PC Power Supply Hacking - 50V Output / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

PC Power Supply Hacking - 50V Output

1 2 3 4 next last

Move Thread

LAN_403

thedatastream

Tue Jan 09 2007, 04:33PM

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

Steve Conner

Tue Jan 09 2007, 05:12PM

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

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.

Marko

Tue Jan 09 2007, 05:13PM

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

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.

Dr. Dark Current

Tue Jan 09 2007, 06:18PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

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?

Marko

Tue Jan 09 2007, 07:22PM

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

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?

Steve Conner

Tue Jan 09 2007, 10:11PM

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

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.

Marko

Tue Jan 09 2007, 10:53PM

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

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.

...

Tue Jan 09 2007, 11:13PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

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 ;)

thedatastream

Wed Jan 10 2007, 08:37AM

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

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:

I've got two that were donated for free
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

Marko

Wed Jan 10 2007, 12:56PM

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

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.

1 2 3 4 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob