Analog Signals: Milliamps vs. Voltage

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In building automation system applications, the most common types of analog signals are milliamp and DC voltage.

DC voltage signals are most often measured using 0-10V signals, but we also sometimes see 0-5V or 2-10V. The reason for the popularity of the voltage signal is it is easy to troubleshoot and almost all controllers will accept this type of signal.

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Depending on the size and the length of wire used, you may run into potential accuracy issues with voltage signals, because of a phenomenon called voltage drop. Using this voltage drop calculator, you can quickly estimate the voltage drop based on wire material and size, conduit material, power factor, voltage, phase, number of conductors, distance, and load current. You can also calculate voltage drop yourself using Ohm’s Law, where Voltage = Current x Resistance.

Other potential shortcomings of the 0-10V signal are that it requires a third wire for power supply, and it is more difficult to detect a wiring problem. The wiring issue may come in when you are receiving a 0V signal. This could be because there is no signal being sent or because a wire has been disconnected.

Milliamp signals on the other hand are typically measured using a 4-20mA signal. Where 0-10V signals require an additional wire for power supply, devices using milliamp signals do not, because there is always at least 4mA flowing through the loop.

Although milliamps signals require a converter at both ends of the wire to convert voltage to current and back to voltage again, voltage drop is not a concern for this type of signal. If amps go into a wire, they must come out the other side. Also, because the minimum signal is 4mA, we know that if we are receiving a reading of 0, there must be a wiring issue.

Because a 4-20mA signal is able to provide more accurate readings over long distances, it has become the preferred analog signal in many building automation devices.

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