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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Newsgroups: sci.electronics.basics,sci.electronics.design,sci.electronics.misc
Subject: Re: Reducing contact resistance for low volt use?
Date: Sat, 14 Dec 2002 20:59:23 GMT
Organization: MindSpring Enterprises
Message-ID: <3dfb9177.7733557@news.pipeline.com>
References: <3df8e0ea.384960@news.pipeline.com> <3df955f5.537575@news.pipeline.com> <71oivu0071hl3hudptqq7q30ulroum3fab@4ax.com> <3df98a08.13869942@news.pipeline.com> <6l9jvu06mor0eul54mq2s3b7qou2qmuaa4@4ax.com> <46djvu45fkek56leq90l2s8198efdg5j7l@4ax.com> <3dfa35e3.4870183@news.pipeline.com> <3dfc493f.7147762@news.texas.net>
Reply-To: jmuchow@SPAMMENOTcamlight.com
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>>>If you're planning on using a MOSFET array, you're going to run into
>>>a HUGE number of problems with getting the Vth's balanced and
>>>tracking to run off a single shunt! MOSFET's have a positive
>>>tempco, so you'd have that going for you in terms of preventing
>>>current hogging, but better you should do this:
>>>[snipped schematic and ckt description]
>>>That'll give you the advantage of having each MOSFET individually
>>>automatically controlled by its own shunt, and everything else gets
>>>easier too.
I love how this has all come around full circle. Started off with a
MOSFET constant-current load wondering how to hold down resistances,
worked through a few wonderful ideas that others contributed, and
ended up back at the original circuit but with the great idea of
setting up lots of identical modules to lower overall resistance. :-)
Seems like the best idea for me to start with...least amount of
up-front purchasing (already have most of the parts) and easy to test
with two indentical "modules" (op-amp, shunt, MOSFET) to verify
operation at 20A discharge current before stepping up to 100A with 10
modules, 10A each.
The biggest challenge would be making sure I connected each module's
wiring (10AWG? 12AWG? Need to look up some DC resistances) up to the
main discharger contacts or contact wire (perhaps 1/0 or so in size).
One question...
Wouldn't I need a "master" current-setting pot and individual
reference voltage pots for each "module" to compensate for differences
in the MOSFETs and shunts? Something like this:
VCC
|
[R1]
| VCC
+------+ +----+ | +--------+---+
| | | | | | | |
[LM385] [POT]<-+ [POT]<------|+\ D | |
| | | | | >--[R2]--+--[R3]--G Q1 | |
GND GND | GND +--|-/ | S | |
| | | [C1 | | C
| | | | | | E
| | GND GND | | L
| +------------------------+ | L
| | | |
| [SHUNT1] | |
| | | |
| GND---+-----+------+
| | |
| VCC | |
+----+ | +--------+
| | | | | |
| [POT]<------|+\ D | |
| | | >--[R2]--+--[R3]--G Q2 | |
| GND +--|-/ | S | |
| | | [C1 | | |
| | | | | | |
| | GND GND | | |
| +------------------------+ | |
| | | |
| [SHUNT2] | |
| | | |
| GND---+-----+ |
| | |
| | |
| | |
| <---- TO FOLLOWING STAGES--> | |
| | |
Or are the differences in MOSFET Vth, shunt resistances and tempco,
and other variables not large enough to warrant calibrating each
module individually?
>>>What I'd do would be to machine the contacts to hold a plug of fine
>>>copper wool against the cell terminal(s) tightly during testing. I
>>>think something like that, which would inherently conform to the
>>>shape of the cell's terminals would be much more likely to give you
>>>the multiple contact points you need. You might want to clean the
>>>plug just before testing by dipping it in one of those instant
>>>copper cleaners and then rinsing and drying it. You could even dip
>>>it in some mercury to make a perfectly conforming all-metal contact
>>>surface, but I don't recommend it unless you've got some very good
>>>way to capture the mercury vapor and you know how to handle mercury
>>>safely.
I was also thinking about using something "mushy" to conform to the
faces of the battery but couldn't think of something low enough in
resistance. Copper wool might work if I use a big enough hunk
(perhaps at one end of a tube that I pushed the battery down to press
against the wad of copper wool?). I thought about using the soldering
iron tip cleaner that is made up of coils of what looks like copper or
brass:
http://www.hakko.com/itnl/products/599.html
It's got beefier hunks/coils of metal to better handle being pushed
around by the battery and would bounce back after each use. I have no
idea if it's conducting though. If it is, perhaps I could smear some
silver conducting grease throughout the stuff to prevent oxidation and
reduce resistance even more. I wish that the black antistatic foam I
store my chips in had MUCH lower resistance. It would be great to
just press the battery terminals into a 1/2" thick block of the stuff
until it "bottomed out" against the discharger contact.
I just had the insane idea of working as much silver conducting
grease as I can into some antistatic foam...It would never work, but
for half a second there I had hope. :-)
I'm pretty sure that I'll avoid using mercury as I have no safe way to
handle the stuff.
John Muchow
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