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.

“Currently” Playing: Connecting A 4-20 mA Constant-Current Signal To Multiple Loads

One of the most popular and long-lived methods for transmitting analog control signals in the HVAC and industrial control worlds is the “constant-current signal loop.” In this scheme, the value of the current flowing in the circuit is the control variable, rather than any voltages that may appear at different points in the circuit. The constant-current loop is very robust and noise-tolerant and can run for long distances. Even in this high-tech digital age, it remains a very popular analog signaling method.

This article will deal with how to connect multiple loads to a constant-current signal loop. Connecting multiple loads to a constant-current signal loop is totally different than connecting multiple loads to a voltage signal source (which is covered in a previously-published article for those interested).

In the HVAC world, the most common range of current used for constant-current signaling is 4-20 mA (1 mA = 1 milliampere = 0.001 amperes of current). 4 mA represents 0% of the variable’s value and 20 mA represents 100% of the variable’s value. Other current ranges can be used, but today we’ll work exclusively with 4-20 mA current loops.

Current loop load “resistance” or “impedance”

Every load to be connected in the current loop will have an electrical characteristic called “resistance” or “impedance.” The load’s datasheet might use either term. Don’t worry about whether the datasheet says “resistance” or “impedance” we’ll take them to be the same thing. In this article we’ll use the term “impedance.”

Impedance is measured in units called “ohms.” If you don’t know what ohms are, don’t worry. All you have to do is use the numbers in some simple calculations later on.

Basic constant current loop configuration

Before we discuss connecting multiple loads to a constant current loop, let’s get familiar with the basic constant current loop containing a single load:

currently figure 1

On the left we have a constant-current source that is regulating the current to some desired value (the desired value is determined by a calculation in a controller, measured physical value in a sensor, etc.). The constant current leaves the source, flows through the load, and returns to the source.

Simple, right? What could go wrong? As usual, “the devil is in the details.”

Two different styles of current sources

There are two different styles of current sources you should be aware of. We’ll call the first style the “locally powered” current source. It has its own source of power for the 4-20 mA output and the only other item needed to complete the loop is the load:

currently figure 2

You will usually find this type of current source on the output of a powered controller.

We’ll call the second style the “loop powered” or “2-wire” current source. It does not have its own power source built-in, but rather depends on a separate power source (usually 24VDC) wired into the loop:

currently figure 3

You will find this type of current source in many 4-20 mA loop-powered temperature, pressure, and humidity transmitters.

Current sources have a “maximum load impedance” they can drive

Current sources like small values of load impedance, in fact they can drive their current straight into a short circuit (zero ohms load impedance)!

Current sources run into trouble when the load impedance gets too large. When the load impedance gets too large, the current source will not be able to make the higher values of current (near the 20 mA end) flow. The current value will plateau at some value less than the desired value.

How do I know the “maximum load impedance” my current source will support?

For a locally-powered current source, you will need to find the maximum load impedance value on the product’s datasheet.

For a loop-powered current source, you may find a parameter called “maximum load impedance (with 24V supply)” on the datasheet. If you do not, you will have to perform the following steps:

1. Find a parameter called “minimum operating voltage” (or some very similar wording) on the current source datasheet.

2. Calculate maximum load impedance as follows:

Maximum load impedance = (Loop power supply – minimum operating voltage) / 0.020

For example, suppose we are using a 24V loop power supply and we see on the datasheet that the 2-wire current source needs a minimum of 11V to operate. Then we would calculate:

Maximum load impedance = (24V – 11V) / 0.020 = 650 ohms

Now that we know something about how a single-load constant-current loop works, let’s see what kind of complications we get into when trying to drive multiple loads!

Connecting multiple loads to the current loop

Rule #1: All loads must be connected in series around the loop, never in parallel.

Below are sketches of the right way (series) and the wrong way (parallel) to connect loads in a current loop:

currently figure 4

currently figure 5

At first glance this requirement seems simple enough. That’s because we’ve drawn all the loads as 2-wire loads that don’t require power supplies. A not-so-obvious problem may occur if the loads do require power supplies. The current will always take the “path of least resistance” back to the current source whatever that path might be. If one of the loads before the last load has a power connection back to the current source, this connection will be the “path of least resistance” and the 4-20 mA will bypass the rest of the loads in the loop:

currently figure 6

If the powered load is a standalone module, you may be able to use a dedicated “floating” transformer or small DC power supply to power the load and resolve the issue:

currently figure 7


If multiple loads in the series connection are powered, each load would need its own power supply that is floating with respect to all the other supplies. For example, if Load 1 and Load 2 were both powered, Load 1 would need a power supply that is floating with respect to Load 2 and the current source, and Load 2 would need a power supply that is floating with respect to Load 1 and the current source. It can get messy quickly if there are several powered loads in the loop!

Consider also that If the powered load is a controller input and the controller is serving other inputs and outputs, it wouldn’t be practical to “float” the entire controller subsystem’s power connections.

