AC DPM's -- are the all DPM'$?

[David Lesher]

I've been seeking some inexpensive AC digital panel meters; but
thus far, there's as hard to find as a popular BP executive.

Marel sells some that are $100 each, gulp.

Or is there a $6.95 AC-DC addon I can use on my pile of old DC
Analog Devices units?

 

[JW]

MPJA is currently out of stock, but they carry this for $8.95

http://www.mpja.com/prodinfo.asp?number=16177+ME

These guys have stock, but you'll pay a bit more if you're in a hurry:
http://www.web-tronics.com/panelmeter.html

 

[David Lesher]

These guys have stock, but you'll pay a bit more if you're in a hurry:
http://www.web-tronics.com/panelmeter.html

Thanks. Those will do fine, but I guess I'll need separate
wallwart power supplies for each one. That, or isolation
transformers which will cost far more.

[Unless you have a spare 8-channel isolated power supply.]

 

[David Lesher]

A 6.3 volt filament transformer, diode & electrolytic capacitor would
be a better choice. They are available wit a 1500 V isolation rating

Not the issue. My point is, each one needs a separate supply. If I could
find very low current 277v primary xfmrs [I'm monitoring a 480 Y system];
then I could use one common supply. But such would be $$$ in quantity.

 

[David Lesher]

Not the issue. My point is, each one needs a separate supply. If I could
find very low current 277v primary xfmrs [I'm monitoring a 480 Y system];
then I could use one common supply. But such would be $$$ in quantity.

Some of the meters use isolated power inputs,

I've not found same at good prices.

You can always build a simple DC to DC converter with multiple,
isolated outputs. Then you can use any reasonable DC voltage
you have available for the input, or a small transformer &
diodes. It isn't rocket science.

Not sure where to find an xfmr with 6-8 identical windings...

I am thinking this would work. Find some old small B+
transformers. Use them to isolate and step down the 277V. Than
an appropriate resistor or two per meter, and a lower range on
the DPM would give me good data.

 

[ehsjr]

These guys have stock, but you'll pay a bit more if you're in a hurry:
http://www.web-tronics.com/panelmeter.html

Thanks. Those will do fine, but I guess I'll need separate
wallwart power supplies for each one. That, or isolation
transformers which will cost far more.

[Unless you have a spare 8-channel isolated power supply.]

Why do you need 8 separate supplies? You should be able
to run 8 (or N) of those DPMs from the same power supply
using the isolated ground configuration.

Ed

 

[David Lesher]

Why do you need 8 separate supplies? You should be able
to run 8 (or N) of those DPMs from the same power supply
using the isolated ground configuration.

I'm less sure of that. I will be measuring 3 legs of 277VAC with them.
Less then good isolation could get....noisy...

 

[ehsjr]

I'm less sure of that. I will be measuring 3 legs of 277VAC with them.
Less then good isolation could get....noisy...

Ok, I can't address how sure or unsure you are, but I can provide
some general information that may assist.

To understand their isolated ground, google on "virtual ground".
It should yield a _very_ high impedance (for practical purposes
you can consider it op amp input impedance) between the DPM's DC
supply and the input (the signal that is being measured) ground.

Also, this datasheet may help:
http://www.intersil.com/data/fn/fn3082.pdf

I guess, for you to be absolutely sure, you'd have to try it.
I have not done it with these DPM's myself.

Ed