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RE: Current sensing

        In industry when measuring large DC currents we typically use a 50
mV shunt.  This is no more than a large resistor machined out of brass or
copper sized to have a 50 mV drop at a specified current.  You then use a 
mV voltmeter.  The resistance is pretty stable with time and temperature.
Also, it is a rigid assembly where your voltage sense connections are
separate from your current carrying connections so mechanical movement,
vibration, loose connections do not affect the calibration.  Corrosion 
have a small long term change in calibration.  (These are available from
Newark Electronics for about $25 each.)
        I would hesitate to use a wire for a shunt because flex and the
conditions of the terminations could affect the resistance.  A shunt wisely
separates the voltage sense connection from current connections.
        The current meter in the GE's themselves uses a crude shunt.  I am
doubtful that they are accurate judging by the multiple voltage sensing
screw positions.  They are probably very repeatable.
        There are also now available Hall effect current sensors.  They are
shaped like a dougnut and you pass the wire you wish to know the current of
through the hole.  Their output is proportional to input, DC or AC.  They
are subject to zero drift due to stray magnetic fields.  There is no 
drop associated with them, whereas the industry standard shunts have 50 mV.
(These are also available from Newark Electronics for $25 to $40.)

-Steve Naugler
-e mail address:  snaugler rodel com

> ----------
> From:         Max Hall[SMTP:maxo iname com]
> Sent:         Saturday, January 02, 1999 7:32 PM
> To:   elec-trak discussion list
> Subject:      Current sensing
> One last question for the Panel of Experts: I want to build a 
> (reasonably)
> accurate current meter for my e15, and for the electric car I am 
> building,
> but I am running into a snag:
> usually, when you want to build a current meter, you just measure the
> voltage across some small resistor in the circuit you want to measure.
> Fine.
> However, at, say, 100 A, the power dissipated, "eye-squared-are" (I^2 *
> R),
> by even a .1 ohm resistor is about 100*100*.1 or 1000W !  The problem is,
> how do you get the resistance to be nice and small, and yet stable, so 
> you
> can measure current without pissing away power? (Thermal variations must
> take over really quickly ...)
> I must be missing the design theory for large current measure, because it
> has to be cheaper and easier than things like....
> - Since the current runs pretty much at steady state, one could, I guess,
> measure the magnetic field around a straight segment of conductor
> - measuring the resistance as a function of temperature of a piece of the
> circuit's conductor, and then compensating the voltage reading across 
> that
> segment by temperature ( thermistor kissing the side of the conductor)
> - give up and call Bob Batson at EVoA and shell out the money for a
> factory
> built one.
> - drink a triple espresso so you can count electrons individually as they
> go
> by (grin)
> Any thoughts? How does a fancy-pants one work? Thanks, allayou.
> -Max
> www.maxmatic.com