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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Sun, 15 Dec 2002 08:31:49 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 15 Dec 2002 08:31:36 GMT
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>>>Yes, indeed. When you evaluate your 20A circuit,
>>>make sure you use
>>>the worst case contact resistance. Then measure
>>>the volts across the fet
>>>at minimum battery voltage and maximum current.
>>>Subtract the fet
>>>and shunt volts
>>>from min battery volts and multiply the result by
>>>5. Subtract this new
>>>number from minimum battery voltage to see what
>>>you'll have across the fets
>>>at 100A. If this is negative, you're hozed.
Thanks for the tips, they're good ones.
>>>The op-amp takes care of the fet Vth. What you
>>>need to tweek out
>>>is the uncertainty in the shunt resistor and the
>>>offset of the op-amp
>>>itself. You may need an offset tweek in addition
>>>Depending on how low you can make the external
>>>and the resultant voltage you have left for the
>>>fet, shunt and internal
>>>losses, you may find you need a VERY good op-amp.
More good tips!
>>>And you still need a place to measure the
>>>current. Either need
>>>another resistor in the parallel path or a hall
>>>The layout of the circuit will be critical.
>>>And you have come full circle. Building a
>>>practical MOSFET load will
>>>be very difficult.
Yea, that current measuring thing is a pain. :-)
I'll probably go Hall as I really don't have much resistance to play
with here. I'll probably go with a 2 or 3 cell (in series) unit
too...get the voltage up.
One nice thing about the MOSFET load circuit is that I already have
all the parts (except for the Hall-effect sensor) and I can easily
work something up.
I have also found 50-milliohm, 850-watt braking resistors (only $7
each!) to use with a 5V, 100A power supply (in current limit).
Hopefully I can get one of the supplies I saw for $20. The seller is
checking if it has a current limiting pot somewhere. It does have
voltage adjust though. I think that could be used in a pinch to
auto-adjust the current, couldn't it? The supply would get HOT, but I
can add another couple of fans.
Time for lots of tests. :-)
Thanks again for your help!
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