Shameless plug for EBay-AC Amp meters - 25 Amp

Dang! Too bad they're AC ammeters -- I'm looking for a DC ammeter (0-25A) and a DC voltmeter, ideally just the meters. Those are nice, Ron. - GWE

Grant, You can make any meter a DC Ampmeter. All you need to do is create a shunt of the appropriate value and connect the meter across it to essentially measure the voltage drop. Typically the raw meter movement is 10 uA (thats Micro Amp) full scale. A bit of ohms law and presto... and yes the shunt for a 25A meter is chunk of copper mayber an inch long. If you google around you can find tables of resistance for various thickness of various metals. If you've ever take apart a good old Simpson 260 VOM you'l see that he 10 Amp shunt was a piece of wire that had a small ubolt like 'thinge' so it was even adjustable to calibrate it.

FWIW the only differance for an AC amp meter vs a DC on is a couple of diodes on the meter side of the shunt.

--.- Dave

You can get cheap 30-0-30 dc ammeters out of a car junkyard ;)

Maybe it's a moving-iron type.

Best regards, Spehro Pefhany

I think that 10 ua you mention is a bit low. There's no need to use a delicate and sensitive movement like that if you're going to shunt it up into the ampere range. I'd wager that the movements in commercial ammeters are at least 500 ua full scale. Even the venerable Simpson 260 VOM you mention below is built around a 50 ua movement.

A bit of ohms law and presto... and yes the shunt for

Agreed, but I think that "proper" shunts are probably made of materials with a lower temperature coefficient of resistance than plain copper.

Come to think of it, the winding in the meter movement is probably copper, so the resistance change of a copper shunt due to ambient temperatures would be canceled out by the change in the meter winding's resistance, wouldn't it?

But, what about the I^2*R heating of the shunt by the high current flowing through it?

Jeff (Furiously backpedaling while trying to save his argument...)

I think they use a manganin alloy or similar low tempco shunt (brass is crappy, but much better than copper, and constantan is pretty good) and put an NTC thermistor in series with the copper movement winding for temperature compensation. 100uA is probably as low as you'd want to go for a panel meter, 1mA is much more rugged.

Best regards, Spehro Pefhany

Marlin Jones (surplus electronics dealer) has 10, 25, and 50 A shunts for $7 each, and a 100 A shunt for $8. They also sell mechanical meters or you can just use a digital multimeter. I picked up a 50 A shunt to add to my anodizing/electropolishing power supply. See

-- Regards, Carl Ijames carl.ijames at verizon.net

Jeff,

No need to back pedal THAT hard... :-)

Yep yer right the ole 260's were 50 uA movments. I've rebuilt a few, those old 1% R's tend to drift after 30 years.. an yeah I'm a sleaze bag I replace those germanium diodes (which turn to wire or go open after 30 years) with some silicon signal diodes (so shoot me). Schottkys are over kill for an old beast like that. Oh an remember to always solder the wires to the D cell... :-))

I was speaking generaly and about most cheap movements you'd find in a surplus meter. And I wouldn't worry too much about heating from the current and gee if ya use a 10 uA meter that shunt *would* be lower R so... ( I'd use an old Ampex VU meter.. and cal it so that the red is overload .... lol) And jeez what about the heating in the rest of the wire leading to the load? ... :-)

--.- Dave

I like the RF Ammeter method myself as the two diodes have to be calibrated. The RF Ammeter still does but gives an RMS value directly - using resistor(s) that heat a thermocouple. The thermocouple drives the meter. It also allows for voltage isolation if it is on an antenna...

Martin