[TriEmbed] Dummy Load

[Fred Ebeling]

I have used similar device in testing power supplies, I would just add 
a fan to cool it down.

Fred Ebeling
ECP Designs



From: Pete Soper 
Sent: Thursday, July 17, 2014 5:12 PM
To: triembed at triembed.org 
Subject: Re: [TriEmbed] Dummy Load

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.

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 nichrome calculator 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.

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:

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

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. :-)

-Pete
(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.)


On 07/17/2014 04:03 PM, Adam Haile wrote:

  Any reason I could use something like this 200W rheostat to act as a test dummy load? http://www.ebay.com/itm/200W-5-OHM-High-Power-Wirewound-Potentiometer-Rheostat-Variable-Resistor-/130697179186?pt=Vintage_Electronics_R2&hash=item1e6e28b032 

  I'd need 25A or so... so, dial it down to 0.2 ohm. And at it's max (5A) it should draw 1A.
  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...

   

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