How can this 'AC true RMS' measurement circuit work? - Page 1
Forum section: electronics / projects

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: August 29, 2022, 07:52:06 pm
content: Just seen this circuit (the watermark shows where) and it purports to be an AC true RMS meter."Potentiometer P2 must be set so that for zero input signal (terminals A and B short-circuited), the bridge is balanced and the meter reads zero. "It shows supply rails as + and -, but unless I'm missing something, with no ground reference how can it measure AC - i.e. a +ve and -ve going voltage?

author: strawberry
author_rank: Super Contributor
timestamp: August 29, 2022, 08:00:48 pm
content: circuit is active rectifier and amplifiertrue-RMS is converted mechanically

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: August 29, 2022, 08:02:14 pm
content: So, it should work with AC pk-pk voltages within the DC supply rails?

author: strawberry
author_rank: Super Contributor
timestamp: August 29, 2022, 08:32:24 pm
content: one half period Q2 Q4 form differential amplifierthen other half period Q3 Q4input signal seems 0,3V

author: Andy Watson
author_rank: Super Contributor
timestamp: August 29, 2022, 08:41:53 pm
content: Quote from: 6SN7WGTB on August 29, 2022, 08:02:14 pmIt has an attenuator - R1 - R7. Although the resistors are not valued, I assume that the figures along side represent the voltage range - i.e. the maximum voltage is determined by the attenuator resistor and it appears as if they intended this one to go to 300V.I believe the intention is to produce a current in the collectors of Q2 and Q3 that represents the input voltage squared. The meter movement will mechanically average the currrent, and I assume it will have a scale that performs the square-root function. So you have the square Root of the Mean of the signal Squared - RMS.Edit: There's a video by Barry Gilbert in which he explains, very clearly, the fundamentals of using transistors to multiply signals. Unfortunately, my brain gets a bit fuzzy when it comes to the practical implementation.youtube.com/watch?v=LQNJVtcFrCcEdit 2: Actually, a link to the original article might have saved some head scratchinghttps://www.eeeguide.com/true-rms-voltmeter/

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 29, 2022, 09:20:57 pm
content: Agree, Andy.It is a log_n function on each half cycle :The base voltage increases through zero at signal zero crossingI_coll = Is * e^(V_be *q/K*T)There is a somewhat similar circuit and derivation in Burr Brown Op-Amp book at article 7.5.5 (1971 version here !)Called "Variable Transconductance Multiplier"

author: Warpspeed
author_rank: Contributor
timestamp: August 29, 2022, 11:02:38 pm
content: That should work, and the subsequent averaging required will be performed by the mechanical meter movement.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: August 30, 2022, 06:58:49 am
content: Quote from: Warpspeed on August 29, 2022, 11:02:38 pmJust so I am clear, this does indeed need an analogue meter to work as described?

author: strawberry
author_rank: Super Contributor
timestamp: August 30, 2022, 09:42:23 am
content: circuit is not great great precision ~5% mechanical part could achieve ~0,5% up to ~0,05% for big scale with parallax mirroror true-RMS ICor heater and temperature sensor

author: Andy Watson
author_rank: Super Contributor
timestamp: August 30, 2022, 10:14:53 am
content: Quote from: 6SN7WGTB on August 30, 2022, 06:58:49 amAn analogue meter is the easiest option. I suppose you could use a digital meter that has a filter to perform the averaging. But consider the following:The scale of the meter must provide a square-root function - easily done with an analogue meter, but a digital meter would require extra processing.The output of this circuit is a current of a few mA max - that's at the lower range for most cheap digital meters.

author: Warpspeed
author_rank: Contributor
timestamp: August 31, 2022, 11:01:09 pm
content: Quote from: 6SN7WGTB on August 30, 2022, 06:58:49 amYes, the circuit distorts the original sine wave, and the output must be then averaged over several cycles to produce a constant steady dc that can then be measured as a dc voltage. The simpler alternative is to feed the distorted waveform straight into a mechanical analog meter, and provided the frequency is high enough, the needle should not vibrate or quiver. The averaging function is done mechanically.If this circuit is to drive a digital voltmeter or a microprocessor, the subsequent averaging and square root functions still need to be done somehow.