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It appears the "operating current" of that rheostate is 6 amps. It
may be that by following previous advice to not "hot switch" small
contacts the wiper connection might tolerate 25 amps without
overheating/oxidizing/melting. Notice it's wirewound and there will
be some inductance with this, but somebody like Fred might say what
effect that might have: I don't understand inductance below RF. :-)
Looks like there are enough turns to have one of them correspond to
.2 ohms or pretty close to that, though.<br>
<br>
But the bummer might be the temperature rise. The whole rheostat is
rated at 200 watts, but what's the effect of about 4% of the wire
dissipating 125 watts? Using this <a
href="http://www.jacobs-online.biz/nichrome/NichromeCalc.html">nichrome
calculator</a> it appears if 25 amps is put through wire instead
of 6 amps (for one example) the temperature rise would be roughly 6X
times the 6 amp rise. For one example I tried with the thickest wire
the calculator handles, at 6 amps the temp is about 200 degrees F
and at 25 amps it's about 1200 degrees.<br>
<br>
If you can get by with a fixed value, here's a .2 ohm resistor that
can tolerate 75 amps at 2/3 of the price and it's non-inductive:<br>
<br>
<a
href="http://www.digikey.com/product-search/en?pv1=1341&pv2=15&FV=fff40001%2Cfff80488&k=resistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25">http://www.digikey.com/product-search/en?pv1=1341&pv2=15&FV=fff40001%2Cfff80488&k=resistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25</a><br>
<br>
Also, looping back to the discussion going on while I was out of the
country, the honking big DC buses I dealt with in the 80s involving
an early SMP machine used drilled and formed copper bars and/or
strap vs wire. But looking at much more recent gear, just find the
power supply hardware in a medium size Itanium system. Ought to be
plenty of surplus by now. :-)<br>
<br>
-Pete<br>
(I couldn't exactly model your .2 ohm, 5 volt scenario with that
calculator, as guestimating 500 inches of wire on that rheostat from
the (ostensibly *wrong example photo*), it would seem to call for
thicker wire than the calculator supports. I could well have made a
big mistake.)<br>
<br>
<div class="moz-cite-prefix">On 07/17/2014 04:03 PM, Adam Haile
wrote:<br>
</div>
<blockquote
cite="mid:CAG8g-TZ7CsaE7Luh=cP6ye3Z6c9AJD=Q0LdDHp7hqWs4m_RQjQ@mail.gmail.com"
type="cite">
<div dir="ltr">Any reason I could use something like this 200W
rheostat to act as a test dummy load? <a moz-do-not-send="true"
href="http://www.ebay.com/itm/200W-5-OHM-High-Power-Wirewound-Potentiometer-Rheostat-Variable-Resistor-/130697179186?pt=Vintage_Electronics_R2&hash=item1e6e28b032">http://www.ebay.com/itm/200W-5-OHM-High-Power-Wirewound-Potentiometer-Rheostat-Variable-Resistor-/130697179186?pt=Vintage_Electronics_R2&hash=item1e6e28b032</a>
<div>
<br>
</div>
<div>I'd need 25A or so... so, dial it down to 0.2 ohm. And at
it's max (5A) it should draw 1A.</div>
<div>People always talk about constant current loads... but I'm
testing a constant voltage so I believe what I really need to
be varying is the resistance...</div>
</div>
<br>
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<br>
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