Switch mode PSU

argonrepublic · 16th October 2009, 05:33 AM

I have trouble with this. I have done a few by swapping in caps but this one is blowing fuse. I am by no means skilled in electronics but would like to be. Only failures and successes are going to teach me. Any advice would be greatly appreciated.

Enzo · 16th October 2009, 05:48 AM

IMPORTANT
These switchers run directly off the mains - your incoming AC power is directly rectified into high voltage DC with a ton of current behind it if it wants. These are extremely dangerous to service. ALWAYS use an isolation transformer at the very least.

Whe I have a small switcher for service, first thing I check for is shorted semiconductors. Check all the secondary side rectifier diodes. Be aware ther are often load resistors to provide minimal loading, so you might see 100 ohms appearing across a diode, but a short is a short usually.

I always check the switching transistors. They can fail on their own, or becuase of something shorted elsewhere. Certainly the main primary side rectifiers.

bountyh · 18th October 2009, 11:08 PM

If you are blowing fuses, you have to replace AT LEAST:

Switch transistors

Output rectifiers

IC that drives the switches

Switcher filter caps

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16th October 2009, 05:48 AM	#2
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	IMPORTANT These switchers run directly off the mains - your incoming AC power is directly rectified into high voltage DC with a ton of current behind it if it wants. These are extremely dangerous to service. ALWAYS use an isolation transformer at the very least. Whe I have a small switcher for service, first thing I check for is shorted semiconductors. Check all the secondary side rectifier diodes. Be aware ther are often load resistors to provide minimal loading, so you might see 100 ohms appearing across a diode, but a short is a short usually. I always check the switching transistors. They can fail on their own, or becuase of something shorted elsewhere. Certainly the main primary side rectifiers.

18th October 2009, 11:08 PM	#3
bountyh diyAudio Member Join Date: Sep 2009	If you are blowing fuses, you have to replace AT LEAST: Switch transistors Output rectifiers IC that drives the switches Switcher filter caps

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