Electronics > Beginners
Panel meter help
tecman:
Many panel meters will have, in small print at the bottom of the faceplate, something like "FS=1ma". This would indicate that the base meter movement is 1 ma full scale. beyond that, there could be series resistors internally for a voltmeter. For ammeters, some have internal shunts, generally for 1 amp or under. Others will require an external shunt.
As stated, use a series resistor to get a reading. Once a resistor value in combination with a know voltage you can determine the movement's scaling. Typical meters, w/o internal resistors or shunts, are 1ma or 100 ua FS.
My uncle worked for Weston as a first job until he retired. As a result I acquired quite a few analog meters of various sizes, scales and shapes. His work highlights were assembling and testing panel meters for the Gemini and Apollo capsules. He received a commemorative medal for his work made from aluminum that was taken from some part of the spacecraft that orbited the moon. The aluminum was melted with a quantity of virgin metal and used to produce the medals.
Paul
Zenith:
--- Quote from: nnills on March 06, 2019, 04:41:14 pm ---I tested a first one and think it might be 5K ohm, is that reasonable?
I can't seem to add images in my post but here are 2 of them, first is 4K7 second 5K1
https://imgur.com/a/VRJKHvX
Edit: Vin=5V
Edit2: No no no... :palm: more like 6K. 5K1 and 270 in series is still a bit over
Edit3: When hooking up a PSU it seems more like a 100mV full scale
--- End quote ---
If the meters you are talking about are 5V full scale deflection as in the last two photos, 5K is reasonable. 5 volts divided by 5KOhms = 1mA. 1mA is a likely full scale deflection for the meter movement.
Buying a lot of old panel meters like that often produces a mixed bag. Specialised scales, odd things like three phase voltmeters and hot wire meters. Some of those are moving magnet ammeters, they have a very non linear scale but were tougher and cheaper than moving coil meters. You're quite likely to find some of the meters don't work and occasionally you can fix them.
Just proceed with caution as Jeroen79 described above. As for connecting an ammeter directly to the terminals of a power supply, it's possible that unless it was a current limited PSU, things could go bang. Look before you leap. In your photos there's a 20-0-20µA centre zero meter and you could easily damage that if you were careless.
When I bought a lot of about 50 old panel meters years ago, with a bit of fiddling and fixing I found about half were good and likely to be useful, so it was worth the money.
jwrodgers:
Have a look at:
https://myclassbook.org/darsonval-meter-movement-principle/
I have also attached some old class notes from my Uni days circa late 90's. The equations don't seem to render correctly now, but there is useful information in there to steer you in the right direction.
Also, wikipedia has this:
https://en.wikipedia.org/wiki/Galvanometer
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