Reply to: Measuring Starting Current
There is a MUCH CHEAPER way to measure high currents, than the clip-over type
probes that Pat discusses. The cheap way is called a "Meter Shunt". A Meter
Shunt is nothing more than a high-power resistor of known, low, resistance.
You wire it in series with the current to be measured. A voltage appears
ACROSS the shunt, and you measure this with a voltmeter. For example, if the
resistance of your shunt is .01 ohms, then a 400AMP current flowing through it
will produce four volts across it. BTW, this is a graphic illustration of the
importance of clean, low-resistance battery and starter-cable connections.
Now, it so happens, that for resistances in this range, you don't have to
go down to the electronics shop to buy them. You can make them yourself, using
plain old copper wire. Engineering handbooks usually have a neat table called
the "copper table". A copper table lists the resistance per foot of all the
various gauges of copper wire. In general, resistance is inversely
proportional to the square of the wire's diameter. I don't have a copper table
handy here at my software development station, but offhand, I would suspect
that such a resistor could be made out of 12 gauge wire.
So you build a little test setup with a battery clamp, a battery post, and this
copper-wire "resistor" in between. To measure starting current, pop the hot
wire off the battery, insert the shunt, and stick your voltmeter leads across
the shunt. The accuracy of this technique is only limited by your precision
and patience in cutting the shunt, and the base precision of your voltmeter,
which in the case of a Fluke DMM, is ridiculously high for this kind of
troubleshooting:-).
- Jerry
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