PCB voltage sensing

Thread Starter

Jeffery Vahrenkamp

Joined Jun 16, 2016
46
I built myself a mains power monitor using current transformers. The voltage sensing is done via a 12ac/ac wall wort and a voltage divider. This is expensive, large, clunky, and inconvenient. Since I am just measuring voltage with minimal current draw (the divider is using resistors in the kilo ohm range) I figured I could use a transformer that has fewer turns of copper. The goal is to get a component that is $1 or less per piece for this job. Finding 10:1 or 20:1 windings in this range has been challenging. I can order ferrite toroids for cheapish, and could wind my own with low gauge copper wire, but if I did that, I'd like a little guidance on how many turns would be needed for the draw which would be in the low milliamp range.
 

Thread Starter

Jeffery Vahrenkamp

Joined Jun 16, 2016
46
Hum... From reading it looks like these are just essentially what I was talking about doing, a power transformer with low loads. That one you linked looks like it is just an isolating transformer (1:1 ratio) and you supply a power limiting resistor and then measure the drop over the second resistor.
 

Reloadron

Joined Jan 15, 2015
5,502
That's how they work, the ZMPT101B and the ZMPT107B. You likely saw this:
Mains Trans.png

They also come on module boards which allow an offset adjust of the board output but transformer alone I have seen for under a buck, I think on Ali Express. The module board versions allow offsetting the AC sine wave to 2.5 volts.

Ron
 

Thread Starter

Jeffery Vahrenkamp

Joined Jun 16, 2016
46
That's how they work, the ZMPT101B and the ZMPT107B. You likely saw this:
View attachment 209475

They also come on module boards which allow an offset adjust of the board output but transformer alone I have seen for under a buck, I think on Ali Express. The module board versions allow offsetting the AC sine wave to 2.5 volts.

Ron
So Hypothetically an audio transformer would work just as well right? I mean, the only difference really is just that the ZMPTs are potted quite heavily, i'm guessing because the isolating audio transformers aren't meant to see as high of voltage as 120-220V.
 

nsaspook

Joined Aug 27, 2009
7,354

Reloadron

Joined Jan 15, 2015
5,502
So Hypothetically an audio transformer would work just as well right? I mean, the only difference really is just that the ZMPTs are potted quite heavily, i'm guessing because the isolating audio transformers aren't meant to see as high of voltage as 120-220V.
There are a good number of considerations in transformer design. The link nsaspook posted covers it well. The AMP transformers mentioned are designed around 50 ~ 60 Hz and use suitable insulation breakdown materials. I have been wanting to maybe get one of the modules just to play around with. Anyway it would be a stretch to say even hypothetically an audio transformer would work. However, I do have an old HF receiver and I run the audio out into a 120 V to 6.3 V filament transformer and drive an 8 ohm speaker just fine. :)

Ron
 

Thread Starter

Jeffery Vahrenkamp

Joined Jun 16, 2016
46
I've been looking at this, and the ZMPT modules are not potential transformers. My original setup with the 10:1 was a potential transformer. If you read that article @nsaspook posted, its talking about something very different than a $.60 Chinese 1:1 transformer. Those are for large commercial systems with extremely high voltage and differential winding ratios. The ZMPT modules are just small potted isolating transformers so that you can do an AC voltage divider without sharing a common ground. My original design did a combination of both of these (stepped down and isolated with the 10:1 transformer) and then used voltage divider using the same ground as the ADC to measure the voltage. But this points me in the right direction.

If the audio isolating transformers explode in my face or catch fire/melt/burn my house down etc... I'll let you guys know so you can say I told you so :).
 
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