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
divided
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
diodes.
73,
Gerald K5GW
In a message dated 12/28/2005 12:21:08 P.M. Central Standard Time,
dezrat1242@ispwest.com 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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