[TriEmbed] Dummy Load

Adam Haile email at adamhaile.net
Thu Jul 17 22:01:56 CDT 2014


Very cool. I think I'll just go with one of those big resistors. Maybe I
could make some sort of decade box with a few various values, that way I
could get at least a few different fixed currents... I'd love to be able to
slowly ramp it up, but this could work too :)
And yes... definitely a big fan :)


On Thu, Jul 17, 2014 at 5:12 PM, Pete Soper <pete at soper.us> wrote:

>  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
> <http://www.jacobs-online.biz/nichrome/NichromeCalc.html> 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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