Convert DC Volt-Ammeter to AC to Monitor Variac Voltage and Current

Thread Starter

kmpres

Joined Jan 24, 2016
21
Hello all,

I am building an isolation transformer/variac/dimbulb tester and want to monitor the input and output voltages and currents using separate cheap panel meters, but I'm finding this a surprisingly difficult task. The usual assortment of Chinese panel meters come in two and four wire variants with the former being powered off the voltage under test and the latter requiring a separate supply to power the meter circuitry thus leaving the meter free to measure the voltage without a built-in power supply loading it down. I prefer the four wire types because the test voltage can go down to zero volts while the two wire variants have a minimum voltage requirement of 20, 50, 60 or even 80 volts before they'll even start to read the test voltage reliably.

Two panel meters are envisioned, one for input to the variac, one for output, and both must measure voltage and current. For some reason there just aren't any in the usual Chinese vendor sites (including eBay and Amazon) that can measure AC voltage AND AC current AND go down to zero. However, numerous examples can measure DC voltages and currents down to zero, including this one. So my question is, how can I safely rectify and filter the AC line voltages going to these meters so I can use DC meters instead and still maintain reasonable accuracy? This is supposed to be an isolated AC power supply so emphasis is on the word 'safely'.

My enclosure space is limited, hence my combining volts and amps measurement into one meter. I'd also like to keep the parts count as low and not bulky as possible. Resolution should go down to single volts and milliamps, if possible. Also, I'm not looking for super accuracy. Just reliable, automatic monitoring of those voltages and currents whenever I turn the device on.

Thanks for your help!
 

Dodgydave

Joined Jun 22, 2012
9,169
You need to make bridge rectifier with Shottky diodes, this will give a voltage drop of approx 0.4 to 0.6V per diode, (0.8V to 1V total).
 

Thread Starter

kmpres

Joined Jan 24, 2016
21
Hello all,

I am building an isolation transformer/variac/dimbulb tester and want to monitor the input and output voltages and currents using separate cheap panel meters, but I'm finding this a surprisingly difficult task. The usual assortment of Chinese panel meters come in two and four wire variants with the former being powered off the voltage under test and the latter requiring a separate supply to power the meter circuitry thus leaving the meter free to measure the voltage without a built-in power supply loading it down. I prefer the four wire types because the test voltage can go down to zero volts while the two wire variants have a minimum voltage requirement of 20, 50, 60 or even 80 volts before they'll even start to read the test voltage reliably.

Two panel meters are envisioned, one for input to the variac, one for output, and both must measure voltage and current. For some reason there just aren't any in the usual Chinese vendor sites (including eBay and Amazon) that can measure AC voltage AND AC current AND go down to zero. However, numerous examples can measure DC voltages and currents down to zero, including this one. So my question is, how can I safely rectify and filter the AC line voltages going to these meters so I can use DC meters instead and still maintain reasonable accuracy? This is supposed to be an isolated AC power supply so emphasis is on the word 'safely'.

My enclosure space is limited, hence my combining volts and amps measurement into one meter. I'd also like to keep the parts count as low and not bulky as possible. Resolution should go down to single volts and milliamps, if possible. Also, I'm not looking for super accuracy. Just reliable, automatic monitoring of those voltages and currents whenever I turn the device on.

Thanks for your help!
Thanks for the quick reply. I assume I should also filter the output with some capacitance, like maybe 10uF? The meters don't require much current so this cap can be small? Can I also assume that I don't need a transformer because the voltage feeding the variac has already passed through my isolation transformer?

Thanks!
 
Last edited:

crutschow

Joined Mar 14, 2008
24,997
I should also filter the output with some capacitance, like maybe 10uF?
Probably, but that will generate the peak voltage of the waveform.
You can use a voltage divider at the meter input to give the equivalent RMS value (.707 of the peak).
 

dendad

Joined Feb 20, 2016
3,537
I would use small transformers for isolation, one to give a constant power for your meter, and another on the Variac output to supply the voltage to be read.
And a current transformer to generate a voltage proportional to the current. This way, all will be isolated from the mains.
A pot on each output is used to calibrate the reading.
 

Thread Starter

kmpres

Joined Jan 24, 2016
21
Attached is my final sketch for this project. Please forgive the pencil-CAD drawing - I'm not yet up to speed on the various computer drawing tools. After a lot of sketching I have decided that four-wire volt-ammeters that can go down to zero volts are not presently available and that converting DC volt-ammeters to work on AC isn't worth the effort. Instead, I have found some very cheap two-wire AC volt-ammeters with CTs that supposedly go down to 20 volts. They, though less accurate, should be adequate for my needs. I really don't know why I can't find digital AC volt-ammeters that can go down to zero volts but I won't argue the point.

Before I begin construction, is there anything in the sketch that I should change?
Many thanks for your help!
 

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MrAl

Joined Jun 17, 2014
7,604
Current transformers are usually used for this with rectified outputs with appropriate load resistors.
 

Thread Starter

kmpres

Joined Jan 24, 2016
21
Interesting. I actually had to look that up. I had never given them much thought before. Always kind of thought of CTs as something like large Hall Effect sensors. OK, my new meters will come with CTs - that'll give me a chance to play with them. As my repair work usually involves tens or hundreds of milliamps at line voltage and not amps, I will probably have to wind a few more turns through the CT to get the meter to display 010 instead of 001. The decimal point will probably be in the wrong spot but, hey, you can't have everything.
 

MrAl

Joined Jun 17, 2014
7,604
Oh yeah that is right if you can wind 10 turns through the center you get 10x the output more or less. Sounds like a good idea for lower current measurements.
 
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