Re: Reliable Moderate HV testing

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Dwight.Crider-at-regulusgroup-dot-com>


The 150 Volt meter does measure current that its design resistance allows
with 150 volt potential.  If there is a series resistor that could be
removed in some meters, it would allow the same current for full deflection
at a much lower voltage. Then it becomes a matter of proper shunt selection
to allow the desired current for full deflection to be correctly divided
between the shunt and the meter movement. If this meter does not have the
series resistor, your options are virtually eliminated. Assuming it does,
the easy method is to measure the current and meter resistance at full
deflection with separate meters. This will give you the meter current( with
resistor in place) and movement resistance (Not including the
resistor).Then remove the resistor. The desired current to be measured
should be say 67% of full scale. So FOR EXAMPLE, if you want to measure
0-10 amps and the measured meter current was 15 ma., you could shunt the
meter so that 15 ma flows through the meter and 14,985 ma flows through the
shunting resistor. So our shunt resistor must maintain the inverse
resistance ratio to the current. If the meter resistance was 1.5 ~ohms, the
shunt should be roughly .0015~ohms.  A shunt can be made of copper or
resistance wire depending on the actual current and resistance desired and
measured. Never attempt to measure rated current without the shunt in
place.


                                                               Ext.
Ohmmeter

v---vvv---v

  +                     Series                             Meter
-        Ext. Milliammeter in series
__________vvvvvv____________vvvvvvvv____________  <------vvvvv------->
                                                                 ^vvvvvv^
            Remove^                             Add^  (shunt)


Dwight Crider