Moving coil ammeters

Whilst passing one of the junk antique shops in a local village this afternoon, I noticed they had a box containing a variety of old analogue panel meters. Amongst them were some small 200/250/500 mA fsd meters, which looked suitable as both cosmetic and functional enhancements to a couple of my arduino projects.

On enquiry I was informed that they had all come from a house clearance and the shop owner believed them to be working. (Though the foundation for that belief wasn't elaborated on.) She did say however that I was welcome to return and test them in situ prior to any purchase.

It now occurs to me - not having had any prior experience of moving coil meters - that I'm a little uncertain on the most straightforward way to check them out. I'm presuming that I can just take along a small 4 x AA battery pack, a couple of low value resistors - say 75R and 100R 0.6W which I happen to have handy - and then connect them up in series with the meter. Or am I being too simplistic?

I'm not looking for precision accuracy. I just want to ascertain they will move around a bit and point vaguely in the right direction. Comments and suggestions welcomed.

Yes that should be enough, it might be that what is shown on the scale is not what the meter actually measures and you need a shunt to get the scale reading to match up. Just try measuring the resistance of the meter, often this injects enough current to make them move.

Good find.

Although they are mechanical, meters can "last forever". Sometimes they can get "sticky", but your odds are good.

...it might be that what is shown on the scale is not what the meter actually measures and you need a shunt to get the scale reading to match up.

Good point! I'd forgotten about that. It's very-unlikely that you can actually safely put 500mA through the meter's coil. It would be a good idea to start with about 1mA (maybe a 1k resistor).

There may be a soldered-on external shunt resistor, an internal one, or it may be remote/separate from the meter and no longer there.

DVDdoug: It would be a good idea to start with about 1mA (maybe a 1k resistor).

I would start with even less than that, e.g. 100uA.

Grumpy_Mike: Good find.

The quality of the find is tempered slightly though by the asking price of a fiver apiece.

Thanks to all for the helpful advice which seems to be, don't trust the dial rating and creep up on them from afar.

Just to check my understanding; the meters may or may not require an additional external shunt. In either case they can be safely checked with a battery and resistor connected in series, chosen such that the current through the meter is less than 500mA 1mA 100uA not very much. (Anyone want to raise on a 5 / (100 / 1000000) = 50k resistor?)

How am I doing so far?

Just for my further edification, should an external shunt be needed, am I correct in thinking it would go in parallel across the meter terminals?

How am I doing so far?

Fine :)

should an external shunt be needed, am I correct in thinking it would go in parallel across the meter terminals?

Yes.

Or you can determine what the full scale reading of the meter is and make a new faceplate.

Most D'Arsonval meter movements will have some small print next to the meter faceplate mounting screws and it will say something like FS 50uA or FS 100 mA or FS 10V or whatever. Shunts are rarely used in ammeters below 10 A. If it is a meter that requires a shunt it usually will be marked.. $7.50 - $8.00 (a Fiver in US dollars) is really a bargain for that type of meter.. The Asian ones aren't worth a fiddlers damn. Cheap, Inaccurate and Unreliable are a few of the adjectives I'd apply. The worst part is the Asian ones are far more expensive than they are worth. The value in a D'Arsonval meter is that it responds nearly instantly to changes in its measurements where "Digital" movements are nice and keeping with the times... When the meter samples once or twice a second whatever happens during and after a measurement until the meter samples again is lost. This holds true even to the expensive ones like my Fluke 179 True RMS meter although the sampling period is much faster. I paid $349.00 with taxes for it 5 years ago mainly because it was the only quality DMM available for the price... Still sells for $277.00 on Amazon... used ones are $169.00, again Amazon and it was discontinued a year or so back.

Doc