How to measure floating high voltage AC with Op-Amp

Published by MKDas on

Measuring high voltage AC is not safe just with resistor voltage divider. Or sometimes, we can not connect one terminal to GND of the measuring circuit. Sometimes, we need to measure a floating high voltage. In this article, we are going to learn how to do that floating high voltage measurement using Op-Amp.

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What is floating voltage?

In most of the cases, one terminal of the measuring point is connected to the GND of measuring circuit. But sometimes, we can not do that. Like if you wish to measure 220V AC using your MCU circuit, you should not connect the Neutral terminal to the GND of your MCU circuit to keep it safe. In this case, you need to keep that Neutral terminal floating.

That means, the floating voltage means, the voltage difference between two points of which are not connected to VCC nor GND.

How to measure floating voltage?

There are different ways to measure this floating voltage. You can use a transformer, Op-Amp or even opto-couplers.

Using a transformer will make it safe and isolated but it will increase the size and weight of measuring circuitry. On the other hand Op-Amp is cheap as well as small in size. Also provide good linearity. But Op-couplers can give you the status of voltage presence but can not maintain good linearity.

Circuit diagram using Op-Amp

Here in this article, we’ll make our circuit with Op-Amp to measure high voltage.

Circuit diagram

As you can see, we used multiple resistors in series with high voltage source. R1, R3, R4, R5 are connected in series. Using multiple resistor provide good isolation to AC high voltage. Also heat due to power loss is significantly reduced.

In this circuit, you can see that we used a voltage reference of 2.5V which is feed to the non-inverting terminal through R10 resistor. Here in this circuit, R9 is working as our feedback resistor.

The gain of this circuit is:

Gain = R9/(R2+R6+R7+R8);

Note that, R1+R3+R4+R5 must be equal to R2+R6+R7+R8. Or distortion in output will arise.

The voltage reference should be selected carefully to obtain full range of voltage. Note that, the output null point is 2.5V. That means if there is no input voltage, there will be a 2.5V DC at output terminal.

Simulation result:

Here is the simulation result:

Simulation result in proteus

As you can see the result in simulation. But I’ve been using this circuit in many products for many years. It works fine. You can easily implement this circuit in your circuit too.

I hope you learned something and enjoyed this project. If you need any help, I’m Mithun here for that. Thank you, Enjoy!


Mithun K. Das; B.Sc. in EEE from KUET. Blog:


enrique · November 4, 2020 at 3:15 am

Excellent explanation as always your work is very good thanks

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