PC power supply

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- onyxx
  
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posted
18 years ago

Fri, Sep 16, 2005 4:42 PM

Hello all, I am working a=E7on a 250W SMPS from a PC. The primary part of the circuit work fine ( or so it seems) but the out put from the secondary is pulsating. I caonnect a small device i made out of 2 small light bulbs (one 5V the other 12V,I use this to quickly check for right voltage output)and i find is pulsating, like on off on off. Can anyone tell me why can this be? I tought of capacitors but they all seem fine. I dont have any equipment to check caps, I used a DMM. Any ideas?

Thanks, Gary.

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- Sam Goldwasser
  
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posted
18 years ago

Fri, Sep 16, 2005 5:14 PM

How much of a load are you lamps? You may need a couple amps on at least the +5 VDC line for stability.

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- Tom MacIntyre
  
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posted
18 years ago

Fri, Sep 16, 2005 10:12 PM

Does it do this when connected to the motherboard? It actually sounds like something is unhappy on the primary side...

Tom

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- Elec_Tech
  
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posted
18 years ago

Sat, Sep 17, 2005 10:39 PM

Is the supply current limiting? It sounds like its trying to start up, and then shutting itself down. If it is, then yes, it does sound like a control side issue. Closely inspect the pcb and components, using a jewelers eyeglass (95% of faults can be found, by careful inspection. To check that a transistor hasnt died, by desoldering one of its legs and measuring it. Also check for dry joints. Good luck.