[mythtv-users] FE replacement recommendations

Brian Wood beww at beww.org
Mon Jun 8 14:41:00 UTC 2009

On Monday 08 June 2009 08:21:32 Tony Brummett wrote:
> On Wed, Jun 3, 2009 at 8:48 PM, Johnny <jarpublic at gmail.com> wrote:
> >> I use a $20 tester to supply a load to the power supply and on top of
> >> that I use a multimeter to test each individual voltage. To me the
> >> leds are useless. This in no way is as good as an oscilloscope
> >> though..
> >
> > Yeah the problem with just testing voltages is that voltmeters or
> > oscilloscopes have ~infinite resistance so they don't show you what
> > the PS will do under load.
> What sort of a load would be an appropriate test?  What about a
> heating element from an electric stove or a ballast resistor from a
> '60s/'70s era car ignition system?

Maybe, if they met the electrical and thermal requirements.

It's a simple Ohm's Law problem:

Let's say the PSU is rated for 30 amps output on the 12VDC rail.

The resistance required would be 12/30, or .4 ohms (R=E/I)

Whatever provides this .4 ohms would have to be able to dissipate 360 watts. 
You might get away with a lower power rating, if you kept the duration of the 
test very short, but you'd have to be careful.

Go through the same calculations for the other power output rails.

At DC, reactance factors are not significant, so, for example. wirewound 
resistors could be used without having to worry about inductive effects.

One thing to be careful of is that heating elements change their resistance as 
they heat up, so the resistance measurement when cold might not be correct 
under operation conditions.

A series/parallel array of commercial high-power resistors is probably the 
best solution, as the resistance would be stable over temperature.

Of course it's probably cheaper to buy another PSU or 2 (or 3) than to buy the 
proper resistors. You would have to load all of the output rails at the same 
time to get a valid test.

Unless you are testing a lot of PSUs, it's probably not worth constructing a 
load test jig.

beww at beww.org

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