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4hv.org :: Forums :: General Science and Electronics
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PC Power Supply Hacking - 50V Output

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thedatastream
Tue Jan 09 2007, 04:33PM Print View
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Posts: 329
EDIT: change thread title to reflect performance achieved.

I want to bodge an AT PC power supply to have a variable output voltage up to 30V (don't really care where the bottom end is). This is for powering my Titch SSTC.

Assuming that the AT has some similarities with this ATX supply and this AT supply in terms of the output and control stage then I should be able to find my way around it. Obvious differences are that there is no 5V standby and no 3V3 supply on the AT.

I intend to

  • Disconnect all rails except the 12V from the transformer
  • Uprate the capacitor and diode on the output
  • Disconnect overvoltage circuit (if any)
  • Locate and alter the feedback components to get maximum duty cycle on the transformer input (with suitable load on output) to figure out how many turns I need to add to the transformer to get my maximum output voltage
  • Add appropriate number of turns
  • Add variable resistor in feedback circuit to adjust output

Things to be concerned about
  • Stability could be affected
  • Saturation of transformer needs to be avoided
  • Fitting more turns ont already crowded transformer could be difficult - perhaps remove existing secondary if possible?

Any suggestions or tips most welcome, or if there is anything obvious that I have overlooked then please let me know.

James
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Steve Conner
Tue Jan 09 2007, 05:12PM
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Hi James

The 12V rail on these things comes from a centre tapped winding rectified by a two-diode full wave rectifier. (Usually in a TO220 package on the output side heatsink: the TO247 is the 5V rectifier.) If you added another two diodes to make a bridge rectifier, got rid of all the 5V rectifier bits, and disconnected the centre tap then the same winding would want to output 24V.

**edit** sorry, that is the standard circuit for an ATX, I can't remember how the AT ones worked. There were all different circuits.

There's a thread on the old board that documents how I "overclocked" a 5V SMPS to 13.8V with some help from Richie Burnett. This might give you some pointers on how to get started.
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Marko
Tue Jan 09 2007, 05:13PM
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Wee. Tradition continues...

Old threads are about ATX modding, though; AT's are almost extinct today.. I don't remember those electronics anymore


- woops, steve already got it.

I may be wrong, but I think that highest voltage that can be 'hacked' from single 12V rail was some 25V with ATX. I don't know if AT will get any better.
-5V could probably be used but windings are almost surely too tiny..


About the transformers, I found it to be a very frustrating thing, unless you are very lucky.

Cores are stuck to their forms with some hardcore epoxy and is practically impossible to get apart in one piece.
Mess isn't closely worth even buying a new transformer.

Best thing I can think off is using a real fullwave rectifier t double the output voltage at start.











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Dr. Dark Current
Tue Jan 09 2007, 06:18PM
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If I'm not mistaken, the 12V output from the xfmr also supplies power to the pwm chip (494), if you up this voltage to 30V, isn't it going to x-plod in your face?
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Marko
Tue Jan 09 2007, 07:22PM
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jmartis wrote ...

If I'm not mistaken, the 12V output from the xfmr also supplies power to the pwm chip (494), if you up this voltage to 30V, isn't it going to x-plod in your face?


494 works well up to 40V, LM393 and 339 up to 36 volts, and judging atx already acheiving to close voltages I don't think that would be a problem.


I'm although, confused with one thing; how TL hets it's startup power?
It looks like transformer T2 does it, but in a rather wierd way (in series with main transformer?) Is thew supply ''kick-start'', or is that transformer actually same core as gate drive transformer? Steve?
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Steve Conner
Tue Jan 09 2007, 10:11PM
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Hi Firkragg

In the ATX power supply schem posted by thedatastream, the 5V standby supply is a separate converter that runs all the time the PC is plugged in. It supplies just enough power to the TL494 to get it started. Once the main power supply is turned on, the +12V output powers the TL494 and driver transistors, through the diode marked D in the schem.

I have no idea how the AT schematic he posted works. I bet it has something to do with the mysterious point A. T2 in that circuit is a current transformer to operate the current limiter.

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Marko
Tue Jan 09 2007, 10:53PM
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I have no idea how the AT schematic he posted works. I bet it has something to do with the mysterious point A. T2 in that circuit is a current transformer to operate the current limiter.


AT thingie was what I asked about.. never mind.

Datastream: ATX'-es are dirt cheap today, and if this thing starts sucking, it may be just easier to buy a new 350W supply for $10 and mod it

You can also series them for more power, but low voltage grounds of all except one supply must be floated.






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...
Tue Jan 09 2007, 11:13PM
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I would recomend buying an ATX power supply, and modding that do do whatever to 18v, then series that with the 12v on the existing power supply ;)
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thedatastream
Wed Jan 10 2007, 08:37AM
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Hi folks, thanks for all the responses

I wonder if the AT engages in a bit of self oscillation to get enough energy through the winding to start up the TL494 (although a quick glance at the circuit doesn't reveal an obvious method).

