psu/monitor problemd

  casper69 19:28 15 Dec 2005
Locked

hi
ive bought a new 500w psu it was connected up all fine and worked perfect but when i turned it on today my graphics card details came up on screen and then the screen went blank and didnt start up. i changed over the psu to my old 200w one and now works fine when started it still shows the graphics card details at first and then starts up fine and i can use my pc fine. ive checked my graphics card and its working fine anyone have any ideas please
cheers

  Gongoozler 19:46 15 Dec 2005

Hi casper69. As long as the new PSU is plugged in properly, then it sounds very likely that it's faulty. What does surprise me though, is that you've made such a big jump in power rating from 200W to 500W. A switched mode supply can only work over a certain power range, and I wonder if you computer that was happy with a 200W PSU has such a low demand when idling that a 500W supply falls out of regulation and switches off. If you have a multimeter you can easily check the volts on a spare drive connector. Although this isn't a complete check, if you get 5V and 12V within 10% at the drive connector than this is a good indication that the supply is working.

  casper69 19:54 15 Dec 2005

ive gone from 200w to 500w as ive got a new case with fancontrol, more fans and led lights and the 200w couldnt take it so i thought a 500w would be ok but the problem above happened do you think i need a 350w or a 400w psu

  Gongoozler 08:32 16 Dec 2005

Hi Casper. I think it is most likely that the PSU is faulty, but it is also just a possibility that it is too big for your computer although I've never heard of this happening. You can get an estimate the power you need here click here. If you can tell us something about your computer we can suggest a suitable PSU.

  rmcqua 08:58 16 Dec 2005

The 500 Watt PSU won't "fall out of regulation and switch off". it should keep its output volts within limits even at very low current demand.
No need to worry about this.

  Gongoozler 09:34 16 Dec 2005

Hi rmcqua, As I said, this is a very unlikely situation, but a switched mode supply does need a minimum load to work, that's how switched mode operates. It chops up the supply with a varying mark - space ratio. Typically the ratio will be between 10% and 90%. If a very large supply is feeding a very small load, then the chop ratio will be outside the range that can be regulated. As I said, I haven't heard of this happening with a computer supply, but it is a fact that a computer PSU will not work without any load.

  rmcqua 09:47 16 Dec 2005

Hi Gongoozler,
Thanks for your comments.
I've worked with and designed SMPUs for many years and, provided that the PC is presenting SOME load to each of the voltage rails, I don't think this is an issue that casper69 should bother about.

  Fruit Bat /\0/\ 09:57 16 Dec 2005

It is a fact that a computer PSU will not work without any load.

True

the load of a HDD or CD or Motherboard is sufficent to switch the PSU output ON.

PSU output voltages can be tested by plugging in one HDD

Unplug the power cord from the power supply, short-out pins 14 and 15 on the power supply main power connector, and plug-in the power cord just long enough to see if the fan is working.
This may not be the best thing to repeatedly do to an ATX power supply (it can damage it).
The ATX12V Power Supply Design Guide click here states that the power-on function should be normally done with TTL (transistor transistor logic) circuitry which pulls pin 14 low.
I use a jumper cable with alligator clips at each end with partially stretched-out paper clips in each alligator clip (an old trick).
Pull the power plug before removing the jumper. If the power supply works, remove the motherboard and see if it is shorted-out by a stand-off or lose screw.
I have seen cases where a particular "good" ATX power supply would not work with a particular "good" motherboard--so much for standards and design guides.

Pin layout

11,12,13,14,15,16,17,18,19,20.
1 2 3 4 5 6 7 8 9 10.


1= +3.3vDC
2= +3.3vDC
3= com
4= +5vDC
5= com
6= +5vDC
7= com
8= PWR_OK
9= =5vSB
10= +12vD

11= +3.3vDC
12= -12vDC
13= com
14= PS_ON
15= com
16= com
17= com
18= -5vDC
19= +5vDC
20= +5vDC

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