages and either AC or DC operation.

If in doubt, the relay’s datasheet should clearly spell out the acceptable voltage range for the coil and whether it’s AC only, DC only, or AC/DC compatible.

Relay coils are generally tolerant of some variation in the coil voltage, a typical datasheet specification is 80% – 120% of rated coil voltage.  So if a 24V transformer is “running a little hot” (putting out maybe 27V) or there is a bit of voltage drop in the coil connecting wires (maybe the coil is only getting 22V) the relay will still function just fine.

The relay contacts will have ratings for the maximum voltage allowed and the maximum current allowed.  Sometimes there are multiple ratings depending on the type of load being switched (for a motor load sometimes the contacts are rated in horsepower for example).  The circuit being switched by the contacts should not put more voltage or current on the contacts than allowed by the datasheet.

Relay Symbols Used in Wiring Diagrams

The relay coil may be drawn several different ways:

The relay contacts are typically shown one of two ways:

Relays go by several possible names on an electrical drawing:

‘R’       (not a great idea since resistors can have this designation too)
‘RL’     (better)
‘RLY’   (better still)
‘K’      (Where did this come from?  We don’t know.  But it’s widely used!)

If there are multiple relays, the name will be followed by a number or a letter as in RLY1, RLY2, RLY3 or RLYA, RLYB, RLYC.  There might be a hyphen (-) between the name and number or letter too.  There is no specific standard on how relays must be named.

Sometimes the relay is shown as a single assembly with coil and contacts arranged in a rectangle:

But sometimes the coil and the contacts are scattered around the drawing:

Relay Terminal Numbering

Unfortunately there does not seem to be a standard for the terminal numbering on relays.

Most of the relays we use in HVAC panels plug into a base socket.  The pins/blades of the relay have assigned numbers and then the bases they plug into have assigned numbers on the screw terminals.  If the same brand of relay and base are used together, the blade numbers usually match the terminal numbers on the base.  If, however, different brands of relay and base are used together, the blade numbers may not match the base terminal numbers even though the two parts are physically compatible (example:  IDEC SH3B-05 relay plugged into Omron PTF11A base).  It’s a good idea to use the same brand of relay and base to avoid confusion.

Some electronic relays such as Functional Devices “Relay In A Box” have color-coded flying leads rather than screw terminals.  Consult the device datasheet to determine the function of each color-coded lead.

Functions a Relay Can Perform

So what useful functions can a relay perform?

  • Use a low voltage and/or low current signal (on the coil) to switch a large voltage and/or large current circuit (on the contacts).
  • Use a DC signal (on the coil) to switch an AC circuit (on the contacts) or vice-versa.
  • Use one signal (on the coil) to switch multiple circuits (multi-pole contacts).
  • Invert the sense of a signal (using the Normally Closed contact).
  • Use a signal from one circuit (on the coil) to switch another circuit (on the contacts) where the two circuits are not allowed to have any direct electrical connection.
  • Use one signal (on the coil) to switch between two alternate loads (on the contacts).
  • Various combinations of the above.

The example below demonstrates a relay performing a combination of several of the above functions.  A low voltage, low current control signal from a controller is switching a high voltage, high current load on and off.  The relay is also allowing a DC control signal to switch an AC load.  There is also electrical isolation between the drive circuit and the load circuit so that the controller is protected from the high voltage 240V power source:

In the next scenario a smoke detector signal is being sent to three different systems simultaneously while maintaining isolation between the systems to make sure there is no unintended interaction between the systems:

Here is an application where we want to reverse the sense of an electrical signal.  We have a foil alarm loop glued to a glass door.  When the foil is intact (current flowing through foil) we want to hold off the Alarm light and horn.  When the foil is broken (current flow stops) we want to energize the Alarm light and horn.  We can get the reverse action we want by using the Normally Closed contact on the relay:

Let’s see how we can use both the N.O. and N.C. contacts to get “alternate action” between two loads.  We decide that in addition to the ALARM light, we also want a NORMAL light for our alarm circuit that is on when the foil connection is intact.  So we want the action of the NORMAL and ALARM lights to alternate, one or the other is on at any time, but never both at the same time.  We can get this alternate action by adding the NORMAL light to the N.O. relay contact:

You may begin to see how versatile relays are.  Even with all the high-tech electronics in control panels, relays will be with us for a long time to come.  The operation is easy to understand and they can be used for many different applications.

Magnetic Latching Relays

There is a special class of relays known as “magnetic latching relays.”  These relays contain one or more small permanent magnets which hold the armature in the last commanded position when the coil drive is removed.  When a coil drive signal is applied, the magnetic field from the coil is stronger than the permanent magnets and can thus move the armature to the opposite position in spite of the permanent magnet’s presence.

Magnetic latching relays must have two coil drive commands, Latch and Unlatch.  Some relays accomplish this with two separate coil windings and two command leads:

Other magnetic latching relays have just two coil wires, but the polarity of the applied DC drive determines whether the command is Latch or Unlatch:

The Latch and Unlatch drive signals are typically meant to only be momentary, you normally would not leave either signal on the coil continuously.

