i got a new 400w psu for my p4 machine. i tried to check the dc voltages using a voltmeter but the meter showed no reading. then i gave a call back to the shop and told him that the meter shows no reading; he replied that i shud connect the psu to a load and then check if its working . why cant i check the voltages with a multimeter without connecting to the load.

No matter what, the power supply will not run without a load. PC Power and Cooling http://www.pcpowercooling.com makes an ATX Power-Tester that supplies a load to one 5V line and then lights an LED if the power_good signal trips. This basically tells you whether the supply is dead or not.

To test the power supply thoroughly, you'd want to load all of the 3.3V, 5V, 5VSB (standby) and 12V lines to the maximum ratings specified by the power supply manufacturer. you'll find information on how to construct power supply loads using common automotive bulbs or heavy duty wirewound resistors.Or leave it in the system to check the rails.

or

thanks very much for upgrading my knowledge. can u tell me that when we buy a psu say of 350watts is there any way we can check the wattage so that we can find out whether the psu given to us is really 350 watts or is it fake say of 300 watts

Sorry I'm late in seeing this one, but I want to add a pennys worth to the thread.

YODA74 has given the perfect answer to the query so I only support what he/she says.

You cannot get a reading on your multimeter because without a load there is nothing to read. As YODA74 says - you must apply a load then as the load draws current from the PSU you can read it.

To read amperage can be done but you will need more sophistocated equipment than you probably have. Trying with a multimeter may well damage it.

Always buy a good quality PSU this is your garantee that it does what it is meant to do.

Thanks :)

Download my diagnostic flowcharts from here
www.erniek.eclipse.co.uk/downloads/sylvanderdiags.zip
and print them.

Note 1 on the SYSTEM chart has web addresses giving information on this topic as follows:
1. http://www.compute-aid.com/atxspec.html = ATX Power Supplies explanation
2. http://freespace.virgin.net/matt.waite/resource/psu/atxpsu.htm = Pin numbers, voltages & wire colors
3. http://www.epanorama.net/links/psu_computer.html = Info on Power Supply
4. http://tinyurl.com/lrkr = ATX Power Supply Tester

The web page at link 1 above explains that under normal circumstances the power supply only supplies full power after it is sent a signal by the [connected] motherboard.
If no motherboard is connected, no signal is received, and the PSU does not supply power.

It is possible to overcome this, but should only be done by those with "extensive electronics training".

The "ATX Power Supply Tester" shown at the webpage link 4 uses a dummy load etc.

thanks Sylvander
the info u provided are very useful. can u please suggest me some site which gives me the info as to how the PSU used for a pc are rated. i mean when we say that this psu is 350 watts and the other is 500 watts what is the difference between them internally . im trying to find out the info thru search engine. hope u might deliver some better information.

Hi gotodreams-
i mean when we say that this psu is 350 watts and the other is 500 watts what is the difference between them internally .
To add some more reading material to Sylvander's excellent lnks, here are a couple more favorite links often used around here (although a little outdated), offering some more information concerning that very question:

http://www.firingsquad.com/guides/power_supply/

http://www6.tomshardware.com/howto/20021021/index.html