Measuring AC current with analog DC panel meter - Page 1
Forum section: electronics / projects

author: garytoosweet
author_rank: Contributor
timestamp: July 17, 2025, 08:36:49 pm
content: Hello,I have a analog DC panel ammeter that I would like to measure the current in an AC circuit with.The circuit is a variable ac voltage source for my repair shop. All DUT will be connected to the box. Current will vary from 0-10A AC.The DC meter is 1mA full deflection.I want reasonable, good accuracy throughout the entire 0-10A range. I may add some shunt resistors to have selectable range on the meter of 1A, 5A, 10A.What is the best way to drive the meter?I was planning to use a current transformer, then rectify the secondary output.The issue is with low current there will be not enough voltage for the diode drop.I am entertaining the idea of a precision rectifier. I also just came upon Instrumentation Amplifiers and am wondering if there may be some added benefit of using one.Is there an easier or more elegant solution that I am missing?The box will have a ~12VDC supply.Thanks in advance. I have bene overthinking this a bit.

author: TimFox
author_rank: Super Contributor
timestamp: July 17, 2025, 09:25:00 pm
content: Typical high-current shunt resistors are designed for 50 mV voltage output at full scale.(There are other voltage values in that range: see data sheet.)I have used current transformers to develop 1 V at full-scale current, see the data sheet for yours.Even 1 V is too low for a simple bridge rectifier, so an instrumentation amplifier would be useful to boost the voltage from 50 mV to, say, 5 V rms.Do you need "true RMS", or is "average responding, RMS calibrated" (precise only for sine waves, but useful for other periodic waveforms) adequate for your purposes?Note that a true RMS indication is problematic at very low levels compared with the full-scale: laboratory analog true-RMS audio-frequency voltmeters (e.g. -hp- 3400A) typically are good for voltages > 10% of full-scale.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 17, 2025, 10:39:21 pm
content: Quote from: garytoosweet on July 17, 2025, 08:36:49 pmIf you just want average AC current, that's simpler.Look at current sense amplifiers. They are now quite low cost, and have very low offsets, and come in many flavours.A current transformer gives you isolation and inherent overload handling.If you have 12V supply, pick one that can operate with 12V Vcc - eg a '199' series can power from 2.7-26VPrecision rectifiers need many parts, a simpler solution can be to just use2 current sense amplifiers inverse connected, and each feeds the meter thru a resistor.Each one manages one half AC cycle and the meter can sum and average for you, or you can add a RC filter as a large cap across the meter.You will need to watch for peak effects, with any rectifier design.eg If you scale things for 1V (appx 1k for your 1mA meter) that gives you 12:1 peak overhead from 12V

author: Benta
author_rank: Super Contributor
timestamp: July 17, 2025, 11:55:56 pm
content: Current transformer followed by an active full-wave rectifier would be my approach. Simple, cheap andsafe.By using an analog meter, you won't even need to do any smoothing/averaging.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 20, 2025, 06:21:10 pm
content: Hi,Use a "moving-iron-instrument" if you can live with a nonlinear scale.Good luck

author: Benta
author_rank: Super Contributor
timestamp: July 20, 2025, 06:56:43 pm
content: Quote from: Roehrenonkel on July 20, 2025, 06:21:10 pmGood idea. They show RMS values, which is nice.https://www.ams-messtechnik.de/en/shop/analogue-meters/moving-iron-meters/