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Re: [Amps] Calibrate a panel meter?

Subject: Re: [Amps] Calibrate a panel meter?
Date: Wed, 28 Dec 2005 14:13:00 EST
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Hi Bill, most of these nice meters begin their life as a much lower value  
movement; 500 microamperes being a common value. If yours is one of these you  
can remove the internal shunt and provide your own calibration circuit.
A commonly used method is to measure the voltage drop across a one  half ohm 
resistor with the meter and a series multiplier resistor. At one  ampere, one 
half volt will appear across the resistor and the series resistor  can be set 
for whatever actual full scale reading you want. For a 500  microampere 
movement the resistance of the meter plus the multiplier would be  1/2 volt 
by 500 E-6 or 1000 ohms. What you don't know is the resistance  of the 
movement but there are ways to determine that. Other values of shunt  
resistance will 
work but in your case the half ohm value will permit use of  shunt diodes 
across the meter to protect it from current surges. Silicon diodes  will clip 
above .7vdc so currents above 1.4 amperes will be safely passed  through the 
Gerald K5GW
In a message dated 12/28/2005 12:21:08 P.M. Central Standard Time, writes:

I have a  Simpson 923A panel meter, 1 amp full scale. It is reading 
low by about 6%.  When it has 800 ma through it, it indicates about 
750 ma. I'll be using it  in my 8877 amp at 800 ma and I'd like it to 
be more accurate.

If it  were reading high, I could just shunt it, but reading low...???

Is  there a way to calibrate it by adjusting anything inside? Moving 
the  magnets around, etc? I'm sure the factory calibrates them somehow 
but I've  never read anything in the ham literature about how to do  it.

Suggestions appreciated.

73, Bill  W6WRT

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