Magnetic latching relays are frequently used in lighting circuits.  One example is the popular General Electric RR7 lighting relay.

Wrap-Up

Some points to remember about relays:

  • Make sure that the relay coil voltage and AC/DC rating matches the coil drive signal in the application circuit.
  • Some relays are available with a “coil energized” light built in. If any troubleshooting has to be done on the panel, these lights can be very helpful.
  • Some relays are available with a manual override button built in. If any troubleshooting has to be done on the panel, these override buttons can be very helpful.
  • Make sure that the relay contact voltage and current ratings are at least as high as the volts and amps that will be applied by the application circuit. If possible, use contacts rated higher than the volts and amps that will be applied.
  • If you need more poles than are available on one relay, you can use multiple relays with the coils connected in parallel.

Using a Multimeter Series – Resistance Measuring Basics

“Resistance is futile, Buwhahaha” exclaimed the villain, rubbing his hands together in glee as our hero struggled to free himself from the ropes that bound him.

Yep, we’ve all seen those Grade B movies.  Maybe in that situation resistance is futile, but in the world of electronics and HVAC controls resistance (electrical resistance) is very useful!

Designing selected values of resistance into the appropriate points of circuits lets us control the voltages and currents so the circuits perform as desired.  The most popular forms of temperature sensors used in HVAC (RTDs and thermistors) operate by varying their electrical resistance as temperature changes.  Resistors are also frequently used to convert the 4-20 mA signal from a transmitter into a 1-5V or 2-10V signal for controller inputs.

Electrical resistance is measured in units of Ohms.  When resistance gets into values larger than 1000 ohms it may also be expressed in Kilohms or just ‘K’ for short (50K = 50,000 ohms).  The symbol for resistance on electrical diagrams looks like this:

We’ll be measuring resistance with the Ohms function of our “multimeter” which can measure volts, current, resistance, and possibly other things too (frequency, capacitance, temperature with accessory probe, etc.).  When the multimeter is set to the Ohms function, it may be referred to as an “ohmmeter.”

Plug the Meter Leads Into the Correct Meter Jacks

Shown below is a typical multimeter face layout.  This is an actual meter Kele uses in our internal training classes.  Note that your multimeter controls may be arranged somewhat differently or completely differently:

To read resistance, plug the black meter lead into the COM jack and plug the red meter lead into the V/ohm jack.

Set the Meter Selector to Ohms

The meter selector switch has different major areas for choosing whether you want to read voltage, current, resistance (ohms), or possibly other things.  You need to move the selector switch to one of the positions in the Ohms area (left side of selector knob on our example meter).

Set Meter Range (Unless You Have an Auto-Ranging Meter)

Our example meter has different resistance ranges to choose from based on the maximum resistance you expect to measure.  Always choose the smallest range that’s higher than the highest resistance you are expecting to measure.  For example, if you are going to measure a resistance you think should be around 10K (10,000 ohms) then on the meter you would select the 20K range (because the 10K we want to check is higher than the next lower range which is 2K).

If you accidentally select a lower range than the resistance you are trying to measure, the meter won’t be damaged.  You’ll get some kind of “overrange” indication on the display.  This can vary from meter to meter.  Sometimes it’s a row of horizontal dashes, sometimes it’s “OL” for overload, or maybe something completely different.

If your meter has an Auto-Ranging function you don’t have to worry about setting the range, the meter will figure it out for you.  It will automatically step through the different ranges until it finds the lowest range that does not result in an over-range condition.

Auto-Ranging is very handy.  The down side is that it can take the meter longer to display a final stable resistance reading because it has to trail-and-error each time to find the right range.  A fixed-range meter manually set to the correct range will stabilize to a usable reading faster since it doesn’t have to experiment to find the correct range.

If the Resistor Is Connected In A Circuit, Disconnect At Least One End of the Resistor Before Taking A Measurement

If the resistor is connected in a circuit, at least one end of the resistor must be disconnected from the circuit before connecting the meter probes.  If the resistor is left connected to other devices at both ends, you will be measuring the “equivalent resistance” of other circuit components in parallel with the resistor of interest.  This will give a false reading that is lower than the actual resistor value.

It would also be a good idea to have the circuit powered down even though one end of the resistor is disconnected.  The connected end of the resistor could be attached to a live voltage or current source and bad things could happen if the probes slipped.

Place the Meter Probes Across The Resistance To Be Measured

Remember to keep your fingers on the insulated probe handles, don’t touch the metal probe tips with your fingers.  Your body is a resistor from hand-to-hand too, and if you grab both metal probe tips and try to read a high resistance value, your body resistance in parallel will introduce a measurement error.

If the display shows an over-range condition, just move the meter ohms selector to the next higher range and try again.

Special Notes On Temperature Sensor Resistance Measurements

If you try to measure the resistance of a thermistor or RTD temperature sensor with a long wire run attached, you may find that the readings jump around on the display.  This is caused by that long wire run acting as an antenna and picking up 60 Hz power line noise:

Some meters may have better noise filters than others, if you have more than one model meter available you might try them all to see which is better at noise rejection.

