PSU is working or not
 

Hi all,

I was going through this link http://compixels.com/51/how-to-check...working-or-not

can any one explain why the 10 ohm resistor is connected ?

Normally you shouldn't start a PSU without a load. While quality PSU can handle this case, some cheap PSUs may be damaged when starting without load. Therefore the 10 ohm resistor is connected to generate a load, just to be safe.

Most cheap PSUs DONT withstand the current or power ratings on them. For example if it says 12 V 15 A, it may only hold up to 12 V 5 A and burn up if you try to take more than that (instantly, or for more than few seconds). Same with watts. A cheap power supply should not be expected to give more than 1/4 - 1/3 of its rating continuously

Cheap power supplies often dont have overload protection. Overload or short circuit - get smoke or sparks, the power supply will continue going. Sometimes there is protection from absolute (high current) shorts and no protection from not-very-high-current shorts and overloads. Avoid shorting your power supply

I have never had a problem running a PSU without a mainboard and without any load resistors (and I powered up several PSUs like that, including really cheap ones) - all PSUs i have seen (including the cheap) allready have the needed resistors inside (usually 47 - 100 ohm not 10 ohm)

In the bad old days PSUs were linear in concept. You fed 240/220* volts into a transformer and pulled 18V and 9V ac off on the secondary windings. This was then rectified, smoothed and regulated to give the 12V and 5V DC needed. Unfortunately to do this, 1.) the transformer was large and heavy, 2.) the DC smoothing needed large electrolytic Capacitors/Condensers the size of soup tins! :(
These PSUs didn't need a load to register output voltage.

Modern PSUs are switched mode. The mains 240/220V* is passed through an isolating transformer; Vin = Vout, then it's rectified to give something like 350V DC (No, I'm not going to work out the correct DC value) It's then passed to an oscillator where it's chopped at about 400Hz, passed to the stepping transformer where it's then stepped down to the required voltages before smoothing and regulation.
What this gets you is much smaller components (though there are more)and greater stability on the output voltage. However... you need an output load when testing, or the oscillator won't work and you'll get no output.

Play Bonny! :hattip:

* substitute 120/110 Volts as appropriate.