Reasons for sticking with using an AT supply rather than buying an ATX are:
  1. I've got two that were donated for free
  2. They are a bit less complicated inside so I've got more chance of finding the correct components easily


Also, I'd prefer not to series the supplies, as I want a single ATX shaped box as the base of the coil - two boxes would be out of proportion and would mess up me feng shui like.

Putting the cores in a pan of boiling water, or perhas in the vacuum oven at work should allow me to separate them OK - should I need to that is. If the secondary is on the outside, I may be able to get away with just unwrapping the layers without dissasembling the core.

UVLO on the TL494 is 7V max, so if I make the bottom end of the adjust range = 8V and stick a heatsunk 7812 regulator between my adjustable output and the TL494 supply then that should be good enough. Too much voltage will affect the current being driven into the main bipolar transistor bases - I don't want to fry the gate drive circuit components.

I'll open up the power supply this lunchtime, have a look inside and let you know how I get on.

Rgds,
James
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Marko
Wed Jan 10 2007, 12:56PM
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Putting the cores in a pan of boiling water, or perhas in the vacuum oven at work should allow me to separate them OK - should I need to that is. If the secondary is on the outside, I may be able to get away with just unwrapping the layers without dissasembling the core.


There is apsolutely no chance you are going to boil the cores off.
The temperature needed for epoxy to melt was some 400C or so, with hot air gun.
I completely charred the plastic form, and core ended broken in several places due to massive overheating.



To get 30V, as already suggested, you don't need to do anything with the transformer;
using a full 4-diode bridge will give you 24V output wich I'm pretty sure for can be boosted to 30V:

You can then use one branch to provide power to supply's electronics without frying it.

This way you could get 50V out of most ATX supplies; you are probably the first one to do the same with AT.

From the moddng side, AT's arent any different than ATX, complexity isn't a factor there.
ATX mods are already done and verified many times.

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thedatastream
Wed Jan 10 2007, 01:55PM
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Posts: 329
Dur... should read more carefully - a full bridge rectifier is ideal. I shall implement forthwith (obviously not using 1N4148s waits for bang)



I hear what you are saying about the epoxy, sounds like I won't have to go down that route now.

Progress so far has been good, I've butchered the PSU and removed almost everything on the secondary power side except the coupled choke and the 12V inductor. It's nice to see that the circuit is almost identical to the AT schematic I linked to in my first post. This means it has been fairly easy to track down the relevant components.

PSU now works with 12V in isolation and no-preload necessary. I've put a 7805 from the 12V to the 5V rail as it powers some of the ancilliary circuitry.

BEFORE


AFTER


REMOVED PARTS


To do
  • Upgrade output diodes to withstand correct voltage and current in fullbridge configuration
  • Remove feedback resistor from 5V line
  • Insert variable resistor into 12V feedback line
  • Test

Thanks again for the help so far
James
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Avalanche
Wed Jan 10 2007, 03:58PM
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Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 829
From my experience separating cores, I've found that there are two different kinds of resin used in ferrite transformers - the white stuff (not a chance, too high a melting point) and then some transformers use a kind of opaque brown resin, that is very brittle and chips off. That kind of resin melts easily into a liquid if you drop it into a pan of boiling water, I separated a transformer from an old monitor like this. Dropped it into a pan and was surprised when 2 minutes later the transformer just fell apart!
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thedatastream
Fri Jan 12 2007, 09:55AM
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Posts: 329
Steve Conner wrote ...
If you added another two diodes to make a bridge rectifier, got rid of all the 5V rectifier bits, and disconnected the centre tap then the same winding would want to output 24V.

The key to this sentence is "disconnect the centre tap", otherwise the windings get shorted out to ground when the bottom winding goes negative - see attached image.



The gain over the conventional half wave rectified output appears to be high at light loads, factor of x2.8 at 10W, although I would expect this to come down as the load increases.

More hacking away this lunchtime, but it's looking pretty good

Rgds all,
James
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Steve Ward
Sat Jan 13 2007, 02:08AM
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Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

The gain over the conventional half wave rectified output appears to be high at light loads, factor of x2.8 at 10W, although I would expect this to come down as the load increases.


Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.

I just recently built another switch mode supply using the old TL494. Its a constant current source for charging NiMH batteries (our robot uses a moster 50x sub-C cell pack). I was happy to see it pumping 38V at 7A with ease. It was actually a rather easy project, the only set back was when i used too small of wire on my ferrite core, and it got rather toasty! The 10awg litz fixed that up .
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thedatastream
Sat Jan 13 2007, 10:49AM
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Location: Yorkshire!
Posts: 329
Steve Ward wrote ...
Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.

