# How do rectifier-based AC meters compensate for diode forward voltage drop?

#### fisherhj

Joined Apr 18, 2016
5
I understand that in principle, a half-wave rectifier-based meter would read 45% of the RMS value. But that assumes ideal diodes. With a circuit using real diodes (1N34A), I get very poor linearity, which I assume is due to diode voltage drop. Is that correct? Since the scales of commercial analog meters seem pretty linear, how do they avoid the results I'm getting?

#### crutschow

Joined Mar 14, 2008
29,785
If they are powered, they likely use an op amp in an ideal rectifier configuration.

#### ronsimpson

Joined Oct 7, 2019
2,063
I have a very old multi-meter. Its low voltage AC scale is nonlinear near zero.

#### MrSalts

Joined Apr 2, 2020
1,762
I understand that in principle, a half-wave rectifier-based meter would read 45% of the RMS value. But that assumes ideal diodes. With a circuit using real diodes (1N34A), I get very poor linearity, which I assume is due to diode voltage drop. Is that correct? Since the scales of commercial analog meters seem pretty linear, how do they avoid the results I'm getting?
A 1N34A is a point-contact germanium diode and the forward voltage drop is very temperature sensitive so you will have instability.

#### ronsimpson

Joined Oct 7, 2019
2,063
I have a circuit that I inherited. It uses a 1000V diode. The voltage is 0.75 volts less that it should be. I found a way to add a matching diode that adds 0.75V to the output. The diodes are next to each other. The temperature stability problem has gone away. The problem has not totally gone away but it is much better. Friday, I shipped board 1 and it has an offset of 0.03V. Board 2 has 11mV offset.