Current to Voltage Conversion for BAS Systems

One of the differences that exists among the various BAS systems that are available today is the type of signals that are acceptable as analog inputs. For example, when it comes to DC voltage and current inputs, some systems can only read a voltage while other systems can be configured to handle either voltage or current. If your BAS computer can read a voltage, but cannot accept a current input directly, you can still take advantage of the many 4-20 mA current producing devices that are available. A simple conversion utilizing only a resistor at the computer input terminals is all that is required to convert a DC current into a readable voltage.

To make the conversion from 4-20 mA current to 1-5 VDC voltage, a 250 ohm resistor is wired across the input terminals at the computer (See Figure 1). This puts the resistor in series with the 4-20 mA current creating a voltage drop at the computer input. Using Ohm’s Law (E=IR) where:

E = voltage in volts
I = current in amps
R = resistance in ohms

we can see that with a 4 mA signal the input would see a 1 VDC drop across the resistor (0.004 x 250 = 1.0). With a 20 mA signal, the voltage at the input would be 5 VDC (0.020 x 250 = 5.0). Other voltage ranges can be reached by using different value resistors. Assuming a 4-20 mA signal, a 500 ohm resistor would produce a 2-10 VDC signal and a 750 ohm resistor would produce a 3-15 VDC signal. The resistor should be selected for an accuracy of at least 1% and be rated for the current loop’s power; 1 watt will handle up to 750 ohms. The maximum load resistance that can be supported by a current producing transducer can vary depending on the type of transducer and the loop power supply. Be careful that your transducer is rated to handle the conversion resistor and any other loads connected to the current loop.

The next step is to convert the measured voltage input into the proper engineering units. The following equation can be used for this purpose:

Sensor Output Value =

where, Sensor Span = Max value – min value Offset = Zero point Vmin = R1 x min amps output Vmax = R1 x max amps output Vout = R1 x measured current output
see figure below

Example: Measured output = 12 mA R1 = 250 ohms Sensor range = 32° to 122°F (Span = 90°) Offset = 32° Vmin = 250 x 0.004 amps = 1.0 VDC Vmax = 250 x 0.020 amps = 5.0 VDC Vout = 250 x 0.012 amps = 3.0 VDC

Check-out of the installation is accomplished by using a voltmeter to measure the voltage across the conversion resistor at the input terminals. The equation above can be used to convert the measured voltage into engineering units which can then be compared to the actual process being monitored.

A 250 ohm, 1%, 3-watt resistor is included with all 4-20 mA current producing transducers from Kele . By simply adding this resistor to create a 1-5 VDC signal, these transducers can be utilized by BAS computers that cannot ordinarily accept a 4-20 mA current signal.

Remember that the use of this resistor could add ±1% error to your sensor. To remove this error, read the actual resistance with a high-accuracy meter and use that value for calculations.