Yes, there is feedback from the output rail. The power supply was operating in burst mode when I made this measurement (buzzing away merrily) so when it is operating properly, this ratio should come down a bit.

Success! Yesterday evening yielded 1.5A @ 48V = 72W into an electronic load - the maximum voltage output. I would hope to be able to get 3A out at this voltage = 144W with the fan cooling running in the chasis (originally a 200W supply). A 100k pot in series with a 1k resistor in the feedback gives me the full adjustment range from 3V to 48V.

However, I still need a regulated 12V supply for the SSTC control electronics, so I've had to install a NatSemi simple switcher to get a 12V 3A rail from the variable output. As its a buck converter, it will only regulate when the input is above 12V. Since the main output can vary lower than this, some kind of "12V good" signal will be required - a zener and bipolar transistor should be good enough.

The intention now is to mount the control cct on the inside of the lid and mount the H-bridge and coil on the top of the case.

Thanks once again for all the assistance in helping me achieve this goal, I will do a full write up on my website before long.

James
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Marko
Sat Jan 13 2007, 10:54AM
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Posts: 3145
However, I still need a regulated 12V supply for the SSTC control electronics, so I've had to install a NatSemi simple switcher to get a 12V 3A rail from the variable output. As its a buck converter, it will only regulate when the input is above 12V. Since the main output can vary lower than this, some kind of "12V good" signal will be required - a zener and bipolar transistor should be good enough.


You can use a buck-boost converter on -5V rail so it will correct it to positive, any voltage you want. I used buck boost in power supply for my little class E coil with good results.

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thedatastream
Sat Jan 13 2007, 06:01PM
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A buck boost, specifically a SEPIC regulator is not a bad idea. The main reason I've gone for a buck is that I already had the circuit built up.

What controller did you use in your application? Obviously something that would run at 5V... I can't use a UC3843 as it has a max turn on voltage of 9V.

James
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Marko
Sat Jan 13 2007, 08:14PM
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Location: Zadar, Croatia
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I used LM2575 since I didn't need anything under 7V, but there are tons of usable regulators around:

I often dig my regulators out of old motherboards and computer trash, and they can be sampled too.

Good luck...
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thedatastream
Sun Jan 14 2007, 10:29AM
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Firkragg wrote ...

I used LM2575 since I didn't need anything under 7V, but there are tons of usable regulators around:
I often dig my regulators out of old motherboards and computer trash, and they can be sampled too.

Cool thanks, I shall investigate my options.

Incidentally, I've come across some more AT PSU schematics on this page - click on the power supply link.

James
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thedatastream
Sat Jan 20 2007, 02:21PM
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Webpage detailing the modifcation is now up

I'm looking at using a NatSemi LM3578 in a SEPIC configuration to provide my 12V rail.

James
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uzzors2k
Sat Jan 20 2007, 06:08PM
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Good job, and well documented! I'm tempted to give this a try again, seeing you were able to keep the short circuit protection intact. Too bad you didn't have an ATX, as modding the seperate 5v standby to 12v would be no problem.
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Dr. Shark
Wed Jan 24 2007, 12:03PM
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Nice work! I have also spend considerable time hacking AT supplys, sorry I did not jump in any earlier. Well, as you have realised AT and ATX are almost identical other than the "extra stuff" in ATX.

I've still got one thing to add though: From my experience it is possible and feasible to rewind the power transformer. Breaking the core is a non-issue since most people have about 10 dead PSUs to pick replacements from, and once the core is open, rewinding is straighforward. Annoingly the primary is the outer layer, so it needs to be removed first, but it is also fairly short. After removing the 5V winding, there is a lot of space on the core for longer, thicker wire, so it worth doing.
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Dr. Shark
Sun Jan 28 2007, 06:09PM
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Whoops, I did it again!


Not much of a Hack really since all I did was take out all parts of the secondary side except for the 12V rail and rewind the power transformer. After a few seconds in boiling water it comes apart nicely. I removed the 5V windings and doubled the length of the 12V wire. I did not mess with the rectifiers and the fan is running off the full 28.7V - this should provide plenty of cooling
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thedatastream
Mon Jan 29 2007, 08:00AM
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Nicely done joe, you could always use a 7812 regulator to drive the fan...

BTW, I'm leaving the LM3578 based SEPIC 12V power supply for a later date, I've blown two chips due to overvoltage on the switch. I'm now going to have to re-think my psu design which will be detailed in a different thread.

Rgds
James
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Marko
Sat Feb 03 2007, 09:43PM
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Hi joe and datastream

BTW, I'm leaving the LM3578 based SEPIC 12V power supply for a later date, I've blown two chips due to overvoltage on the switch. I'm now going to have to re-think my psu design which will be detailed in a different thread.


Using a 40V converter obivously isn't going to work well on former 12V rail you boosted to 50V.