If the resistance value is jumping around, look for the minimum value that ever shows up on the display and the maximum value that ever shows up on the display.  Chances are that the true resistance value will be approximately halfway between those two values.

You might also notice that when you put the meter probes on the thermistor or RTD sensor, the resistance value starts to slowly move (up for RTDs, down for NTC thermistors).  It’s unlikely (although remotely possible) that you have an unstable sensor, it’s more likely you are seeing the effects of self-heating in the sensor.

The ohmmeter passes a small current through the sensor resistance in order to measure it.  This small current causes a slight heating of the sensor material.  Since temperature sensors are specifically designed to give large resistance changes with temperature change, this self-heating  shows up as a slow resistance change:

This is nothing to worry about and does not mean that the sensor is bad.  The initial sensor resistance value (before self-heating takes effect) more accurately represents the temperature of the medium being measured by the sensor.

Temperature sensors with a small thermal mass and slow-moving media are more likely to exhibit self-heating effects than sensors with a large thermal mass and swiftly moving media.  For example, a chip sensor sitting up in the air on two thin wires inside a wall-mount room housing will exhibit more self-heating effect than a duct sensor potted in the end of a metal tube placed in the moving air stream inside a duct.

Continuity Checks Using An Ohmmeter

Sometimes you simply want to know whether two points in a control panel or on a module terminal block are connected directly together.  The ohmmeter is an excellent tool for doing continuity checks.

To perform a continuity check:

  1. Make sure the circuit is powered off.
  2. Place the ohmmeter probes on the two points to be checked.
  3. If the resistance is less than 1 ohm, the two points are very likely directly connected.

Why wouldn’t the resistance read zero ohms if the points were directly connected?  Because all conductors icluding the ohmmeter test leads have some electrical resistance.  You can prove this by shorting the ohmmeter test probes directly together and observing the display, it will show a non-zero (but very small) resistance value.

Take-Away Points

  1. If the resistance to be measured is mounted in a circuit, disconnect at least one end of the resistor from the circuit before measuring .
  2. Noise pickup on long wire runs can make the resistance value jump around on the meter. Look for the lowest and highest displayed values and take the midpoint between the two as the true value. Try a different meter to see if it has better noise filtering.
  3. Slowly changing temperature sensor resistance after connecting the meter is probably self-heating of the sensor element and not a bad sensor.
  4. When taking continuity measurements with an ohmmeter, don’t expect to read 0.00 ohms across two connected points.  Any value less than 1 ohm is a good indication the points are directly connected.

Using a Multimeter Series – Current Measuring Basics

If you are involved in installing or troubleshooting HVAC systems, sooner or later you will be taking electrical current measurements. This tutorial will discuss the basics of how to take those current measurements.

Current is measured in units of amperes, usually abbreviated to simply “amps.” When working with small currents (less than 1 amp) it may be more convenient to describe current in “milliamps,” typically abbreviated as “mA.”

When you see the term “milliamp” or “mA” this means 1/1000 of an ampere. For example:

4 mA = 4/1000 amps = 0.004 amps
20 mA = 20/1000 amps = 0.020 amps

You might be measuring AC (alternating current) or you might be measuring DC (direct current) depending on the situation. AC current is constantly reversing directions whereas DC current is always flowing in the same direction.

Very likely you will be taking current measurements for one of two reasons:

  1. Measuring how much current a load draws from its supply (amps or mA, AC or DC).
  2. Measuring a 4-20 mA control signal value (always DC).

Amp-Clamp Ammeter Versus Multimeter

If a lot of your work is done on high-current AC power circuits, an “Amp Clamp” ammeter will be very useful. An amp clamp ammeter has spring-loaded jaws that simply snap around a conductor (no electrical connection required) and the built-in display (analog or digital) reads the amps of current flowing through the conductor:

ammeter

Amp-clamp ammeters are convenient, but they have some limitations:

  1. Typically an amp-clamp only measures AC amps; it can’t measure DC amps.  (There are amp-clamps with a special kind of sensor known as a Hall Effect sensor which can measure DC amps, but these are not typical of most amp-clamps).
  2. The range covered is usually high (hundreds of amps) so the accuracy on small amp values is poor.

If you are measuring relatively low value AC currents or DC currents, you will likely be using the amps/mA measurement function of a multimeter.   A multimeter can measure other quantities besides amps but today we are just concentrating on amp/milliamp current measurements.

Shown  below is a typical multimeter face layout.  This is an actual meter Kele uses in our internal training classes.  It’s a few years old but the functionality of the controls is the same as a modern multimeter.  Note that your multimeter controls may be arranged somewhat differently or completely differently.  No one likes to do it, but you might want to actually read your multimeter instruction manual if it hasn’t been thrown away/lost by now!

multimeter

To read current, plug the black meter lead into the COM jack and plug the red meter lead into the mA or 10A jack.  Using the mA jack, you can measure currents up to 200 mA on this particular meter.  Other model meters may have higher mA ranges available.  If you want to measure currents above 200 mA on this meter, you would move the red meter lead to the 10A jack.

