... and the fuse remained intact.

[Th3_uN1Qu3]

Yesterday i was making the finishing touches on my +/-35v and +/-20v 600W unregulated SMPS for audio amps, as the amps will be put into full service starting tomorrow.

The PSU is a heavily modified ATX supply with 120mm fan (used to be a 430W Spire based on the AT-2005B platform), the amps are TDA7294 modules, designed to power bi-amp speakers. Two bridges for the woofers and a stereo module for the tweets. A pair of 10000u 50v caps in series in the PSU secondary and another pair on each module. Total of 40000uF at 70 volts, enough that short circuit protection is basically pointless, as it will vaporize 18AWG wire.

I wired everything up and hit the switch. Nothing. Took the PSU apart, and... not again. Primary transistors shorted b-e due to the current surge at power up (the PSU topology remained the same, half bridge with proportional base drive, and the base current at startup was too high and took out the transistors).

I messed with the base resistors a little but this was clearly not the way to go as it affected efficiency too much. A proper soft start needed. So i turned the PSU off by the switch to put in the original base resistors (4.7 ohm) but forgot to unplug it. As i was unscrewing the board the screwdriver touched something and... BANG. Turns out the circuit was live.

Damage:
One of the primary transistors shattered, the other cracked in half. The one that shattered blew away a cap in the base drive circuit too.
One MOV obliterated, blew away one of the primary bleeder resistors as well.
NTC cracked.
One of the diodes in the bridge (4A) shorted.
And my left ear felt like it just received a flashbang.

As i later found out the TL494 was gone too, not sure why as everything else in the secondary was fine.

Obviously, a short of this magnitude popped the main breaker too. However, the fuse in the PSU stayed put. Now it makes sense why - the fuse ended up on the neutral and this was a live to earth short.

One day's work later, PSU is fully operational and with proper soft start. Added a second NTC after the primary caps just in case. The fan in the PSU (thermally controlled 0-100% 30-70C based on primary heatsink temp) spins at like 1000rpm with the amps burning up. All in all, working great.

I have to begin designing my own boards, these ATX mod jobs end up too much of a hack even for me.

[Rulycat]

Nice job. Hope you were wearing safety glasses.

If you design your own boards would you be interested in sharing them? Bear in mind Powmax will be monitoring with great interest ready to copy the design.

Always wondered why nobody on here has made an ATX PSU from scratch, rather than reusing the PCB from an existing design.

Hope you bought new undies.

[Th3_uN1Qu3]

Originally Posted by Rulycat

Nice job. Hope you were wearing safety glasses.

Nope, but there's enough stuff in front of that primary heatsink (like the main transformer), and the heatsink has fins on the top that cover the transistors entirely, so there's no risk of those ending up in my face. I'll definitely wear em if i get into 1kW+ jobs tho... For some reason i *have* to blow some transistors before a good idea hits me, so they'll be necessary in the near future.

Originally Posted by Rulycat

If you design your own boards would you be interested in sharing them? Bear in mind Powmax will be monitoring with great interest ready to copy the design.

Sure. As a matter of fact i'll be sharing my active crossover board tomorrow night. I'd do it today, but there's also that thing called uni to take care of, heh.

Originally Posted by Rulycat

Always wondered why nobody on here has made an ATX PSU from scratch, rather than reusing the PCB from an existing design.

I guess i'll break the ice then. I'm not interested in making ATX PSUs tho, i'm interested in making PSUs for things that make noise. However i'm getting into small form factor systems, and i'm likely to need a custom PSU for what i have in mind. SFF PSUs are both crappy and expensive.

Originally Posted by Rulycat

Hope you bought new undies.

I don't get scared that easily.

[tom66]

I wasn't so lucky. Powered up this TV with a shorted bridge rectifier. The fuse made a very nice pop and sent flying glass across the room. It tripped the main breaker too.

I initially thought, "it's a glass fuse, they don't have great current interruption ratings."

But I put in another glass fuse removed from another TV (so known to be safety certified) and it didn't shatter when it failed (a mistake of mine, I had a solder bridge between pins I didn't adequately clear.)

I guess it was a counterfeit/fake fuse, sold by Maplin. Can't trust anything retail these days, it seems.

[Shocker]

Of course, it could have been worse...

Lesson learned: ALWAYS use double-pole mains switches, especially in countries with non-polarised sockets.

[Th3_uN1Qu3]

Originally Posted by Shocker

Lesson learned: ALWAYS use double-pole mains switches, especially in countries with non-polarised sockets.

Or just unplug the darn thing.