Re: Cheap AC Milliammeter

[Ed Phillips <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla List wrote:
> 
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> 
> Hi All,
> 
>         I picked up a Radio Shack 0-130 VAC panel meter (RS# 22-412)
today for
> $13.  If you put a 1000 ohm 10 watt resistor (10x RS# 271-152)across it,
> you will change the scale from 0-130 volts to 0-130 milliamps.  This would
> be useful for checking the output current of neons without risking too much
> money.  Since neons output thousands of volts, dropping a hundred volts or
> so across a current shunt resistor is no big deal.  Of course, the meter
> may be charged to high voltage so it needs to be well insulated from
> surrounding objects.  Much better to blow up $15 of Radios Shack stuff
> rather than a real nice DVM...
> 
> Cheers,
> 
>         Terry

Terry:

	Suggest that, if you are going to measure current during sparking
conditions, you put a large bypass capacitor across the meter to protect
it from transient voltage spikes.  A 1 ufd CERAMIC bypass would be fine.

	By the way, you can usually pick up conventional AC meters, either
ammeters or voltmeters, at swap meets and they are a lot more rugged
(and probably cheaper) than the RS meter.  You can remove the multiplier
resistors from the volt meter, measure the resistance and full-scale
current, and shunt it to read whatever you want.  The ammeter ranges can
be changed with transformer.  For instance, if you had a 1 amp FS meter,
its effective FS value would be reduced to 0.1 amps if you connected it
to the low-turns side of a 10:1 transformer and connected the circuit to
be measured to the the low-turns side.  Do the reverse to increase the
FS reading.  (Hope I said that right.  Current ratio is inverse to the
voltage ratio.)

Ed