Re: Amp Meter

From: Rich Grise (rich_at_example.net)
Date: 02/16/05 

Date: Wed, 16 Feb 2005 17:25:16 GMT

On Wed, 16 Feb 2005 09:45:17 -0500, John Popelish wrote:
> Nikki wrote:
>>
>> On closer inspection of the meter I have taken the cover off and the needle
>> will not move so I think the meter is no good. I have a second I would like
>> to use this one goes from 0 to 60 On the back it has a + on one of the posts
>> and printed on the back is 120 on the front of the meter is says (its very
>> small print) FS-50mvDC and it also says D C Amperes. Now I know this came
>> from the Phone company and it was on a 48 volt circuit.
>> Nikki
>> "Rich Grise" <rich@example.net> wrote in message
>> > On Tue, 15 Feb 2005 07:58:33 -0500, Nikki wrote:
>
> Amp meters have a sensitive movement connected in parallel with a low
> resistance current carrying shunt that produces the small voltage
> required to move the meter (50 mV in this case). The open question
> with any amp meter is whether that low resistance shunt is sealed
> inside the meter or is assumed to be external to the meter. Measure
> the resistance of your amp meter.

DO NOT DO THIS WITH AN OHMMETER!!!

Use two potentiometers.

Start with one in series with the meter and, say, a 1.5V battery. Start
with the pot at max. resistance. Turn it down until the meter reads
full-scale. Start with at least 10K. If a 1.5V battery won't give full-
scale deflection even with the pot turned to 0, then clearly the meter
expects the full 60A through itself, and you'd stick it in series with
the supply. Otherwise, go to step 2:

2: Take the second pot and put it in parallel with the meter (while
leaving the pot and battery, that have your meter deflected full-scale).
Adjust the second pot until the meter deflects exactly one-half. Take that
resistor, and measure _its_ resistance with the ohmmeter, and that will
tell you the internal resistance of the meter.

Then, it's just Ohm's law. :-)

Good Luck!
Rich

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