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<DIV>I have used similar device in testing power supplies, I would just add
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<DIV>a fan to cool it down.</DIV>
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<DIV>Fred Ebeling</DIV>
<DIV>ECP Designs</DIV>
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<DIV style="font-color: black"><B>From:</B> <A title=pete@soper.us
href="mailto:pete@soper.us">Pete Soper</A> </DIV>
<DIV><B>Sent:</B> Thursday, July 17, 2014 5:12 PM</DIV>
<DIV><B>To:</B> <A title=triembed@triembed.org
href="mailto:triembed@triembed.org">triembed@triembed.org</A> </DIV>
<DIV><B>Subject:</B> Re: [TriEmbed] Dummy Load</DIV></DIV></DIV>
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style='FONT-SIZE: small; TEXT-DECORATION: none; FONT-FAMILY: "Calibri"; FONT-WEIGHT: normal; COLOR: #000000; FONT-STYLE: normal; DISPLAY: inline'>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>
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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
href="http://www.ebay.com/itm/200W-5-OHM-High-Power-Wirewound-Potentiometer-Rheostat-Variable-Resistor-/130697179186?pt=Vintage_Electronics_R2&hash=item1e6e28b032"
moz-do-not-send="true">http://www.ebay.com/itm/200W-5-OHM-High-Power-Wirewound-Potentiometer-Rheostat-Variable-Resistor-/130697179186?pt=Vintage_Electronics_R2&hash=item1e6e28b032</A>
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<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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