> ... 0.01 ohm will provide 0.2V at full load.

and dissipate 4 watts, so take care of resistor wattage used.

> be able to use a short piece of brass or steel wire (though steel is
> hard to solder to if not impossible).

It's not hard to get 0.01 ohms with copper. Do however be aware that
temperature rise can affect the resistance. You need to either limit rise *
to a level which is acceptable for accuracy OR accept such inaccuracies.
There are metals available that have approximately aero temperature
coeffricient, but these are liable to be overkill in this application. (*
thick wire, well air cooled ot even heat sunk).

The hall cell based Silicon Chip circuit that Jinx mentioned

>    Here's a constructional article for an 80A ammeter using an F84
>    http://www.siliconchip.com.au/cms/A_30551/article.html

Has the advantage of using a general purpose hall cell - this is NOT rated
for any particlular current but depends on how it is used.
Despite the small extra complexity of the gapped core this circuit is well
worth considering due to it's low power loss and flexibility.

Allegro linear hall cells here

        http://www.allegromicro.com/hall/linear.asp


The "trouble" with the current sensors already suggested from Allegro at

        http://www.allegromicro.com/hall/currentsensor.asp

is that, in exchange for simplicity of use and integration, you get less
control than if you "roll your own".
The Silicon Chip circuit can have its sensitivity doubled or multiplied by N
by winding more turns of the current carrying wire through the core. Even a
relatively small core can carry quite a few turns of wire carrying eg 20
amps. (Look at 10A fuse wire - use something MUCH thicker :-) ).




        Russell McMahon

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