Fuse Eating Variac.

[Ptron]

The way the needle moves (steady, oscillating, jumping around) gives you additional information that just looks like an unstable display on a digital meter.

I'd have to agree. You can watch trends on a digital meter, but a swinging needle I think gives a much more informative visual as to the relative steepness of the curent ramping.

Good points. I actually had an analog current meter on it before I built the box. The problem I had with it was, as a 10A meter, most things I used it with barely moved the needle...if it moved at all. With the digital I should theoretical be able to see currents down to .01A with a .01A resolution. Perhaps this is of no real concern as the most important function of the meter is to show when the current is "taking off". I suppose I could use parallel shunts and add a range switch.

I do also like the visibility of the LED meters.

I wouldn't think of any terminal on that meter as "ground." The 5v supply should completely float just as the 5VAC winding for a rectifier tube does. Then it doesn;t matter which side of the meter sees neutral and which sees hot. Unless I am missing something

And I think I would have been all right if I was only using one meter or they were both wired "backwards", and indeed, I tried each seperately first and they worked, but together the output of the variac was shorted to neutral through the meters and the 5V supply. Too bad it didn't just burn up a trace or a wire...or better yet a fuse if I had been smart enough to add one. It did take out the trace that connects the 5v "GND" and measurement "GND", but before this happened it also blew a big hole in the controller chip with some smoke and a nice smell to go with it. The ammeter looks ok but when I power it it shows a dim -18.88 instead of a bright 0.00.

[Steve Conner]

All digital meters are like that. The ground of the +5V supply isn't quite exactly "connected" to either measurement terminal, but if you let the two grounds get more than a volt or two away from each other, you'll let the smoke out, as you discovered.

In more geeky terms, the input is a differential amp, running off the +5 rail and an internally generated -5, so you don't want to go outside of that range. A separate, isolated supply for each meter is where it's at.

[Ptron]

All digital meters are like that. The ground of the +5V supply isn't quite exactly "connected" to either measurement terminal, but if you let the two grounds get more than a volt or two away from each other, you'll let the smoke out, as you discovered.

In more geeky terms, the input is a differential amp, running off the +5 rail and an internally generated -5, so you don't want to go outside of that range. A separate, isolated supply for each meter is where it's at.

Actually, in the case of these meters the 5V gnd is directly tied to one of the measurement terminals. There's a trace between pins on the meter's PCB...or at least there was a trace, it turned into fire and smoke on the voltmeter. It's still there on the ammeter.

I like your idea of separate 5v supllies. I think I'll go with that. Or at least a fuse this time.

To my surprise, they got back to me right away and offered replacements for the cost of shippng. They didn't tell me what that was though. They're so cheap, shipping is probably 90% of their price.

[Ptron]

So after a few back and forths, they have not only offered to replace the meters but they asked me to tell them what they should change the datasheet to say! I don't know if they'll seriously take my advice but in case they do, how's this:

Original line: "The power supply and the signal being measured can have the same GND or the power converter can be separate one."

New line: "The power supply and measurement GND are internally connected (pins 3 & 4). They must connect to the same potential or the power supply must be isolated."

Does that get the point across? It's deceptively tricky to say it in a sentence or two. I suppose technically, the power supply GND aren't necessarily GND and they have to be connected to the same potential whether the supply is isolated or not.