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Changing output voltage of a commercial PSU, such as ATX PSU or a XBOX PSU.

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David_

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Hello.

I have a 12V @ 16,5A XBOX 360 power supply(but this question do concern any commersial PSU) but I need at least 24V output.

I know that ATX power supplies often can be adjusted by locating th feedback resistive divider and changing its values but I want to pose you a question.

Lets say that I open up my XBOX 360 PSU, where do I begin looking in order to determine if it can handle a doubling of output voltage?

Do any of you know theoretical if a ATX PC PSU can be trimmed from 12V to 24V.

Regarding the XBOX PSU first I guess I should try and determine the topology used, then if we think of the transformer, it shouldn't care about 12V/24V output voltage should it?

Of course I need to change or at least check all output caps and such so they don't break down but can you foresee any more subtle pitfall in an endeavour like this?

I will open the XBOX PSU later tonight and then come back and tell you about what I found.

Regards
 

Prototyp_V1.0

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My guess is that it might be possible to tweak the voltage up - but it might not be capable to keep the voltage under load. Also be sure that there isn't components that is rated for lower voltage than you tries to get out of the power supply.

Warning: Most probably there is safety circuits - but don't trust that. It might start to smoke/burn if the output tranistors saturates.
 

David_

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Okey, that might actually work for the intended usage.

Its for a inductor core saturation device and the power supply is really only meant to charge up the big bulk capacitor in between the activation of a MOSFET that allows the current through the inductor to flow.

But why would that be, the load problem you mentioned?

I thought that it should be pretty straight forward to double the voltage while halving the load current but what I don't know could create and fill a hole new universe.

I have been wanting to ask you guys/galls about ATX power supplies, a ATX power supplies have in one variation or another a 3,3V rail, 5V rail and 12V rail. In a 300W unit I have at home the 3,3V & 5V rails seems to gobble up half the maximum power while the other half goes to the 12V rail(if maxed out).
I have jet to look closr at such a device but I have been thinking that maybe just maybe the construction of such a ATX PSU could allow you to scrap the secondary side 3,3V, 5V and 12V rails and build your own for example 24V @ 300W rail. Or make a 12V rail that can utilize the full 300W...

Does any one know if I am simply day dreaming and what I think might be possible is really impossible, or if it could be done. the point is ti use as much of the original PSU as much and not demanding to fiddle around with the transformer and any such major design intervention.
That might sound strange since I am talking about scraping pretty much the hole secondary side but...

Regards
 

c_mitra

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I believe it should be possible. Yes, 5V and 3.3V take away 50% of the capacity. Theoretically, you can combine the 3.3, 5 and 12V windings in series and adjust the feedback and you should be able to get 24V at 300W. (Perhaps not: it may stop above 150-200W)

The few I have looked at are so poorly done (constructed) that if you take out one component, you will not be able to put it back (the foil comes off the PCB and it is single side PCB). I have tried to salvage the transformer and the bobbin several times (no success: if you know some way, please tell me).

The secondary side is rather simple. There is a power good line you must defeat that (the orange wire, I think). It is the primary side that is dirty.
 

Easy peasy

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At 24V there will probably be an over-voltage circuit that will kick in, the excess stress on the output rectifying device will be close to killing it, and if you draw the same o/p current you will be trying to draw double the design o/p watts, so the thing will trip on over-temp if you are lucky (just plain BANG if unlucky), the output caps may not be rated for 24V either...

- - - Updated - - -

2 x ATX psu's with the 12V connected in series will give you 24V at design o/p current...
 

c_mitra

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At 24V there will probably be an over-voltage circuit that will kick in, the excess stress on the output rectifying device will be close to killing it, and if you draw the same o/p current you will be trying to draw double the design o/p watts, so the thing will trip on over-temp if you are lucky (just plain BANG if unlucky), the output caps may not be rated for 24V either...

2 x ATX psu's with the 12V connected in series will give you 24V at design o/p current...
I am not so pessimistic. If you just cut off the 3.3 and 5V circuits, you are unlikely to exceed the designed power rating for the transformer.

The output rectifiers do not see the voltage, they only feel the current. The current limit will not be exceeded. The reverse voltage ratings will be 100s of volts (approx).

I doubt the ATX outputs are floating; you cannot connect them in series.

I am attaching an ATX schematics (downloaded from the internet, where else) http://obrazki.elektroda.pl/9000351900_1456131551.png

It is messy but not undecipherable. The original is here: http://www.smpspowersupply.com/ATX_power_supply_schematic.pdf
 

FvM

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You can either refer to general SMPS properties or analyze the specific power supply circuit. 10% voltage variation should be easily possibly, probably 20 %. +100% would be excluded both by the transformer windings ratio and component ratings.
 

Easy peasy

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If you just cut off the 3.3 and 5V circuits, you are unlikely to exceed the designed power rating for the transformer.
ooh, this is good, so suddenly we magically make the 12V winding wires bigger to handle the extra current?

The output rectifiers do not see the voltage, they only feel the current
ooh, another good one, so they do not have to block any voltage huh? how hot do they feel when they "feel" twice the current? twice as hot?

The reverse voltage ratings will be 100s of volts (approx).
ooh, you are so knowledgeable (??) why do you even need to post on here in the 1st place? The diode shown in the schematic is only 45V? the 12V is coupled to the 5V o/p making life a little tricky for running the +12V only...

I doubt the ATX outputs are floating; you cannot connect them in series.
Ooh you doubt? In other words you don't know..., many people have... how would you test for this? the transformer providing complete isolation is a bit of a clue here...

Welcome to the Homer Simpson school of ATX power supply confabulation....

- - - Updated - - -

Oh yes, the 12V o/p caps are 16V as per the schematic, so bang straight away for 24V out, as FvM says the turns ratio would not allow 24V anyhow except at a few watts...
 

David_

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I have spent a few hours dissembling the XBOX PSU only to then discover something that I am hopeful about, look at this picture taken from a random ATX PSU I had laying around:
ATX_10%.jpg

I maybe able to use those two 12V rails... Or not, now that I am writing I realize that the are probably not isolated, why would them be?

Regards
 

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