# Measuring AC Current at High Voltages (up to 600VAC)

I've been looking at various ways to measure current for a while now, and am stumped on how to read high-side AC Current Measurements ohmically (no Hall Effect sensors). My initial thought was to use PNP and NPN BJTs in parallel (one of each), connecting them across a 0.1 Ohm current sense resistor (basically making a differential amplifier). I can't seem to make a circuit that works in this configuration, however, and differential amplifier ICs can't handle the high voltages I'm looking to measure (up to 600VAC). I've seen a op-amp from Analog Devices that can handle common mode voltages up to 600V, but am trying to make a circuit just from discrete circuit elements to start.

My question is mainly this: is it possible to measure high-side AC current with just a few BJTs and/or MOSFETs? My ideal circuit would do the following:

Vin=600VAC, 0.1Ohm current shunt resistor on the high side, Current=1 Amp AC. --so-- V1=600VAC V2=600VAC-(0.1 Ohm) * (1 A) = 599.9VAC

BJT circuit takes V1 and V2 as inputs, and gives 0.1VAC (V1-V2).

Thanks for all your help! The replies don't have to be an answer to the question, more trying to understand how something like this could be possible.

• The usual way is with a current transformer, then your problem reduces to measuring an isolated 5A (usually) AC current. It could be done a shunt, precision amplifier and some means of signal isolation- either analog or digital (transformer, optical, RF, capacitive etc.) but you also need to create a power supply that lives on the 600V rail. Rogowski coils are also used sometimes. Feb 13, 2020 at 3:04
• Why not measure neutral currents? Feb 13, 2020 at 3:54