Old Panel Meters

[Wilson Lamb]

Sounds fishy to me, but...
For reading voltage:  Ifs=Efs/ Rseries+Rmeter, Rtot=Efs/Ifs, 
Rseries=Rtot-Rmeter
For a 1mA meter at one volt, E/I=1/0.001=1K, so Rtot=1K/Volt, for whatever 
range you want, minus Rmeter.

Current is harder, because of the parallel connection,  so use 
Emeter=Imeter*Rmeter to get the voltage needed across the meter to make it 
read FS.  Then Rtot=(Rshunt*Rmeter)/Rshunt+Rmeter
In other words, the desired FS current develops enough voltage across the 
meter and shunt resistances in parallel, of drive the meter to FS.  A more 
elegant solution is to look at conductance.
Rshunt=(Imeter/Ifs)*Rmeter.

Check these, I have to get going!
WL
----- Original Message ----- 
From: "Bob Jackson" <bob at NOFROWNS.NET>
To: <BOATANCHORS at LISTS.TEMPE.GOV>
Sent: Monday, May 03, 2010 9:05 AM
Subject: Old Panel Meters


I bought several OLD panel meters at a recent hamfest. All are DC milliamp 
types in the range 30-500 mA full scale. I've taken the movements out of 
each to check the works before attempting to apply any voltage and all seem 
to be OK, i.e. pointers move freely, no signs of rust, corrosion broken 
connections, etc (cases are battered and filthy so I thought I'd better 
look).

My question  is this, not having any prior experience with same, is it 
correct that these old (some with military markings) have internal 
resistances that are VERY low, e.g. ranging from  0.4 ohms in the 30 mA one 
to about 4.5 ohms in the 500 mA one, with the other ranges in between (all 
measured with a very high quality DMM)? What sort of shunts (or multiplier 
resistance in the case of voltmeter applications) do you use with such low 
internal resistances? For example, using the 30 mA meter in a 3A AC circuit 
(full scale reading to mean 3A)?

I got these CHEAP (and they look really neat!) but I'm a bit baffled by the 
low internal resistances. I've reused meters before, e.g. from defunct test 
equipment) but those internal resistances were a LOT higher.

Thanks,

Bob  AG5X

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