I've used the MAX 472 to measure 100 amps, using a 0.5 milliohm sense
resistor. These are commercially available products commonly called
"shunts".  Take a look at the Blue Seas Systems web site:

http://www.bluesea.com/Products/ME/ME.html

I'm sure there are a lot of other suppliers as well.

Shunts are typically rated as "amps for 50 mv", since they were originally
designed for use with analog meters, which typically required 50 mv for
full scale deflection. My shunt is 100 amps for 50 mv (thus the 0.5
milliohms). With the MAX 472, you can set the gain with the resistors to
basically any shunt value.

When I first started playing with measuring current, I didn't have a proper
shunt, so I made one out of a few feet of 8 or 10 gauge wire. Using copper
wire tables, I figured out about how long a piece I needed, and cut the
wire a bit longer. Then I put a measured 10 amps through the wire. Using a
digital voltmeter, I clipped on lead to one end of the "shunt" and clipped
a needle to the other end of a probe, and pushed it through the insulation
on the shunt in various places until the DVM read the proper voltage (in my
case I wanted 100 millivolts for 10 amps). Then I stripped a bit of
insulation off the shunt at that point and soldered a wire to it to use as
a sense wire.

The problem with using copper wire (or a copper trace) is that copper's
resistance changes with temperature - a shunt is built to have a
close-to-zero temperature coefficient. But for most hobby purposes, it
isn't a problem as long as the wire gauge used is large enough that the
current flowing through it does not cause it to heat up.

Larry


At 12:10 PM 11/21/2002 +0000, you wrote:
>Hi list
>
>Due to the discussion on the current sense amps a few days ago ( week or so
>? )
>I have some samples.
>
>However my application calls for measuring > 10 amps at 24v which would
>require a
>sense resistor of 5 mOhm. How on earth (apart from a pcb track which I don't
>think
>I will be able to measure or make) do I get 5 mOhm ?
>
>If anybody has worked with these IC's would they be kind enough to help me
>out
>with the resistor values for the above parameters ?
>
>Thanks
>
>Steve

Larry Bradley
Orleans (Ottawa), Ontario, CANADA

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