DPM digital panel meter power supply issues

[festus]

Hi,

I am trying to add several (4) DPM's to an old power supply.  2 are configured to read DC volts, 1 for DC milliamps, 1 for AC voltage.

I am having trouble powering all of the meters from a single (completely separate) power supply, which is a standard 5v switching power supply independently powered from the mains.  The problem is that apparently somehow the DPM's short out the circuit when you try to power more than 1 of them from the same power supply. 

Or, I should be more accurate and say:
- connecting the 2 DPM voltmeters works fine, apparently because they share the same (-) location in the circuit
- connecting 1 dc voltmeter with either the milliamps meter or the AC voltmeter, then the circuit gets shorted out.

And yes, they are connected correctly except for the power supply problem, which apparently is based upon how the DPM is designed.  When I power each DPM from a separate battery, everything functions as intended, but I cannot use batteries for my project.

What is the trick to power all these DPM from the same single power supply and without shorting out the circuit?  Right now I am mixing & matching DPM's, PM-128E set for 5V common and a ML-5135 Sure Electronics AC DPM.  Ideally I would like to use all the same meter, probably pick all the same type from Sure, maybe something small such as the "Red LED Digital Volt Panel Meter" that you see on ebay (Item 160460021855, for example).

Appreciate your expertise, thank you

[alm]

You need a separate power supply that's floating relative to the measured voltage for each DPM, a tiny transformer with two secondaries can power two DPMs. If two DPMs have one terminal in common (eg. measure current on the low side and voltage relative to the same point), you might get away with powering these two from the same supply.

[Wim_L]

Assuming the negative power supply for the meter connects straight through to the negative sense lead, you could connect the negative sense leads of a DC voltmeter and DC milliamps meter together, then connect the power supply you're trying to measure with the positive side to the + of the voltmeter, and the negative side to the negative sense leads of both meters.. Output power would then be taken between the +sense leads of the two meters. Of course, the voltmeter would not be reading the true output voltage then, because there would be the burden voltage of the milliamps meter. If the milliamps meter can measure negative current, connecting the power supply between the positive sense leads of the meters would be better.

I'm probably missing something though. Why would you add two DC voltmeters, an AC voltmeter, and a milliamps meter to a single power supply?

[david77]

I had to wrestle with a similar problem some time ago. I upgraded my trusty dual PSU with digital volt and ammeters based on the ICL7106. In the end I had to use a dedicated transformer and an isolated DC/DC converter per DVM to get satisfactory results. Otherwise the readings on the meters were unstable or the circuits would short themselves out. I have not analysed it further but I'd guess it's got something to do with the floating inputs of those DVM's. Even powering them from a separate isolated winding on the PSU transformer wouldn't work properly (though using the isolated DC/DC converters!). I never figured that one out. The 7106 is a bit touchy it would seem and many standard panelmeters still use it.

[festus]

David, you know it.  Same problems here, I may have to give your solution a try, or just forget the idea altogether, the analog meters ain't that bad.

[grenert]

I really prefer analog meters for current.  When you see the needle jump to the right, you immediately know to shut things down!   

[david77]

Usually I have no problem with analog meters, they work just fine. But when I built that PSU I really had no money to spare on expensive meters so I used the cheapest meters I could find. They were really properly cheap at about 3,00 EUR a piece. And that's also how good they were . Absolute shite.
So I ended up with a DMM permanently connected to the output to monitor the PSU voltage, wich was really annoying.
So I decided to do something about it and as good analog meters are more expensive than gold (at least here in Germany) I went digital.