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Old 29th April 2013, 11:33 AM   #1
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Default Crude idea to modify PC ATX PSU for Tube Amp

The idea to use salvaged ATX PSU for heater and B+ pops up once in a while and the answer is usually that the main transformer has to be re-wound which is difficult especially for higher voltages.
Looking at the schematics (a collection can be found at AT and ATX PC computer supplies schematics) I wonder whether it might be feasable to leave the ATX PSU as is, load the 12v rail with tube heaters and just connect an additional similar transformer to the AC output of the one already in place - albeit IN REVERSE.
Would that give me the desired 300 volts or so - after rectification and filtering of course ?
Or would this play havoc with the PSU if I tried ... ?
Regulation might be poor w/o further modifications but still ...
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Old 29th April 2013, 11:36 AM   #2
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take B+ from PFC output. That should be around 350-400VDC.
Then take heater from 12v or modify feedback so 5v becomes 6.3v
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Old 29th April 2013, 02:56 PM   #3
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powerflux,
is there any isolation, if he would do so?
Without isolation is very dangerous!
Payloads idea of using another trafo should work.
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Old 29th April 2013, 03:23 PM   #4
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most of the PFC's used in ATX power supplies are isolated. However, you can't be too sure. Always check, never trust the internet.
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Old 29th April 2013, 05:03 PM   #5
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Quote:
Originally Posted by powerflux View Post
most of the PFC's used in ATX power supplies are isolated. However, you can't be too sure.
Hi,

I'm quite sure that PFC's in general are not isolated. Usual topology is a rectifier bridge connected directly to the mains, at it's best via a NTC, then followed by the PFC arrangement which provides a raw DC voltage of ~400 volts for the switching devices.

Best regards!

Edit: The idea of rewinding the isolation tranny probably tends to fail due to the brittleness of the core ferrite. I've never managed to dismantle such a xformer yet.

Last edited by Kay Pirinha; 29th April 2013 at 05:06 PM. Reason: supplement
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Old 29th April 2013, 06:00 PM   #6
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Excuse me. You are absolutely right. My statement was based on that the secondary does float.

see, never trust the internet
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Old 29th April 2013, 08:15 PM   #7
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PFC out is certainly on the mains input side and I do not want to touch that at all.
Actually the entire ATX PSU would stay as is, the B+ trafo being just add-on.

I measured one of the salvaged transformers for inductance and came up with 4mH on the primary and 60uH center tapped for 12v secondary (170uH for 5v).
This is about consistent with what I found here: http://danyk.cz/s_atx01u.png.

Assuming I connected an identical transformer backwards like in the attached drawing I should be back to 300v ... or maybe not ?

The turns ratio is sqrt(4000uH/60uH) = 8 which may not be enough.
This is why I connect across the center tap of the 12v output.

Before I try that I would appreciate some opinion whether it is worth doing and whether there might be any adverse effects on the PSU itself ?
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Old 29th April 2013, 08:19 PM   #8
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Please don't forget the half bridge configuration of the primary side of the power transformer. This means that the voltage across it is, at it's best, 300 Vpp! So you'll need a voltage doubler.

Best regards!
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Old 29th April 2013, 08:46 PM   #9
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Quote:
Originally Posted by Kay Pirinha View Post
Please don't forget the half bridge configuration of the primary side of the power transformer. This means that the voltage across it is, at it's best, 300 Vpp! So you'll need a voltage doubler.

Best regards!
I didn't. Thats why I connected the additional trafo across both halves of the original center tapped.
4mH -> 120uH (60+60) -> 60uH -> 4mH . I guess thats a doubler in winding ratios already. Actually I would be happy with 250v.
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Old 30th April 2013, 09:11 AM   #10
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Did the experiment today - it works ! surprisingly well !

355 V @ 225 mA / 80 W when hooked up to the 12V AC rail

and regulation is not poor either, 366v @ 150mA, 380v @ 75mA

other voltages are also possible, 250v on the 5V rail, 200v on the 3.3v rail

I'd call that a success ...

Another good thing: the B+ can be floating and who says I cannot hook up 4 transformers for the 4 floating PSs for a classic circlotron
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