Can anyone give a link to a site which covers modifying an ATX PS 12 volt rail to 13.8v in reasonably simple terms? I Googled for a couple of hours last night and found answers from the bizarre to ridiculous. The +12v output is rated at 20 amps and I only need 8 max.
I'm pretty sure that RadCom did an article on doing this not so long back. With licenses now available on the backs of cornflake packets, I guess it would have to be pretty simple. I used to keep all my RadComs, but took to chucking them out over the last couple of years. I'll have a look anyway, as I still tend to keep them until there's a pile, then lob 'em. If it wasn't in RadCom, it was in Elektor.
Jacking these PSUs up a couple of volts is not usually that hard. There's often an 'adjustable zener' IC on the primary side, which is fed from the secondary side via an opto coupler. This zener serves as the reference to set the PWM for the desired output voltage. Altering that voltage is just a case of changing one resistor to alter the reference voltage.
Arfa
Another resource.
The above schematic could be frigged about with by selecting a new value for R25.
Typical 13.8v project.
The one I have to hand is a bit more modern and uses an all in one SG6105 supervisor IC.
There are reference and feedback inputs at pins 11,12,13 &14 on that IC, and if you examine the internal schematics, you will see that behind those pins are the 'adjustable zeners' which are often external devices on PSUs which use a slightly less sophisticated management IC. I expect that if you played with the resistors attached to those pins, you could alter the output voltage of the supply. One thing to note is that this IC also has under and over-voltage lockouts, and if the feature has been implemented on your PSU, then cranking the 12v up to 13.8v, may result in an unwanted shutdown, so you might need to disable this feature.
Just as a matter of interest, what are you hoping to run from this PSU ? Requiring 13.8v would suggest that it is something designed for mobile use. However, most such devices will happily run from 12v, as this may be all the vehicle voltage that's available, if the engine isn't running.
Arfa
It is indeed. However I'd like the correct nominal voltage for prolonged soak testing.
In message , "Dave Plowman (News)" writes
Ahh, grasshopper, to modify the 12v rail first you must find the voltage reference for the 5 v or 3.3 v rail, your journey will take you into the realms of voltage dividers and the TL431.
Basically, the PSU will regulate the 5v or 3.3v rail and all the other rails are set by the transformer ratios. You need to alter the voltage divider for the 5v or 3.3v rail and all the other rails will change proportionally (with the exception of the +5v SB as this is not generated by the main switcher and maybe the -12 and -5v as these are often provided by 79xx linear regulators). Don't forget, you may need to add some load to the 5v and/or 3.3v rails to make the PSU stable and don't mess around with it unless you're sure the main reservoir capacitors are discharged.
Ain't got one of those.
Without trying to be negative about the project, ATX power supplies are 'jelly bean' items. The only common points between different brands and models are the case dimensions, the pinouts of the output leads, and the AC in connector.
You are going to have to continue what you have started. Analyse the circuit, locate the voltage regulator and over-voltage protection and adjust the output voltage by changing the appropriate resistors.
Depending on the availability of a suitable power supply, your electronics skills, and time available, you might be better off taking a simpler approach. This supply would be much easier to work on.
PlainBill
On Tue, 04 Aug 2009 08:23:07 +0100, "Dave Plowman (News)" put finger to keyboard and composed:
I've done this with an AT PSU. All I did was to modify the voltage sensing resistors at the feedback pin of the error amp. In my case the PSU regulated by sensing a weighted average of the +5V and +12V supplies, so I removed the +5V sense resistor and reprogrammed the
+12V resistive divider. You may also have to modify the overvoltage sense circuitry, in my case a zener diode. You could also remove the components associated with the other rails and then fudge the POK circuit. I also added an aluminium clad dummy load resistor and mounted it in the fan's air stream.- Franc Zabkar
I've certainly found that out. ;-)
I'm not going to waste much time on it - given the cost of new units that do what I want.
But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a lot, if I decide to spend some cash.
Yes it has, Dave. With that particular supervisory IC, it's behind the reference and feedback pins, and not an external discrete device, as I previously explained ... If you're not understanding the principles of how these things regulate, then I don't think you are going to get too far with this project.
Arfa
The most popular official voltage is apparently 13.5VDC, not 13.6VDC.
I bought a pair of these, for bipolar stepper motor power:
The output is tweakable from 12V to 15V, which is handy for also using it as a battery charger. It belches some HF RFI but not too horrible. Yeah, I know it delivers more amps than you need, but the extra power might be useful for something else.
On Tue, 04 Aug 2009 13:46:35 +0100, "Dave Plowman (News)" put finger to keyboard and composed:
Datasheet for SG6105:
There is an application circuit on page 8.
It seems to me that your +13.8V modification would be very easy.
Just remove the 5V sense resistor from the SG6105's IN pin. Then reprogram the 12V and SS resistors so that the voltage at the IN pin is equal to +2.5V when the 12V rail is at +13.8V.
Gnd o-- 2K5 --o-- 11K3 --o +13.8V | | IN
Remove the connections to the V33, V5, and V12 over/undervoltage sense pins and then construct a potential divider (see below) with the
+13.8V output rail at the top end, and taps at +12V, +5V, and +3.3V connected to each of the sense pins. In this way you would be monitoring only the +13.8V rail and faking the others.Gnd o-- 3K3 --o-- 1K7 --o-- 7K --o-- 1K8 --o +13.8V | | | | | | V33 V5 V12
- Franc Zabkar
On Wed, 5 Aug 2009 01:37:49 +0100, "Arfa Daily" put finger to keyboard and composed:
AISI, the main rails are regulated via the error amp at pins 16, 17, and 18. The OP would need to remove/modify the resistors at the IN pin.
The TL431's at the FB2 and FB1 pins regulate the +3.3V and +5VSB supplies. I don't believe they need to be touched. I suspect, though, that the +3.3V regulator will be working flat out to bring the output down, if indeed it can.
- Franc Zabkar
And it's a TL431? If you mean a shunt regulator that would be ok.
I doubt I'm alone in that. Try Googling on 'modify ATX 13.8v' and see just how many different approaches there are. Which is why I asked here.
13.8 is the usual voltage on a car with the engine running and things stabilised. But of course most equipment will tolerate a wide range and still work.
Google gave this hit:-
Which says to use 1N4448 in the 3.3, 5 and 12v error sensing rails, which I did. No difference. Tried changing the resistor feeding 'IN' with a pot and altering that from the fixed value cause shut down.
I like 13.7.
Right - having read the spec more carefully it does indeed state it contains two of these. So please accept my apologies.
Well, if I were an expert on them I'd hardly need to ask here. It's not for some life support mechanism, you know.
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