Hello Bob: the meter has a shunt resistance in parallel with the "real"
instrument to extend its current range.
You must take it out, then measure the full scale deflection current of
the meter. I guess it may be a 1 mA full scale instrument but you must
find the real full scale deflection current by yourself.
Arrange a series circuit consisting of a variable power supply (PS,
3..15 V) in series with an amper meter (in milliamper range first) and a
15 K resistor with the modified meter (that is, the 0..5A meter without
the shunt). Check all the polarities are correct. Also attach a
voltmeter in parallel with instrument.
Varying the PS voltage (start at the lowest output voltage) you can see
when the needle reach full scale and then you can read full scale
current (Im) and voltage drop across the meter (Vm).
If the meter is a lower current one (100uA or so, I think not, but...),
you must increase resistor value to 150 K.
Finally, if you plan to measure 5000V full scale, you have to connect a
resistor in series with the modified instrument; you calculate the
series resistor as follows:
Rseries = (5000V - Vm) / Im
Actual resistor power dissipation is: Im*Im*Rseries
Take a higher value of dissipation for safety and isolation purposes.
I believe also Vm will be negligible compared to 5000V, so you may take
also an Rseries value of 5000V / Im.
Once obtained this value of R, keep in mind it will be high, and for
safety and durability use many smaller value resistors in series (also
for isolation purposes in each resistor body), the best are with
vitreous (or ceramic) bodies. I suggest to use no less than 10 in
series.
Hope all this helps.
Good Luck
Guillermo.
w5rg@postoffice.swbell.net escribió:
>
> Hi..I need help..I just can't get thru my head..I have a P/S that I'm
> building and want to put a volt meter on the output side..The meter I
> have for this is a 0-5 dc amperes..this is the only extra meter I have
> at this time...I want to read 0-5000 volts..the current is 2 amps...I
> hate to ask anyone help but I'm stuck on this one..Thanks Bob
>
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