In cases where a floating power supply for each load is not practical, it’s time to employ a signal isolator module such as the DT-13E from Kele:

currently figure 8


The DT-13E signal isolator has its inputs isolated from its outputs, and both inputs and outputs are isolated from the power terminals. So with the DT-13E installed, it just doesn’t matter where Load 2’s power source comes from, it will not corrupt the 4-20 mA current in the loop.

Rule #2: The sum of all the load impedances must be less than the maximum load Impedance the current source can support.

When the mA loads are wired in series (as they must be) the total load impedance seen by the current source is the sum of the individual load impedances. So if each of the three loads in the drawing below is 250 ohms, the total load impedance seen by the current source would be 250 + 250 + 250 = 750 ohms:

currently figure 9


Suppose your current source datasheet shows that it can only drive 650 ohms maximum impedance? Houston, we have a problem…

What can you do if your total load impedance adds up to more ohms than the current source can handle? This is another way that the DT-13E signal isolator can sometimes help. Many HVAC 4-20 mA loads have impedances of 250 to 500 ohms, but the DT-13E mA input impedance is only 125 ohms. So by isolating one or more loads using DT-13Es, not only do you remove grounding concerns but you may also lower your total loop impedance to a value the current source can handle:

currently figure 10


Now the current source only sees 250+125+250 = 625 ohms which is lower than its maximum load impedance, and Rule #2 is satisfied.


Multiple loads inserted in a 4-20 mA current loop must be connected in series. If the series-connected loads are powered devices, “sneak paths” between the load power supply connections and the current source may misdirect the 4-20 mA so that not all the loads receive the signal. Isolating the series-connected load with a DT-13E or similar signal isolator will solve this problem.

The sum of the series-connected load impedances may be too high for the current source to handle. The DT-13E signal isolator’s impedance of 125 ohms is less than that of many common HVAC loads. Isolating a load with the DT-13E may lower the total loop impedance to an acceptable value.

Connecting A Voltage-Output Signal Source To Multiple Loads

The Tech Support crew here at Kele frequently gets quizzed by customers who have a voltage-output signal source which must drive multiple voltage-input loads. Requests for “voltage signal replicators” come in frequently. Sometimes extra hardware is needed, but sometimes it isn’t! So we thought it might be a good idea to write an article addressing this topic. Here we go…

*** A note about the terms “resistance” and “impedance”

When looking at datasheets for HVAC control modules, you may see the terms “resistance” or “impedance” used in describing signal input/output loading characteristics.  Strictly speaking, “resistance” and “impedance” are different entities (impedance = resistance + reactance) but for a slow-moving control signal which is what we usually have in the HVAC world, the two terms can be used interchangeably.  So don’t worry about whether the datasheet says “resistance” or “impedance” we’ll take them to be the same thing.  In this article we’ll use the term “impedance” because it’s one letter shorter than “resistance” and we’ll save keystrokes! J

Impedance is measured in units called “ohms.”  If you don’t know what ohms are, don’t worry.  All you have to do is poke the numbers into some simple calculations later on.  You may see a suffix “k” or “K” appended after the ohms value on a datasheet.  This is important to note!  The “k” or “K” means “times 1000” so an impedance of “100K” means 100,000 ohms and that’s the number you would put into your calculations.

On with the show…

The first thing we must do is answer a few questions about the signal source and loads:

  1. Do the Signal Commons on the different loads have to be isolated from each other?
  2. Does the Signal Common on the voltage source have to be isolated from all loads?
  3. What is the “minimum load impedance” the signal source can drive (datasheet item)? (Alternately, this could be specified as “maximum output current”).
  4. What is the “input impedance” of each of the voltage-input loads (datasheet item)?

The first two questions are sometimes difficult to answer, and are application-dependent.  If you cannot find concrete answers, you can always assume that every device must have isolated signals.  This approach will always work, but it will also cost more money as signal isolator hardware is required.

With regards to question #3, the signal source datasheet may specify a “maximum output current” value instead of “minimum load impedance.”  The “maximum output current” will be expressed in units of mA (milliamps).  No worries, we can calculate what we need by this equation:

Minimum load impedance = Largest output voltage required/maximum output current

As an example, suppose the signal range of interest is 0-10V and the maximum output current available from the signal source is 2 mA (0.002 amps):

Minimum load impedance = 10V / 0.002 amps = 5000 ohms

Calculating “equivalent load impedance”

When connecting voltage loads in parallel, you’ll need to calculate the “equivalent load impedance” of all the loads connected together.  There are a couple of ways to do this:

If all load impedances are the same—

Equivalent load impedance = impedance of one load / number of loads

If the load impedances are different –

Equivalent load impedance = 1 / (1/Load1 impedance + 1/Load2 impedance + …)

