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Panel volt,amp meters

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markelectro

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Hello all
I have a question on meters the ones found inserted into panels with a needle marker, mine are around 2" in diameter, some are in Volts some Amperes some Milli amperes and some in Micro amperes. All have various readings on the dials 0-5, 0-100, 0-50 and so on.

I have to use two or three in a project but I don't have three with the same readings. I am looking for a simple reading on the dials of 0-40v, 24v is the max voltage used in my project. I think that all can be used and altered to suit my requirements but this is where I get stuck. I could make new dials but I want to be sure I have them set up right and I don't want to overload them.

Can you please help?

Regards Mark
 
hi mark,
Some where on the meter, sometimes the very lower part of the face scale it will state the FSD mA or uA of the meter.

I'm assuming that they are moving coils meters.

Once you know the FSD current you can calculate a series resistor to scale it for a Voltmeter, these are usually FSD = 100uA

or a ammeter and calculate a Rshunt resistor, these are usually FSD in mAmps.

You can remove the front scale face and use 'lettra-set' numbers to remark them.

If you cant find the FSD marking its possible to use another calibrated meter, in series, with a battery and series resistor to measure the FSD.
 
Eric Thanks for the reply.

I want them to read volts as you know.For one example one meter reads from-0 .2 .4 .6 .8 on the upper scale the lower reads 0-1 the fsd saysFSD=1ma.500mv

how would I convert this to read 0-40v and what resistor do I use?

Yes all are moving coils meters mainly ex military.

Regards Mark
 
markelectro said:
Eric Thanks for the reply.

I want them to read volts as you know.For one example one meter reads from-0 .2 .4 .6 .8 on the upper scale the lower reads 0-1 the fsd saysFSD=1ma.500mv

Resistance of meter = 0.5/.001 = 500R

So to have 1mA flowing thru the meter and series resistor

Rt = 40/.001 = 40K0
The Rtotal = Rseries + Rmeter, so Rseries= 40K0 - 500R = 39K5
Use a 39K0 0.1% MF resistor and a 1K0 cermet trim pot. [to cal at 40Vdc]



how would I convert this to read 0-40v and what resistor do I use?

Yes all are moving coils meters mainly ex military.

Regards Mark

Does this help?

EDIT: As the Rmeter + Rs = 40K, for a 40V FSD, then you have 1000R/Volt, so you must allow for the 1mA drawn by the meter when you actually measure a Voltage. Do you follow this part??
 
Last edited:
ericgibbs said:
Does this help?

EDIT: As the Rmeter + Rs = 40K, for a 40V FSD, then you have 1000R/Volt, so you must allow for the 1mA drawn by the meter when you actually measure a Voltage. Do you follow this part??

Eric
Think I follow the diagram-

+10V is halved by the two resistors so +10v is now +5v (without the meter connected) With the meter contected taking into account the draw from the meter(40k) the reading will be +3.3v because +10v/3 = +3.3v.

So far have I got it???

Regards Mark
 
hi mark,

If you wanted to make a test meter with this 1mA meter you could have a simple amplifier [OPA] powered by a 9V PP3, that would increase the input resistance to the meter to around 1megohm, that is 25K/Volt.

This would make it on a par with old AVO8/9 meter, which IIRC was about 20K/Volt.
 
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