Be aware of meter probe properties in the mA/amp mode!

When the meter probes are plugged into COM and mA/amps jacks, the meter’s internal resistance is very low, just a fraction of an ohm.  In the current mode, the probe-to-probe path through the meter looks almost like a straight piece of wire!

This has important implications.  If you put the current meter probes across a power source, you will short out the power source!  This makes sense if the probe-to-probe path looks like a straight piece of wire, right?

Many power sources have the capability to produce very large fault currents if their output terminals are shorted directly together.  If you do this with your current meter probes, very large currents can flow through the meter!

don't do this

Because it’s so easy to do this accidentally, most meter manufacturers put an internal fuse in series with the meter’s mA/amp jacks.  In that case, hopefully the fuse will blow and protect the meter and the power source.  Still, it’s a pain to open up the meter, locate the blown fuse, and replace it.

Some very cheap meters do not have a fuse in series with the mA/amp jacks.  If you send large fault currents through one of these cheap meters, the foil on the circuit board will blow apart in lieu of a fuse, rendering the meter a candidate for the trash can.  Hopefully your meter does have internal fuses on the mA/amps jacks.

Set the Meter Selector to mA

The meter selector switch has different major areas for choosing whether you want to read voltage, current, resistance (ohms), or possibly other things.  You need to move the selector switch to one of the positions in the mA area (lower right area on our example meter).  If you are measuring large currents (amps instead of mA) the meter selector may have a separate position for that, or it may still use the mA position since there is a separate jack for amps.  Check your meter’s user guide.  Our example meter above still uses the selector switch set to mA even though we are using the 10A jack for large currents.

Set Meter Range (Unless You Have an Auto-Ranging Meter)

Our example meter has different current ranges to choose from when using the mA jack based on the maximum current you expect to measure.  Choose the smallest range that’s above the highest current you are expecting to measure.  If you are not sure what range the current will be, start on the highest range and take a measurement.  Then if you can move down to a lower meter range, it will give you better accuracy.

If you accidentally select a lower range than the current you are trying to measure, the meter won’t be damaged unless you put a really large fault current through that blows the fuse.  Typically you’ll get some kind of “overrange” indication on the display.  This can vary from meter to meter.  Sometimes it’s a row of horizontal dashes, sometimes it’s “OL” for overload, or maybe something completely different.

If your meter has an Auto-Ranging function you don’t have to worry about setting the range, the meter will figure it out for you.  It usually starts on the most sensitive range and, if it sees an over-range condition, moves to the next higher range etc. until it finds the most sensitive range that does not result in an over-range condition.

Auto-Ranging is very handy.  The down side is that it can take the meter longer to display a final stable current reading because it has to trial-and-error each time to find the right range.  A fixed-range meter manually set to the correct range will stabilize to a usable reading faster since it doesn’t have to experiment to find the correct range.

Set Meter for AC or DC As Needed

If you set the meter to read DC mA and put the probes in a circuit with AC current, the meter will read essentially zero mA (the readout might jump around the zero reading a bit).

If you set the meter to read AC current and put the probes in a circuit with DC current, the meter display will jump up momentarily then “coast” back down to essentially zero current over time.

So be careful – an incorrect AC/DC meter setting will give completely bogus results!

If you are probing a “mystery circuit” and you’re not sure whether the current between two points is AC or DC, you can try both settings on the meter to see which gives you a non-zero value.

It’s Time To Place the Meter Probes in the Circuit

Now this is very important:  to take a current measurement, you must disconnect an existing wire in the circuit and connect the meter current probes to take the place of the original connection.

The idea is that you must make the current that would normally flow directly between the two points take a detour and flow through your current meter on the way to its destination:

 

Measuring SupplyMeasuring Supply Current

Measuring 4-20mAMeasuring 4-20 mA Signal

 

Remember to keep your fingers on the insulated probe handles, don’t touch the metal probe tips with your fingers just in case the connection contains high voltage.  If you have jumper wires with alligator clips, these can be handy for connecting the meter probes into the circuit hands-free.

We’ve shown the current meter connected at the load in the drawings above, but it could just as easily have been connected at the power source/signal source end instead.

In the case of AC current there is no polarity to worry about, the signal will always read positive on the display no matter which way the probes are placed.

In the case of DC current, the red probe should go on the more positive point and the black probe should go on the more negative point.  But if you should get it backwards no harm is done, the meter will just display a negative current value, the magnitude of the reading will still be correct and you will know that the point with the black probe is actually the more positive point.

If the display shows an over-range condition, just move the meter current selector to the next higher range and try again.

Average-Reading  Versus True-RMS AC current meters

Not all current meters are created equal when it comes to measuring AC current.  There are two different measurement techniques in use.

“Average-Reading” AC current meters only give a correct reading if the AC current is a sine wave.  Less expensive meters tend to use the average-reading measurement technique.

“True-RMS” AC current meters will give a correct reading no matter what the wave shape (does not have to be a sine wave).  This technique is typically reserved for the more expensive meters.