The second calculation is most easily done using the 1/X key on your calculator.  Just do a 1/X calculation for each load impedance and add the terms together.  Then do a final 1/X calculation on the first result.  Here’s an example—

Load 1 = 10,000 ohms (10K)

Load 2 = 50,000 ohms (50K)

Load 3 = 100,000 ohms (100K)

Equivalent load impedance = 1 / (1/10000 + 1/50000 + 1/100000)  =  7,692 ohms

The two rules that must be satisfied

There are basically two rules that must be satisfied when driving multiple loads from a voltage source:

  1. If electrical isolation is required, you must add a hardware signal isolator module to each device to be isolated.
  2. At each voltage output-to-input interface, the equivalent load impedance of the loads connected together must be higher than the minimum load impedance of the signal source

Case 1:  It’s OK for all devices to have their Signal Commons tied together 

This case can be done using direct wiring (NO extra hardware) if the equivalent load impedance (calculated above) is greater than the signal source’s minimum load impedance:

direct connect method

That’s great UNLESS… your equivalent load impedance is smaller than the voltage source’s minimum load impedance.  Then what are you going to do?

When the equivalent load impedance is too small for the signal source to drive directly, you will need to insert a “signal booster” device of some sort between the original signal source and the loads.  The signal booster doesn’t need to boost the voltage (you typically want the same voltage out that’s coming in).  It needs to boost the available current so there is enough to drive all the loads connected together.

The Kele UAT is a good device to use as a signal booster in this application.  A look at the datasheet shows that its voltage output can drive up to 20 mA of current.  Let’s assume that the signal range of interest is 0-10V so the maximum voltage we ever need to output is 10V.  Then the minimum load resistance supported by the UAT would be:

UAT minimum load resistance = 10V / 0.020 amps = 500 ohms.

This is plenty of drive power for most parallel-connected voltage loads.

Now when we insert a signal booster device, we must insure that the input impedance of the booster itself is not too low for the original signal source.  We see that the UAT input impedance on the 0-10.9V range is 156K ohms (156,000 ohms).  If your UAT datasheet shows 156 ohms input impedance, don’t panic!  It’s really 156K, some datasheets have a typo.  Our apologies.  So the UAT input impedance of 156K is far higher than the signal source’s minimum load impedance of 5K, and it’s all good:

signal booster method

Now you should understand that the UAT does not provide signal isolation as it has one Common terminal for both signal input and output.  If you require isolation between devices, read on…

Case 2: Load Signal Commons can be tied together, but voltage source needs isolation

In this case you will need a signal isolator device such as the Kele DT-13E.  On the DT-13E, the input signal terminals are completely isolated from the signal output terminals and both of those terminal sets are completely isolated from the power terminals.

We look at the DT-13E datasheet and see that the input impedance on the 10V input range is 13.3K.  This is higher than the signal source’s minimum load impedance of 5K, so there is no problem driving the DT-13E input from the original signal source.

Looking at the DT-13E voltage output, we see that the maximum current available is 6 mA.  For a 0-10V signal, the max voltage we need to drive out is 10V so the DT-13E’s minimum load impedance will be:

DT-13E minimum load impedance = 10V / 0.006 amps = 1667 ohms.

If we have the same three 10K loads paralleled as in the previous example (3333 ohms equivalent load) then the DT-13E has enough drive and this setup should work just fine:

isolated signal source

Case 3: Load Signal Commons must be isolated from each other

In this situation, each load will need its own signal isolator module like the DT-13E.  The most obvious setup to use is shown in the next figure.  When we do the load calculations, however, we find that three DT-13E inputs in parallel will present an equivalent load impedance of 13.3K / 3 = 4433 ohms.  Oops, that is lower than the minimum load impedance of 5000 ohms specified for the signal source:

individually isolated loads

How do we fix this problem?  Here are some possibilities:

  1. Use a different signal source with more drive capability.
  2. Use different signal isolators with higher input impedance.
  3. Try rearranging the wiring using the devices that we have.

Let’s see what we can do with option #3.  Realizing that inputs and outputs are isolated on the DT-13E modules, it’s perfectly legal to take one of the DT-13E inputs and drive it from another DT-13E’s output.  Let’s check the math to see if this would work.

With only two DT-13E inputs paralleled on the signal source, the equivalent output impedance would be 13.3K / 2 = 6650 ohms.  This is higher than the signal source’s minimum load requirement of 5000 ohms, so that works.

With the new arrangement, one DT-13E output will drive a parallel combination of the original 10K load and one DT-13E input (13.3K).  The equivalent load impedance will be:

Equivalent load impedance = 1 / (1/10000 + 1/13300) = 5708 ohms.

This is higher than the DT-13E output’s minimum load impedance of 1667 ohms so that works too!  And this is the final configuration:

idividually isolated 2


By following the two interface rules discussed above, multiple voltage-input loads may be successfully driven by a common voltage-output signal source.  The math is simple algebra and can be done on any calculator.  Happy interfacing!