Re: Amp Meter

From: John Popelish (jpopelish_at_rica.net)
Date: 02/16/05 

Date: Wed, 16 Feb 2005 09:45:17 -0500

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
> news:pan.2005.02.15.18.55.11.112456@example.net...
> > 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. If it is less than an ohm, the
shunt is almost certainly inside the meter, and it is safe to pass
amperes through the meter terminals. If higher than an ohm, the meter
is supposed to have an external shunt resistor connected to it, and
the load current is supposed to be passed through that resistor
through an extra pair of terminals. Here is what an external shunt
resistor might look like:
http://www.carrel.co.nz/shunt/main.htm
Note the big terminals for the load current and the smaller ones for
the meter connection.

A more detailed review of ammeters:
http://www.allaboutcircuits.com/vol_1/chpt_8/4.html 

-- 
John Popelish

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