Meter movement.

[Dave Plowman (News)]

I have a car revcounter, and would like to know what the basic movement is

- like 1mA FSD or whatever. Any easy way to determine this?

[The Natural Philosopher]

If its Smiths, 10mA was the Rule.

Try a 1.5v battery and a 150 ohm resistor. Should be about full scale.

Or a 5v supply and a 1k resistor. should be half scale.

[Graham.]

You put a calibrated ammeter or multimeter on the appropriate range in series with the movement and a potentiometer (say 5K Ohm) adjust the pot for FSD on the movement under test and the calibrated meter gives the current.

I can't help but add I was surprised you asked this.

[Dave Plowman (News)]

If it were say 50 micro amp, I'm not sure my DVM would measure this.

[The Natural Philosopher]

why would you take an expensive and fragile 50uA meter and put it in a car?

They are IIRC 10mA.

[Andy Dingley]

Take it out and look.

You could have an ammeter in there (Smiths were 10mA for a long time, but I think they were more originally). You could also have an "air core" gauge, which is two opposed fields (more stable needle position against mechanical shocks and cancels supply voltage variations), usually found on other gauges though. More recently it will be a stepper. If it's fairly old post-war Smiths ('40s) it could even be a synchro (fecking rare and valuable on the spares circuit, used in some Bristols and Astons, AFAIR), from when they first started recycling their aircraft instrument lines to go into cars.

Most of mine are stepper motors. If something breaks, it's easier to rebuild a new mechanism into the old shell than it is to pay watchmaker prices to get a Smiths mechanism repaired. This is also why I have what's probably the only pre-war MG with an OBD-II diagnostic connector.

[Dave Plowman (News)]

It is a spare. I'm experimenting with different electronics.

It's a moving coil type with a scale of approx 120 degrees. Late '70s. And it's not 10 mA.

[Dave Plowman (News)]

It's not. The original electronic type may well have been.

[dennis]

Well even a really cheap DVM will do 50 microamps if you put a suitable resistor in series with the test item and put the DVM across the resistor and use volts.

[Newshound]

Real motorbikes were still using these in the 50's and 60's. Could never figure out how they worked, and never managed to fix a broken one. Admittedly my smallest tool was a 1/4 Whitworth spanner.

[<me9]

I doubt 50microamp, not robust enough! Stick a variable resistance[1] in series with the meter and 12volts, adjust for FSD, measure value of resistor with mutleymeter and I=V/R

[1] If 50 microamps start with at least 270K.