Building Automation Systems

Building Automation Systems

Post Date: December 11, 2015 Topic: Controls Author: Gary Arlet
One of the differences that exists among the various BAS systems that are available today is the type of signals that are acceptable as both analog inputs and analog outputs. 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. This also generates one of the most commonly asked questions, “How do I convert between a voltage and a milliamp signal?” The answer is quite simple. 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 and vice a versa.

To make the conversion from 4-20 mA current to 2-10 VDC voltage, a 500 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

Figure 1

You can see that with a 4 mA signal the input would see a 2 VDC drop across the resistor (0.004 x 500 = 2.0). With a 20 mA signal, the voltage at the input would be 10 VDC (0.020 x 500 = 10.0). Other voltage ranges can be reached by using different value resistors. Assuming a 4-20 mA signal, a 250 ohm resistor would produce a 1-5 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.