DISCLAIMER: This is all still quite experimental. Even if it wasn't, modifying power supplies that work with AC, whether they're switchers or otherwise, is serious business. It's quite easy to blow something up or seriously injure or even kill yourself by screwing around with these things. I take no responsibility for any damage or injury that happens to yourself or others as a result of following these instructions.
I've been really ticked about the modded computer supplies that keep showing up that just don't have a high enough 12V for the games they're supposed to go into, so today I'm experimenting to take matters into my own hands. I just successfully modified a mini-switcher to have adjustable voltages. The hardest part is finding the feedback resistor; once you have it, you can just put a pot inline with it and boom, adjustable switcher.
The catch is, every switcher is different. Some are self-regulating; some have a regulator IC in the way. So it's not as simple as "trace X components back from the optocoupler"; you need to put some thought into it. Even worse, some REALLY bad designs don't regulate at all; you can't mod those as detailed below, because there's no feedback circuit to overdrive.
Here's some preliminary instructions:
1. Unplug it, take it out of the machine, and take the thing apart. (If you didn't already know to do that much, you REALLY REALLY shouldn't be trying this.) You need to get far enough that you can easily see both sides of the PCB.
2. Find the input/output divide. For safety reasons, every switcher is divided into the input section, that produces high-voltage high-frequency AC, and the output section, that filters this AC into DC voltages. Sometimes the divide between the two is clearly marked with a big white line; other times not so much. Nonetheless it's easy to spot because there are necessarily absolutely no traces crossing it.
3. Find the optocoupler. They're easy to spot because they'll be the one and only IC that crosses the input/output divide, doubly so because they're usually in a distinctive 4-pin DIP package.
4. Now, trace the circuit from the optocoupler back across the output side. You'll either hit an IC, or come across some resistors that bridge directly to the output rails.
5a. If you wind up at resistors going directly to one or more output rails, without an IC in the middle, it's dead simple. Lift one leg of one of these resistors, and solder in a potentiometer in series with it. (I recommend 5K or less.) You can now adjust the voltage with the pot.
5b. If you hit an IC, It's (usually?) still a matter of lifting one leg of a resistor and putting a pot in series with it, but exactly which resistor is, well, complicated. You have to find the feedback input pin of the IC and trace that back. I'm still working on more concrete instructions on this.
Keep in mind that this usually moves ALL voltages and they probably won't change proportionately. You can easily wind up with 5V@5.9 and 12V@12.2 with this mod.
If you need to move the voltage DOWN, you can replace the resistor you lifted with something a little smaller. I wouldn't go more than 10% though; at some point, you'll fry the feedback circuit.
Here's how it works: The resistor we targeted is what provides the sense voltage back to the feedback circuit. If you move its resistance down, the sense voltage goes UP, and the input side dials down to compensate, lowering voltages. Likewise, if you move the resistance up, the sense voltage goes down, and the input side dials up to compensate.
I've been really ticked about the modded computer supplies that keep showing up that just don't have a high enough 12V for the games they're supposed to go into, so today I'm experimenting to take matters into my own hands. I just successfully modified a mini-switcher to have adjustable voltages. The hardest part is finding the feedback resistor; once you have it, you can just put a pot inline with it and boom, adjustable switcher.
The catch is, every switcher is different. Some are self-regulating; some have a regulator IC in the way. So it's not as simple as "trace X components back from the optocoupler"; you need to put some thought into it. Even worse, some REALLY bad designs don't regulate at all; you can't mod those as detailed below, because there's no feedback circuit to overdrive.
Here's some preliminary instructions:
1. Unplug it, take it out of the machine, and take the thing apart. (If you didn't already know to do that much, you REALLY REALLY shouldn't be trying this.) You need to get far enough that you can easily see both sides of the PCB.
2. Find the input/output divide. For safety reasons, every switcher is divided into the input section, that produces high-voltage high-frequency AC, and the output section, that filters this AC into DC voltages. Sometimes the divide between the two is clearly marked with a big white line; other times not so much. Nonetheless it's easy to spot because there are necessarily absolutely no traces crossing it.
3. Find the optocoupler. They're easy to spot because they'll be the one and only IC that crosses the input/output divide, doubly so because they're usually in a distinctive 4-pin DIP package.
4. Now, trace the circuit from the optocoupler back across the output side. You'll either hit an IC, or come across some resistors that bridge directly to the output rails.
5a. If you wind up at resistors going directly to one or more output rails, without an IC in the middle, it's dead simple. Lift one leg of one of these resistors, and solder in a potentiometer in series with it. (I recommend 5K or less.) You can now adjust the voltage with the pot.
5b. If you hit an IC, It's (usually?) still a matter of lifting one leg of a resistor and putting a pot in series with it, but exactly which resistor is, well, complicated. You have to find the feedback input pin of the IC and trace that back. I'm still working on more concrete instructions on this.
Keep in mind that this usually moves ALL voltages and they probably won't change proportionately. You can easily wind up with 5V@5.9 and 12V@12.2 with this mod.
If you need to move the voltage DOWN, you can replace the resistor you lifted with something a little smaller. I wouldn't go more than 10% though; at some point, you'll fry the feedback circuit.
Here's how it works: The resistor we targeted is what provides the sense voltage back to the feedback circuit. If you move its resistance down, the sense voltage goes UP, and the input side dials down to compensate, lowering voltages. Likewise, if you move the resistance up, the sense voltage goes down, and the input side dials up to compensate.