My idea, from upper post, was to use a 5V rail, or even -5V as simpler buck-boost converter. From your site, I figured you removed all dides and stuff from all outputs.

Could you revive the -5V rail to run a buck-boost from it?

In worst case, you can always use a small transformer to power your TL494, fan and coil electronics.

It's not elite like switching converter but is very sturdy and robust.



I've still got one thing to add though: From my experience it is possible and feasible to rewind the power transformer. Breaking the core is a non-issue since most people have about 10 dead PSUs to pick replacements from, and once the core is open, rewinding is straighforward. Annoingly the primary is the outer layer, so it needs to be removed first, but it is also fairly short. After removing the 5V winding, there is a lot of space on the core for longer, thicker wire, so it worth doing.


I boiled my transformers for about half an hour but none of them wanted to budge.

Hot air gun is only method I found to work, but it torches a hell out of ferrite and it's no wonder that it just falls apart after... simply not worth effort.

If I needed to wind a transformer, I would just get a large enough flyback core, or, if I need specific cores, order some from cws.










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thedatastream
Mon Feb 26 2007, 05:03PM
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I've changed the output rectification scheme as I found that two diodes can be used instead of four.

This grounding of one end of the transformer gets round the problem of trying to run the KA7500 control voltage from one of the transformer taps and having the ground return currents messed up causing the power supply to trip out at higher voltages.

It also gives me a 1/2 Vcc output fromt the old grounded centre tap of the transformer to run my SEPIC off for the control circuitry for my Titch tesla coil.

updates soon

James

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thedatastream
Thu Mar 01 2007, 10:46PM
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OK, the half wave rectification seems to have resolved a lot of the stability problems I had, but I now have a problem using this modified power supply to power my Titch SSTC.

I've tested Titch on a bench dc power supply (30V, 2A linear) with no issues but when using the modified PC supply, ramping the power up to more than what I estimate to be a maybe 10 watts (coil running interrupted at 50Hz, about 20% duty) the power supply cuts out in a similar fashion to when the output is shorted out unintentionally.

I've measured the output voltage - With 4 x 820uF, low impedance capacitors on the output, there is about 0.5V to 1V of ripple when the coil turns on. I haven't measured the ripple current, although I suspect that the pulsed currents are enough to trip whatever over current protection exists in the supply. I've tried decoupling the output with a 3300uF additional low ESR cap but to no effect.

To try:
* Increase decoupling and filtering using the above large capacitor and a big series inductor
* Find and either slug off the response or disable the over current protection circuit

Does anyone have any other suggestions of things to try?

Thanks
James
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Steve Conner
Fri Mar 02 2007, 10:37AM
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Yes, I'm not convinced the halfwave rectification is a good idea. It will put a high average DC current through the transformer and saturate it, which is probably what's tripping your overcurrent protection on the primary side.

The other hookup with the 4 diode full bridge is what higher voltage commercial SMPSs use. I just bought a 400W SMPS for an audio amp that delivers +/-80V, and it has the 4 diode setup. The centre tap of the transformer is grounded, and there are two output filter inductors, one on each output of the bridge.

If you use the 4 diodes and apply a filter inductor to the centre tap (ie replace D5 in your before schematic with an inductor) it should give you half the main supply voltage. You also need a filter inductor on the main +50v rail, but I assumed you left the original one in. Hooking the bridge output straight to a capacitor will cause high peak currents and trouble with the regulation.
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Marko
Fri Mar 02 2007, 12:40PM
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Yes, I'm not convinced the halfwave rectification is a good idea. It will put a high average DC current through the transformer and saturate it, which is probably what's tripping your overcurrent protection on the primary side.


Lots of forward converters use halfwave rectification (with a free wheeling diode) and seem to work fine.

If only a single switch is used then there must be a tetriary winding with a diode wich rectifies the stored energy from the core back to supply rails and prevents saturation.

Computer PSU is also basically a forward converter except it uses a halfbridge, and emmiter-collector diodes are supposed to rectify the energy from 'dead' cycle and prevent saturation much in same way.

It may be misuse of halfbridge and it's qualities, max possible output power is reduced but it looks like it should work. Steve?

PS. just one more question, for Steve: in all time, only thing I haven't completely figured out yet is that yellow-toroidal output inductor thingie. It couples all rails strongly together but itself is very lossy, so it could have no real benefit in forward operation (real forward inductors are wound on ferrite rods and are in series with it).
?

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Steve Conner
Fri Mar 02 2007, 01:17PM
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Firkragg, you could be right. I'm confused now The Astec SMPS I modified to power my ham rig was exactly that kind, it used a diagonal half-bridge (a la coilgun) feeding a transformer that fed a half-wave rectifier with freewheel diode identical to James's. I somehow assumed that its transformer would need an airgap, but when I think about it more, I guess it doesn't.

*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.
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