A lesson in power and efficiency.
We all know that cheap PSUs blow up at anywhere between 175 and 350W. Most are only safe to use long-term at sub 150W. Most of the time it's the primary transistors that blow.
When you ask someone uneducated in PSU design as to why they blew, he'll probably answer "crap transistors". Someone slightly more educated would answer "crap heatsinks". But this is only half the truth. Of course, low efficiency and small heatsinks don't mix, but what if we could make the efficiency higher?
There's an easy way to do that and that's upgrade those god-awful secondary diodes. If you've bothered to recap a low quality PSU then at least don't make it a waste of caps. Use diodes with at least twice the current rating of the rail, and the lowest voltage drop you can find. Most cheapies are silkscreened for both TO-220 and TO-247 (or those horrible round-diodes-on-a-bracket things) so there are plenty of choices. Watch for reverse voltage too, general rule is 40-45v for 5v and 3.3v rails, 100v for 12v. This will dramatically increase the efficiency and thus the power output that you get from that supply.
Next in line are small and thin-wired inductors and transformers, but you're better off buying a new, good quality PSU than going to this extent to get more out of a cheap one, unless you're planning to do it for a different application than powering a PC.
FYI: At 85% efficiency a pair of TO-247 13009s will do over 600W output. I got to 650W with room to spare, but my transformer and inductor are only good for 600W long-term so i had to stop there. The TO-220 variety will do 400-some watts. So in a sense the power ratings of cheap PSUs aren't a complete lie, the primary can do it. But only if the rest of the supply had the right diodes, inductors, transformer and heatsinks...
When you ask someone uneducated in PSU design as to why they blew, he'll probably answer "crap transistors". Someone slightly more educated would answer "crap heatsinks". But this is only half the truth. Of course, low efficiency and small heatsinks don't mix, but what if we could make the efficiency higher?
There's an easy way to do that and that's upgrade those god-awful secondary diodes. If you've bothered to recap a low quality PSU then at least don't make it a waste of caps. Use diodes with at least twice the current rating of the rail, and the lowest voltage drop you can find. Most cheapies are silkscreened for both TO-220 and TO-247 (or those horrible round-diodes-on-a-bracket things) so there are plenty of choices. Watch for reverse voltage too, general rule is 40-45v for 5v and 3.3v rails, 100v for 12v. This will dramatically increase the efficiency and thus the power output that you get from that supply.
Next in line are small and thin-wired inductors and transformers, but you're better off buying a new, good quality PSU than going to this extent to get more out of a cheap one, unless you're planning to do it for a different application than powering a PC.
FYI: At 85% efficiency a pair of TO-247 13009s will do over 600W output. I got to 650W with room to spare, but my transformer and inductor are only good for 600W long-term so i had to stop there. The TO-220 variety will do 400-some watts. So in a sense the power ratings of cheap PSUs aren't a complete lie, the primary can do it. But only if the rest of the supply had the right diodes, inductors, transformer and heatsinks...
Half of the computer problems is caused by bad contacts 
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