Date: Mon, 01 Mar 2010 10:47:14 -0500
From: Tim Long <tlong8@earthlink.net>
Subject: Re: [Amps] Manganin wire
Hello All,
Forgive me if this has already been said.
The resistance of manganin wire is extremely stable with respect to
temperature and this is why it is so useful as a meter shunt.
An ammeter is nothing more than a voltmeter that measures the voltage drop
across a meter shunt.
## The v drop across the meter is typ .055 vdc. Your typ 0-1A DC
ammeter [for a plate current meter] consist of a 0-1 ma DC ammeter,
that has a typ 55 ohm internal resistance.
## A shunt is placed in parallel with the meter [ aprx .055 ohms].
When 1000 ma flows..... 999 ma flows through the shunt, and only
1 ma flows through the 0-1 ma meter itself. The meter face is
changed to read ....'0-1A'
The resistance of the shunt and the meter scale
determine the range of the ammeter. There is no reason to use manganin
wire to connect the power supply to the meter unless the designer is
using this connection AS the meter shunt. Just because the meter scale
reads amps, doesn't mean that there is a shunt internal to the meter.
## I'd be using heavy duty HV wire..from B- to metering. I treat the
B- like it was hot.
### Most dc ammeter's up to 50A will use an internal shunt. After that,
they are external. Just connecting a shunt in parallel with a meter is a
chore.
As u can see, just a tiny bit of extra resistance from a lousy solder
connection,
or not enough torque, will throw the calibration off.
## Easy enough to check... insert a fluke dvm in series with the suspect
meter.
Later...... Jim VE7RF
Good luck & 73, Tim, W2UI
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