True-RMS current readings are based on what heating effect the current would have if passed through a resistance.  The fact is that many AC-to-DC power supplies draw their AC supply current in short, high-current pulses which are not a sine wave.  Since the hookup wire has resistance, and fuses are designed to open based on heating of the fuse element, True-RMS current measurements are more meaningful for applying the correct wire size and fuse size on an AC-to-DC power supply input.  True-RMS current meters are therefore preferable over average-reading current meters.

Final Thoughts

The most common complaint from the field is that a current meter is reading zero.  The most common problems are:

  1. The meter selector switch is not set for current (mA/amps).
  2. The meter leads are not plugged in the correct jacks (COM and mA/amps).
  3. The internal meter fuse is blown.
  4. The meter’s AC/DC switch is set opposite to the type of current actually present.

Using a Multimeter Series – Voltmeter Basics

If you live in the world of HVAC design/installation, sooner or later you’re going to need to take measurements on a circuit using a voltmeter (even if it’s “not your job,” we all know how that goes).

So we thought it would be a good idea to put together some basic instructions on using a voltmeter. Even if you’ve been using a voltmeter for years, there might be some tidbit of information here that you hadn’t thought about before. :-)

Voltmeter or Multimeter?

These days you’d be hard pressed to find a test meter that just measured Volts and nothing else. Everyone manufactures multimeters which measure volts, current, resistance, and possibly other things too (frequency, capacitance, temperature with accessory probe, etc.). But today we’re just going to concentrate on making voltage measurements with our multimeter.

Plug the Meter Leads Into the Correct Meter Jacks

This seems obvious, but this author has failed to do this many times. Shown below is a typical multimeter face layout. This is an actual meter Kele uses in training classes. It’s a few years old but the functionality of the controls is the same as a modern multimeter. Note that your multimeter controls may be arranged somewhat differently or completely differently. No one likes to do it, but you might want to actually read your multimeter instruction manual if it hasn’t been thrown away/lost by now!

To read voltage, plug the black meter lead into the COM jack and plug the red meter lead into the V/ohm jack.

Do not mistakenly leave the red meter probe in the mA or Amps jack and try to measure voltages between two points in a circuit. In the mA or Amps mode the meter leads essentially look like a direct connection and you will be shorting out the two measurement points in the circuit. Bad things can happen. Since we are discussing voltage measurements today, that’s all we’re going to say on the subject. You have been warned.

Set the Meter Selector to Volts 

The meter selector switch has different major areas for choosing whether you want to read voltage, current, resistance (ohms), or possibly other things. You need to move the selector switch to one of the positions in the Volts area (upper right area on our example meter).

Set Meter Range (Unless You Have an Auto-Ranging Meter)

Our example meter has different voltage ranges to choose from based on the maximum voltage you expect to measure. Always choose the smallest range that’s higher than the highest voltage you are expecting to measure. For example, if you are going to measure 24V then on our meter you would select the 200V range (because the 24V we want to check is higher than the next lower range which is 20V).

If you accidentally select a lower range than the voltage you are trying to measure, the meter won’t be damaged, you’ll get some kind of “overrange” indication on the display. This can vary from meter to meter. Sometimes it’s a row of horizontal dashes, sometimes it’s “OL” for overload, or maybe something completely different.

The only way you should damage your voltmeter from overvoltage would be if you exceeded the maximum rating for the meter. That value should be in the instruction manual and it’s almost always printed on the meter face too. It’s pretty high, typically something like 750V or 1000V, values you will probably never encounter in HVAC work.

If your meter has an Auto-Ranging function you don’t have to worry about setting the range, the meter will figure it out for you. It usually starts on the most sensitive range and, if it sees an over-range condition, moves to the next higher range etc. until it finds the most sensitive range that does not result in an over-range condition.

Auto-Ranging is very handy. The down side is that it can take the meter longer to display a final stable voltage reading because it has to trail-and-error each time to find the right range. A fixed-range meter manually set to the correct range will stabilize to a usable reading faster since it doesn’t have to experiment to find the correct range.

Set Meter for AC or DC Volts As Needed

 If you set the meter to read DC volts and put the probes on a AC voltage source, the meter will read essentially zero volts (the readout might jump around the zero reading a bit).

If you set the meter to read AC volts and put the probes on a DC voltage source, the meter display will jump up momentarily then “coast” back down to essentially zero volts over time.

So be careful – an incorrectly set meter will make you think there is no voltage present when there really is.

If you are probing a “mystery circuit” and you’re not sure whether the voltage between two points is AC or DC, you can try both settings on the meter to see which gives you a non-zero value.

Place the Meter Probes on the Circuit Points To Be Measured

To make a voltage measurement, you do not need to disconnect anything in the circuit. You do that for current or resistance measurements (which we are not covering today).

Remember to keep your fingers on the insulated probe handles, don’t touch the metal probe tips with your fingers. You might know the voltage is supposed to be low (24V) but why take a chance in case you’re mistaken or there’s a short in the wiring?

In the case of AC voltage there is no polarity to worry about, the signal will always read positive on the display no matter which way the probes are placed.

