[mythtv-users] FE replacement recommendations

Brian Wood beww at beww.org
Thu Jun 4 03:02:56 UTC 2009

On Wednesday 03 June 2009 19:48:21 Johnny 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.

Well not infinite, the input impedance of an oscilloscope is usually measured 
in megohms, and if you have a good low-capacitance 10X probe the loading is 
minimal. Certainly not "infinite", and low enough to cause problems for the 
unwary at high frequencies.

But your point is correct, in terms of DC measurments you can assume an 
infinite input resistance and get away with it in computer PSU 
troubleshooting applications.

It takes some good high-power resistors to properly load a supply, and even 
then the impedance(s) presented to the PS are not the same as a mobo and HDDs 
would present.

The only really practical test for the home user is substitution. You can 
measure the voltages at the Molex connector for "under load" conditions, of 
course if the load (mobo) is bad...

Resistor bank test jigs are just not practical outside a shop or lab. I 
wouldn't spend the money to build one for home use.

beww at beww.org

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