The "proper" AT-to-split-supply mod!

[Th3_uN1Qu3]

Re: The "proper" AT-to-split-supply mod!

If I knew this PSU was this capable (in terms of mods), I would have definitely fixed it.

Awesome work, keep it up.

Any PSU using tl494 is a good candidate for mods. The gutless ones are better coz there's less stuff to remove.

What isn't clear to me yet is how the 494 is powered. Vcc is a steady 28 volts regardless of the output voltage of the former 12v rail. I didn't find any extra windings on the main transformer so i suspect it's powered from the base drive trafo somehow.

We'll see what happens when i install the new main transformer. If the supply doesn't start at all it means i was wrong.

[Pyr0Beast]

Re: The "proper" AT-to-split-supply mod!

Self-oscillation after which TL494 takes control.

[Th3_uN1Qu3]

Re: The "proper" AT-to-split-supply mod!

I know that. But i don't know where exactly the 494 is powered from.

[Pyr0Beast]

Re: The "proper" AT-to-split-supply mod!

12V rail probably. Follow the supply trace.

[Th3_uN1Qu3]

Re: The "proper" AT-to-split-supply mod!

Once i overcome my laziness i will... But i already said that the Vcc pin of the 494 holds steady at 28v while i vary the 12v rail from 7v to 28v. It only goes down (to ~5v) when the protection trips hence my suspicion it's somehow pulled from the driver transformer. If there is a buck-boost converter, i don't see it. And i highly doubt there is.

Edit: Confirmed. It has a little self-oscillating affair feeding the 494, which like i thought is coming from the driver transformer.

[Th3_uN1Qu3]

Re: The "proper" AT-to-split-supply mod!

Update. The driver transformer only starts the thing... the TL494 power is actually derived from the main transformer winding, and i learned it the hard way. I'll spare you the long story but the TL494 wouldn't start and its power supply were at 1.38v or so. I thought there was something wrong with that, but nope... i had a shorted 494. Now having a good idea as to why it died, i replaced it and added a 30v zener (actually two 15v in series) between TL494 Vcc and ground. Success!

The power supply now starts up just fine. I have to go to the post office to get a package (12 ATX PSU boards with everything on them except the wires, $15 for the whole lot ), when i'll be back i'm wrapping the whole thing up and taking pics.

[kaboom]

Re: The "proper" AT-to-split-supply mod!

The '494 gets its VCC after the 12V rectifier but before the output inductor. Hence no pulse length integration-to-variable-voltage. That's why you had 28V on the '494 no matter what the 'output' was.

When power is first applied, the primary switches are slightly biased into conduction. Almost immediately, one saturates. Current flows through the power transformer, as well as a one turn winding in the driver tx. The driver tx couples this winding to two windings to the bases of the switchers, one base winding 180 degrees out from the other. Shortly after, the driver tx nears saturation and this is coupled to the other transistor. The one that was saturated drops out and the other picks up. After a few cycles of self-oscillation (by way of driver tx), the cap(s) feeding the '494 charge up. Since that point is before the output inductor, this happens even before the 'big' output caps are fully charged. Now the '494 has power and drives the bases of the 2SC945s feeding into the driver tx. At that point, the supply has bootstrapped and the bases of the switches are fed from their own output via the driver tx. Add a 4148, some small caps and resistors, and the driver tx can give a turns ratio derived reflected voltage that represents the primary current. After all, the primary current flows in the one turn 'kicker' winding in the driver tx.

So that 'driver' tx has the possibility of doing triple duty- nice for parts saving and leads to 'interesting' circuits.

Have fun.
-Paul

[momaka]

Re: The "proper" AT-to-split-supply mod!

Any PSU using tl494 is a good candidate for mods. The gutless ones are better coz there's less stuff to remove.

In that case, I'm good then . Got quite a few old Deers.

12 ATX PSUs for $15? Not bad. I'm assuming most of that $15 is for the shipping .

[Th3_uN1Qu3]

Re: The "proper" AT-to-split-supply mod!

@ kaboom: Thanks a lot for the explanation.
@ momaka: Yeah, shipping was $8. Just the bare boards, but i had enough housings anyway. Some fans would've been nice tho.

I'm leaving this away for a while because i'm experiencing some issues that point to transformer saturation, even though this shouldn't be the case, and another transformer from a half bridge supply (not the original one as unfortunately i broke that when i tried to take it apart), behaves the same. There's something funky going on but i don't exactly know what.

I've decided to first make a PCB for the supply i am designing and building from scratch, and then come back to this one.