In the case of DC voltage, the red probe should go on the more positive point and the black probe should go on the more negative point. But if you should get it backwards no harm is done, the meter will just display a negative voltage value, the magnitude of the reading will still be correct and you will know that the point with the black probe is actually the more positive point.

If the display shows an over-range condition, just move the meter voltage selector to the next higher range and try again.

Average-Reading Versus True-RMS AC voltmeters

Not all voltmeters are created equal when it comes to measuring AC volts. There are two different measurement techniques in use.

“Average-Reading” AC voltmeters only give a correct reading if the AC voltage is a sine wave. Most AC voltage signals we read in the HVAC world are sine waves (or very close) so this is typically acceptable. Less expensive meters tend to use the average-reading measurement technique.

“True-RMS” AC voltmeters will give a correct reading no matter what wave shape (does not have to be a sine wave). This technique is typically reserved for the more expensive meters.

Do Newbie Practice On Low Voltages

If you’re new to taking voltage measurements with a voltmeter, we recommend you start with a low-voltage source just to keep things really safe. A step-down transformer with a 24VAC secondary or a bench power supply with a low voltage DC output would be great.

Follow the recommendations above and taking voltage measurements with a multimeter should be second nature in no time at all!

UFT Helpful Application Notes

Kele designed the Universal Flow Transmitter (UFT) for use with Badger/Data Industrial flow sensors, but in reality the UFT can be used with many other makes and models of flow sensors if their signal outputs are compatible with the UFT signal input.

Kele frequently gets asked the question “will this Model XYZ flow sensor work with the UFT?”  So we decided that it might be a good idea to write an article addressing this topic and at the same time provide other application information beyond that shown on the UFT data sheet.

UFT Power

The UFT requires 24VDC +/- 10% at 80 mA maximum for operation.  Note that the UFT cannot be powered with 24VAC.

UFT Flow Sensor Input Circuit

The UFT will accept pulses from the following types of flow sensors:

  1. Sensor output that is open-circuit in the “high” state and conducts to common in the “low” state.  Most Badger/Data Industrial flow sensors operate this way.  For this type of sensor output, install the “PWR XDCR” jumper located near the PWR LED across both pins of the header.  When the sensor contact is open, 8V will appear across it.  When the sensor contact is closed, 8 mA of current will flow through it.

    Figure 1: Switch Style Flow Sensor

  2. Powered sensor output that drives to a “high” value of +5 to +24VDC and drives to a “low” value of 0 to +2VDC.  For this type of sensor install the PWR XDCR jumper on just one pin of the 2-pin header.

    Figure 2: Voltage Drive Flow Sensor

Up to 2000 feet of sensor cable 20AWG or larger can be used with the UFT, but to reduce noise pickup, please use the shortest length of sensor cable actually needed for the application.

The flow sensor must put out no pulses (0 Hertz signal) with no flow.

To be compatible with the UFT Span adjustment range, the flow sensor must put out pulses between 15 Hertz and 150 Hertz at full flow velocity (max GPM).

There is an XCDR SIG IN LED that indicates the state of the input signal from the flow sensor.  When the input is “high” the LED is on, when the input is “low” the LED is off.  When the pulse rate is fast, the LED may appear to stay on continuously even though it is actually going on and off very rapidly.  If the sensor is working properly, you should always be able to see the LED flashing when the flow is first starting up or ramping back down to zero.  At no flow, the XDCR SIG IN LED may be either on or off depending on the resting state of the flow sensor output.

The UFT can be factory modified to handle higher input frequencies (up to 1000 Hertz) at max flow velocity.  It cannot be modified to handle any frequency lower than 15 Hertz at max flow velocity.

4-20 mA Output Circuit

The 4-20 mA output represents the instantaneous (averaged over about 5 seconds) flow rate.  The UFT powers (sources) the 4-20 mA internally, an external power supply should not be inserted in the 4-20 mA loop.  The output will be 4 mA at 0 GPM (no flow).  The output will be 20 mA at whatever maximum flow GPM the UFT is calibrated for.

The 4-20 mA output is designed to drive a maximum load impedance of 750 ohms.  If the mA output is open-circuited, approximately 20V will appear between MA SIG OUT and COMMON.

Figure 3: 4-20mA Output Circuit

The 4-20 mA output normally comes from Kele pre-calibrated for the sensor model and maximum flow rate specified on the customer order.  When Kele calibrates the UFT, a label is attached to the top of the board specifying the calibration parameters. 

Field Calibration (Not Recommended)

The UFT can be field calibrated if a steady maximum flow rate can be maintained (see following procedure).

Caution:  once the SPAN pot is turned in the field, it will be impossible to fall back to the factory calibration setting that was done at Kele.

  1. Stop flow completely (or disconnect sensor wire) and trim the ZERO pot for 4 mA output.
  2. Establish steady max flow rate and trim SPAN pot for 20 mA output.

The 4-20 mA output changes slowly when trimming the ZERO and SPAN pots, you must be patient and wait for the output to stop changing with each pot adjustment.

Pulse Output Circuit

The UFT pulse output circuit can be used to drive a mechanical/electronic totalizer or an automation system binary input point for gallon totalization in software.

