How to monitor AC voltage and current with your RMS unit.

Our remote voltage monitoring boards have built-in DC voltmeters that can measure +/- 100 volts. If all you need to do is sense the presence of AC power, then this is easily done by using a common AC to DC wall transformer as shown below. Plug the transformer into a wall outlet or generator outlet, then attach the wires to any of the RMS voltmeters to monitor if AC power is available. NOTE: Make sure the transformer outputs DC voltage not exceeding 100 volts.

Wall wart Transformer

But what if you need to monitor the actual AC voltage or current?

The aim of this project is to show you how to monitor AC voltage and current using the RMS remote monitoring system. Please read the disclaimer at the bottom of this web page before proceeding.

Measuring AC voltage

Two major problems to overcome are:

The voltmeters on the RMS board can only measure DC voltage.
The voltmeters on the RMS board has a limit of +/- 100 volts.

What is needed in this case is a way to convert the AC voltage to a DC voltage that can be measured by the RMS board. To do this we need to use a transducer. A transducer is a device that converts one type of energy to another. The transducer we tested with is the model number AVT-300c from Ohio Semitronics shown below.

The AVT-300c can take 0 - 300 volts AC input and will output a relational 0 - 10 volts DC. This makes measuring AC voltage by the RMS board easy. Hook up the AC wires to terminals 3 and 4 on the transducer. Hook up the output voltage wires from the transducer terminals 1 and 2 to any of the RMS DC voltmeters. To figure out the AC voltage, simply take the measured voltage from the RMS voltmeter and multiply the reading by 30. The RMS board makes this easy by providing an adjustment multiplier for each voltmeter.

Several transducers are available from Ohio Semitronics that have different AC input voltage ranges. AC voltage ranges include 90, 150, 300, and 600 volts.

Measuring AC current

Measuring AC current is also easy using a current transformer (CT). The current transformer shown below is a non-invasive split core current sensing device. Several models are available that measure up to 60 amps AC and output 1 volt DC. Clamp the CT around one power wire of the current source you want to measure. Hook up the output wires from the CT to any of the RMS DC voltmeters. To figure out the AC current, simply take the measured voltage from the RMS voltmeter and multiply the reading by the maximum input of the CT. So in the case of a 25 amp CT that outputs .5 volt DC, you would multiply the .5 by 25 to know that 12.5 AMPS of current are flowing through the CT. Some inexpensive current transformers can be found here. Be sure to purchase models that output DC voltage.

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