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Subject: Re: Reducing contact resistance for low volt use?
Date: Thu, 12 Dec 2002 16:27:10 -0800
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There are a whole bunch of practical issues
building something in a box
that can be carried off the bench and hooked up
with real wires.
To get the measurements I posted,
I had to use my parallel-gap spot welding head and
hook it up with
#2 cable just to stay within the dynamic range of
milliohm meter. 12 milliohms is a damn small
number in a practical
pressure contact test configuration.
Here's what I suggest.
Get a +5V 100A power supply, just cause they're
2V is probably plenty.
Modify the supply to adjust the current limit over
range of 0 - 100A. DON'T modify it for lower
you reduce the input voltage. You keep the heat
in the supply
down by running it at it's max voltage.
Sit the battery on top of the +5V.
Hook a 50 milliohm 500W+ resistor from the top of
the battery to ground.
You want the resistor to sink the 100A at as high
a voltage as possible.
Making the resistor smaller will run it cooler,
but the heat will go into
the supply instead. Keep it outside in the
Assuming your resistor is stable, you can measure
the current as the voltage
at the resistor. If not, put a calibrated meter
shunt in series.
Measure the voltage differentially across the
battery with a separate
kelvin connection. It's already a floating
measurement, so sitting at 5V shouldn't be
a problem. You can put the battery "under" the
supply, but I'd rather not
have to float the whole thing...dpends on what
supply you get.
Build a comparataor at 5.5V or so to abort the
test when the cell voltage
hits 0.5V...or pick a number.
The low tech solution is to put two 100A diodes
from the battery + to the resistor
and one 100A diode from the - of the battery to
the top of the resistor.
Will need a lower resistance resistor to
compensate for the diode drop.
That will limit the disharge to half a volt or
so. If you can't get the parasitic
resistances low enough, use one diode in each leg.
Probably sufficient if it's an attended
measurement and you just want to prevent
an explosion if you get distracted.
Having 5V to burn dramatically simplifies the
You can use wires that you can flex by hand. But
remember, that doesn't
relax the requirement for a low resistance contact
point. You don't want
heat at the contact.
And you don't have to build any active circuitry
or worry about heat sinks.
As for the brass brush connection, watch out for
To get reliable contact, the bristles need to
contact at an angle so each
has it's own springiness/compliance. Copper is
not good for this cause
it's too ductile.
I'd worry about too much pressure with current
In my welding experiments, I've blown holes in the
Learned very early that I needed more current for
shorter time to prevent this.
A hot contact area under pressure can bend
John Fields wrote:
> On Thu, 12 Dec 2002 20:05:32 GMT, jmuchow@SPAMMENOTcamlight.com
> (John Muchow) wrote:
> >I probably have to do some tests to see exactly what kind of current I
> >can realistically pull through the circuit. Just use a length of
> >wire, a couple of MOSFETs switched fully on (and allowed to stabilize
> >their temperatures), and a 1.2V cell...see what happens.
> You might be able to get away with a single logic-level MOSFET.
> Go here:
> and get the data sheet for the IRL1404 for a nice surprise.
> John Fields
> Professional circuit designer
Bunch of stuff For Sale and Wanted at the link
Tek 2465, TFT projector, ham radio, 30pS pulser
Tektronix Concept Books...
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