[RndTbl] partial PS use?

Gilles Detillieux grdetil at scrc.umanitoba.ca
Wed Feb 17 12:24:52 CST 2016

On 02/17/2016 11:54 AM, Trevor Cordes wrote:
> On 2016-02-15 Hartmut W Sager wrote:
>> The -5V and -12V are probably fine with no load, but the +12V line
>> should have at least the minimum required load (whatever that might
>> be).  Just use a suitable power resistor (power resistor means it
>> won't fry into a molten lump) on the +12V line.  In the absence of
>> info on the PS label, I'd go for loading the +12V line with about 10%
>> of its maximum power/amps.  Use V = IR and P = IV to do the algebra
>> to get the right resistance (ohms).
> Thanks for the tip!  The old PS indeed says a minimum on the +12V of
> 1.3A.  I paralleled 2 15ohm power resistors (5%; the big huge white
> rectangle ones) to approximate a 1.3A or higher (1.6 actually) load.  I
> only had a limited selection of power resistors around here.
> I can get the whole unit to power on now, and seems to try to boot and
> blink some LEDs, yay!
> BUT the power resistors get REAL hot REAL fast.  Like too hot to touch
> in under 30s.  Does that sound right?  Not sure I can deliver that to
> customer like that.  Any other ideas?  Different resistors?  Maybe try
> a "real" load, but what?

If you do the math, you'll see each resistor should be putting out 9.6 
watts of heat. If they're not rated for that high a wattage, they could 
burn out. I'm not sure, but I imagine it's possible their resistance 
would drop as they overheat, causing them to draw more than the 0.8 amps 
they should draw at 12 V, on their way to burning out (which would make 
them put out even more heat). Even if they're rated for more than 9.6 
watts, that's still a pretty significant amount of heat you'd get out of 
them. Together they'd put out almost half the heat of a 40 W 
incandescent bulb. Any load you use will put out that much heat for the 
same amount of current.

It seems to me a decent switching power supply should be able to 
regulate voltage with much less draw than that, though. I'd try a single 
15 Ohm resistor and see how well it holds the +12 & +5 V, and if it's 
good, try even higher resistor values than that. You might be able to 
get the current draw down below 100 mA (1.2 W) without any loss of 

Gilles R. Detillieux              E-mail: <grdetil at scrc.umanitoba.ca>
Spinal Cord Research Centre       WWW:    http://www.scrc.umanitoba.ca/
Dept. of Physiology and Pathophysiology, Faculty of Health Sciences,
Univ. of Manitoba  Winnipeg, MB  R3E 0J9  (Canada)

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