The UFT pulse output is optically isolated from the remaining UFT electronics.  The pulse output does not drive any voltage of its own, the voltage must be provided by the load.  The pulse output connections are polarity sensitive.

In the “high” state the pulse output is open-circuit.  In the “low” state the plus and minus terminals are connected together with approximately a 0.7V difference between them.  This will be seen by most automation system binary inputs as a contact closure. The UFT pulse output can operate from 1-40VDC in the “high” (open) state.  The UFT pulse output can carry as much as 200 mA in the “low” state.

Figure 4: Pulse Output Driving Totalizer Module

Figure 5: Pulse Output Driving BAS Binary Input

The pulse output is jumper-selectable for divide-by-10 or divide-by-100 operation.  For divide-by-10 operation, one complete (high and low) output pulse is generated for each 10 sensor pulses.  For divide-by-100 operation, one complete (high and low) output pulse is generated for each 100 sensor pulses.  There is no other calibration for the UFT pulse output (no trimpots). There is a PULSE OUT LED which indicates whether the pulse output is open-circuit (LED off) or conducting (LED on).  Note that if the pulses stop coming from the flow sensor, the output could stop in either the open-circuit (LED off) or conducting (LED on) state.

Testing the UFT Without a Flow Sensor

  1. Disconnect the sensor wire (if present) from the XDCR SIGNAL IN screw.
  2. Install the PWR XDCR jumper on both header posts.
  3. Move the divide-by-10/100 jumper to the 10 position.
  4. Connect a jumper wire to the Common screw.
  5. Tap the other end of the jumper wire on the XDCR SIGNAL IN screw.

Figure 6: Testing UFT Without Flow Sensor

As you tap the wire, you should see the XDCR SIG IN LED go on and off.  The PULSE OUT LED should cycle on and off for every few taps of the wire.  The mA signal should rise above 4 mA and the faster you tap, the higher the mA should go. If the UFT behaves as described above, it is functioning properly.

Figure 7: Complete Application Diagram

Divide & Conquer Those Hard-to-Read Flow Meter Pulses

Some HVAC applications require reading and totalizing pulses from flow meters.  This sounds simple enough, just take the pulse output from the flow meter, connect it to a Binary Input (BI) on your controller, and set up the program logic to count pulses coming in on the BI.  What could go wrong?

Unfortunately things are not always as simple as they appear.  HVAC controllers frequently operate on the “scan” principle where the inputs are not read continuously, but only once per controller scan.  Controller scan time could be fast or slow depending on the controller design.

The controller only “sees” a pulse if the input is low on one scan and high on the next scan.  This means that:

  1. A pulse whose duration is longer than the time between controller scans should always be reliably detected:

    upd1
  2. A pulse whose duration is less than the time between controller scans will sometimes be detected and will sometimes be missed:

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NOTE:  some controllers have one or two “high-speed counter inputs” in addition to their regular Binary Inputs.  High-speed counter inputs have a much higher scan rate and may be fast enough to detect your flow meter pulses reliably with no additional hardware needed.  Always check the controller datasheet to see if your controller has any high-speed inputs you can use for connecting your flow meter.

What can I do if my flow meter pulses are too narrow to be reliably detected by my controller?

Kele sells a Universal Pulse Divider (UPD-2) that can divide the incoming meter pulses by a selectable divisor and output a wider pulse that your controller can reliably detect.

What kind of pulse signals can the UPD-2 accept on its input?

The UPD can accept pulses from a simple contact closure, a transistor switch, or a driven 0-5VDC signal:

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In the Low state, the output of the flow meter must be able to sink approximately 2.3 mA of current.  This will be compatible with most flow meters.

What kind of output does the UPD provide?

The UPD output is an optically-isolated electronic switch.  It is polarity sensitive.  In the High state it is open-circuit, in the Low state it closes the circuit.  In the Low state, the output can sink up to 6 mA of current.  In the High state, it can tolerate up to 30VDC.  The UPD output is compatible with most controller Binary Inputs which are typically configured as an internal reference voltage with a pull-up resistor as shown below:

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What is the minimum high or low pulse time the UPD will detect on its input?

The UPD will reliably detect any pulse with a high or low time of at least 10 milliseconds.  This limit is programmed into the UPD’s microcomputer firmware, the hardware could actually detect shorter pulse durations.  But HVAC equipment lives in electrically noisy environments, and we don’t want to start counting noise glitches as flow meter pulses, so we set the minimum valid pulse duration at 10 milliseconds.

How do I set the Divisor value on the UPD?

The UPD has an 8-slider dipswitch for setting the divisor.  Each slider has a divisor value assigned thus:

  • Slider 1 = divide-by-1
  • Slider 2 = divide-by-2
  • Slider 3 = divide-by-4
  • Slider 4 = divide-by-8
  • Slider 5 = divide-by-16
  • Slider 6 = divide-by-32
  • Slider 7 = divide-by-64
  • Slider 8 = divide-by-128

The total divisor value is the sum of all the sliders which are turned on.  For example, if sliders 2 and 3 are turned on, the total divisor is 2+4 = 6.

