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Measuring 120/240 VAC (App Note)

Warning

The voltages discussed in this app note are dangerous to people and equipment.  Only qualified individuals should engage in these measurements.

How can I measure 120 VAC (or other high AC voltages)?

The first question is what do you want to measure?  Voltage, current, power?  Do you want the waveform, or perhaps you just want a singular value that represents RMS, true RMS, maximum, minimum or frequency?  The most common answer is that someone simply wants to get a value such as "120.0" that is the RMS of the AC voltage.

Quick answer

The easiest and safest solution is to use a sensor or transducer that connects to the AC voltage and outputs an isolated DC signal that is proportional to AC RMS. Same goes for current or power measurement. Some possible sources are listed below.

Measuring high AC voltage

You need to reduce the voltage to the analog input range of the LabJack which is usually ±10V.

  • Transformer: Provides isolation, but accuracy is often a problem.
  • Resistive Divider: Simple but no isolation. Can even be done with a series resistor in front of an LJTick-Divider.  The lack of isolation provides added danger, so do not attempt unless you know what you are doing.
  • Sensor or Transducer: The best option is to find a sensor or transducer made to do just this.

Isolation is recommended between AC mains and the LabJack.

Some transducers will output a DC signal that is proportional to the RMS of the AC voltage. This is very simple to measure. Other transducers will simply output an AC signal that is an attenuated version of the original signal, in which case you have to acquire the waveform and then do the desired math to get the value you want (e.g. RMS).  T-series devices have AIN-EF features that might be able to do the desired math (e.g. RMS) for you.

Measuring current on a high voltage AC line

You need to convert current to the analog input range of the LabJack which is usually ±10V.

  • Current Transformer: Low cost solution that provides isolation.  AC output signal requires math to get RMS.
  • Resistive Shunt: No isolation and can be expensive. The lack of isolation provides added danger, so do not attempt unless you know what you are doing.
  • Sensor or Transducer: The best option is to find a sensor or transducer made to do just this.

Isolation is recommended between AC mains and the LabJack.

Some transducers will output a DC signal that is proportional to the RMS of the AC current. This is very simple to measure. Other transducers will simply output an AC signal that is an attenuated version of the original signal, in which case you have to acquire the waveform and then do the desired math to get the value you want (e.g. RMS).  T-series devices have AIN-EF features that might be able to do the desired math (e.g. RMS) for you.

Is your intention to multiply RMS current times RMS voltage (measured or assumed 120VAC) to calculate power?  That is often a decent approximation, but realize it is only valid if both waveforms (voltage and current) are perfect sinusoids and both waveforms are perfectly in phase.  The correct way to measure power is to acquire both waveforms and multiply them point-by-point to get the power waveform.

Sources for high voltage AC sensors

Here are some possible sources we found for sensors.  Feedback is welcome on what anyone else finds or uses:

http://panelmeters.weschler.com/
http://www.ohiosemitronics.com/
https://www.pc-s.com/
https://www.flex-core.com/
http://transdatainc.com/
https://new.abb.com/