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PC Power Supply Hacking - 50V Output

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Marko

Sat Mar 03 2007, 12:44AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Em, hi guys...

Well, best I can think of that it is a simple overload occuring during the conduction cycle since only one cycle is used instead of two. If you're worried about that you can scope the halfbridge, current transformer and the output and see how it looks.

*edit* As far as I know, the yellow toroid thingie is a proper filter choke and would (does, even) function as a forward inductor. The multiple windings thing is just a cheap money saving trick. Because all the rails come off the same transformer, the voltage waveforms on them are identical in shape, so the same inductor can (sort of!) smooth them all, if you make the turns ratio on the inductor windings the same as on the transformer windings.

That's what I guessed at first, but existence of ''normal'' ferrite inductors in series with it is controversal for the low-cost theory. Nobody would put two different types of inductors if he wants to save money..? (as in schematic

)

Yellow inductor is also quite lossy at these frequencies, but this probably only comes out at very high output currents. So meh..

Steve Conner

Sat Mar 03 2007, 11:08AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'd guess that the two series inductors should have a capacitor to ground between them, which would make it a LCL filter. The schematic guy probably forgot it.

Marko

Sat Mar 03 2007, 01:25PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I'd guess that the two series inductors should have a capacitor to ground between them, which would make it a LCL filter. The schematic guy probably forgot it.

He didn't; what you say is true only for 5V rail, and schematic seems right compared to other schematics I have and real PCB. It's probably some kind of factory thinkerness I don't want to be bothered with, so thanks anyway.

thedatastream

Sun Mar 04 2007, 10:13AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

The yellow coil is a powdered iron toroid which is used as a coupled choke. This aids cross regulation between the outputs when one output is loaded and the other isn't. Basically it is a current sharing choke.

From what I've seen in PC power supplies, they don't use an LCL, they use: transformer > diodes > coupled choke > single choke > large capacitor > maybe another capacitor or a preload resistor

I'm not sure that the output rectification makes a difference but I'll keep it up my sleeve if nothing else seems to do the job. Will have a tinker when I get back to work and see what we can do!

Thanks
James

Ken M.

Sun Apr 08 2007, 03:10AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Wow this info helps alot! Thanks guys! Well It worked great until I discovered my PSU HAD a pot that let me adjust 12v+ between 11-13v I replaced it with a bigger pot, found the short circuit protect transistor, PARTIALY disconnected it and could get 11-18v+ and -12v for 11-30v..at least till the golden rule of electronics was broken (I can't remember the name of the rule) "ALL Electronics run off smoke! IF you let the smoke out the device will no longer work!"

So as a result the excesive +V split the transistor in 1/2.

ragnar

Sun Apr 08 2007, 04:50AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

I hacked my first TL494 supply last week... it was too easy -- I just put a fixed resistor in series with the existing feedback on pin1, then replaced the smoothing capacitor on the output with one with a more suitable voltag rating. Voila, 18V at 5A, and they only cost me $20. Now I can run my IXDD414s from it.

Though in reality, I'd really much prefer to have 24V, the transformer starts squealing at 19V, so I think a rewind is in order.

^_^

uzzors2k

Sun Apr 08 2007, 09:43AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Yeah, PSU modding is surprisingly simple. However to get any lifetime out of it, a rewind and lots of component replacement is necessary, which makes building your own smps from scratch worth the trouble.

thedatastream

Mon Apr 09 2007, 01:22AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

My modification was performed with no rewinding of the transformer - only the choke had turns removed and the output rectification was changed.

Glad to see that this mod is coming in useful

ragnar

Mon Apr 09 2007, 05:45AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

I hope I don't sound too thick when I ask: why did you take turns off the choke and alter the rectification? Did you put a voltage doubler on the rectifier?

thedatastream

Tue Apr 10 2007, 01:57PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

wrote ...
I hope I don't sound too thick when I ask: why did you take turns off the choke and alter the rectification? Did you put a voltage doubler on the rectifier?

If you look at

you can see that the rectification has changed so that it uses each end of the transformer outputs instead of the centre tap. I didn't use a voltage doubler

I didn't remove turns from the choke, I removed windings - specifically the 5V, -5V and -12V ones. I used the 12V winding unmodified as the output choke.

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