** After changing the dipswitch sliders, be sure to cycle the UPD power as the dipswitches are only read by the microcomputer chip at power-up.

How does the division logic work on the UPD?

It’s important to understand how the UPD division logic works.  The UPD counts all the up-and-down signal transitions on the incoming pulse.  For every N up or down transitions on the input signal, the UPD makes 1 transition on the output signal (where N is the total divisor selected).

This is one of those times where a picture is worth a thousand words.  Shown below is the division action when the total divisor is set to 3 (sliders 1 and 2 On, all others Off):

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How do I know what the duration of the output pulse will be?

That totally depends on the durations of the high and low portions of the original input signal.  Let’s say that, on the diagram above, the input signal high portion is 15 msec long and the input signal low portion is 25 msec long.  If we add those values to the diagram, it’s easy to calculate the duration of the output signal high and low portions:

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You will note that the durations of the low and high parts of the output are not identical.  That’s because we used an odd divisor.  If you use an even divisor, the output low and high durations will be identical.  If you use an odd divisor, the output low and high durations will be close, but not identical.

You should be able to find some documentation about the flow meter output pulse timing on the flow meter datasheet.  Typically the high portion of the pulse is fairly constant and the duration of the spaces between the pulses changes as the media flow rate changes (faster flow = shorter spaces between the pulses).

The flow meter datasheet may specify a nominal pulse width and have a calculation to give the output frequency versus flow rate.  This is no problem, if we know the nominal pulse width and the frequency we can calculate the duration of the low part of the signal as follows:

Period of signal = 1/frequency

Duration of low part of signal = period of signal – duration of pulse

For example, say the nominal pulse with is 20 msec and the frequency at max flow rate is 20 Hz.

1/frequency = 1/20 Hz = 0.050 sec = 50 msec period

50 msec – 20 msec pulse width = 30 msec duration for low part of signal

So the overall approach to using the UPD is this:

  1. Use flow meter datasheet and anticipated maximum flow rate to figure the shortest duration pulses/spaces that should be coming from the flow meter.
  2. Use controller datasheet to find the shortest pulse/space that the controller is guaranteed to reliably detect.
  3. Draw out the flow meter pulse train on paper, including the durations of the pulses and spaces.
  4. Look at the drawing and start adding together the high and low durations of the meter signal until the sum exceeds the minimum pulse duration required by the controller.  If the sum should fall exactly on the minimum spec for the controller, add one more section from the picture as a safety margin.
  5. Count the number of up-and-down edges for the sections selected in step 4 ignoring the initial rising edge.
  6. Set the UPD divisor to the number of up-and-down edges counted in step 5.
  7. In your controller logic, multiply the raw count collected on the Binary Input times the UPD divisor to arrive at the true meter pulse count.

How about an example?

  1. The flow meter datasheet says the nominal pulse width is 15 msec and the calculated frequency at our max flow rate is 15 Hz.We figure the duration of the low part of the signal as:
    1/15 Hz = 0.067 sec = 67 msec period for the signal
    67 msec period – 15 msec pulse = 52 msec duration for low part of signal
  2. The controller datasheet says that the minimum pulse width/space that can be reliably detected is 100 msec.
  3. We draw a picture of our flow meter signal showing the 15 msec and 52 msec times:upd10
  4. We start adding high and low durations together until we exceed the 100 msec. minimum pulse detect time specified for the controller:upd11
    So we see that the output pulse duration will need to be a minimum of two high portions and two low portions of the input signal.
  5. Now count the number of up-and-down edges for the selected sections ignoring the initial rising edge:
    upd12
  6. In step 5 we counted 4 pulse edges (ignoring the initial rising edge) and so we set a divisor of 4 on our UPD by turning on slider #3.  And this is the final result of our efforts:
    upd13
  7. Keep in mind that every pulse counted by the controller input actually represents four pulses from the flow meter.  So to get the true flow meter total counts for your application program, you need to multiply the controller counts x 4.

What happens if you set the Divisor = 1?

You simply get a replica of the original input signal.  But remember, the output is electrically isolated from the input so you could use the UPD as a simple pulse signal 1-for-1 isolator.

What else should I know about the UPD-2?

The UPD-2 has an on-board 24VAC isolation transformer for its power supplies so you can get your 24VAC from any convenient source without worries about ground interactions.

Each UPD-2 contains two completely independent divider sections electrically isolated from each other.  One UPD-2 can serve two separate flow meter/controller setups or you can parallel the UPD-2 inputs from one flow meter and get two electrically isolated output pulses:

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Conclusions

Flow meter output pulses are sometimes too narrow to be reliably detected on a controller’s binary input if the controller scan time is slow.  Inserting a UPD-2 Universal Pulse Divider between the flow meter and controller creates wider pulses that the controller can reliably count.  There is a well-defined method for deciding the correct divisor to use on the UPD-2.

The UPD-2 is dual-channel device which can serve two separate flow meter/controller setups or a single flow meter can drive both UPD inputs to create two separate electrically-isolated pulse output signals.