On Fri, 24 Apr 2009 05:05:07 -0700 (PDT), wrote:



And I would humbly disagree re ohmmeters vs ammeter's - its all to
easy to fry an ammeter, the 10a range on most DMM's is rated for a few
seconds only, then the copper wire shunt disengages itself....besides,
10a is leakage current in a vehicle...and DMM's are now cheap, throw
away items, but a decent ammeter is still expensive....

Andrew VK3BFA.

Replacing fuses in a DMM used in ammeter mode can get old after a
while. Been there, done that.

For those inclined to tinker a bit, as I think Andrew might be, a
solution is to make some shunts of known (low) resistance. Connect
shunt in series with load, measure the voltage across the shunt,
calculate the current. Weston shunts of yesteryear were 50 millivolt
shunts. 100 amps at 50 millivolts is 5 watts, no big deal but not
negligable either. The shunt can be made of convenient material --
iron, steel, stainless, brass, whatever. Nichrome and carbon are
readily-available materials with fairly low temperature coefficients.
Constantin and manganin are much lower but not readily available.
Just make it big enough so it doesn't get spit-sizzle hot and
fuggedaboudit. Don't need 1% accuracy to be useful, right? My shunts
are made of copper, stainless, brass and steel. They work for me.

I calibrate/trim the shunts by measuring both current and voltage with
a controllable bench supply so as not to cremate the ammeter. Once
calibrated, the shunt and a voltmeter become my robust ammeter.