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Old 06-16-2020, 10:23 AM   #1
Dannyx
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Red face Modifying power brick output voltage question

Good day folks. With the joint I work in starving for parts, I use everything at my disposal and stock everything as much as I can. Take this situation: not a single 12v brick around ! Instead, I scrounged up two HP printer bricks. They're rated at 36v 2A, which is useless for anything other than powering the printers they're designed for. What do we do ? Hack them, of course. I did this in the past successfully on a Vaio adapter, with the help of some of our wonderful chaps here, but this one gave me a bit of trouble and I was wondering what I'm doing wrong. I'm aiming for a large drop here as well: 12v out of a 36v adapter, unlike the previous one where I only dropped it from 16v to 14v, so I'm not even entirely sure this is possible. It's not so much about whether I need it or not (that surely doesn't help ), but mostly about learning how such a circuit works.

It was the nicest and most wonderful thing to take apart: 3 screws holding the two halves together ! Pure bliss - if only all adapters were put together like this

I had a look around the board and identified the AS431 reference IC, responsible for driving the single optocoupler. I drew a schematic of the feedback loop on the primary, though please note that the values you see are the actual numbers written on the SMD resistors - I know I should've marked them as they are on the board, but it's just a personal choice when drawing schematics involving SMDs, because my brain automatically processes the first thing it sees, rather than the part number on the silk screen.

Based on the schematic, which we assume to be correct at this point, the resistor I chose to modify was R49 (originally a 01D resistor, meaning 100kOhms). Since it goes between the output V+ and the REF pin of the AS431, lowering this value should also lower the output of the brick, since the transistor inside the AS431 conducts more, turning on the LED in the optocoupler nice and bright, which prompts the primary to decrease the frequency.

To check this, I started off with a 20k trimmer pot turned all the way "up" in place of the resistor: 20k instead of 100k did indeed lower my output, but now it was TOO low - 8v. I eventually found a 31kOhm resistor in place of the trimmer, which brought my output closer to what I was expecting: 13.5v.....sort of. See the trouble I'm having is that it's not stable for one thing (it fluctuates between 13v and 13.8v, so regulation is off) and it also lacks current: the slightest load immediately kills it and it goes all over the place. The green power LED I marked as P.LED in my doodle also pulses regularly, like twice every second and I think the actually issue is here: I gave that LED no second thought, but then had a closer look and noticed it's what actually powers the optocoupler, if I'm not mistaken. My theory is that with the circuit tuned for 36v, with the output now at 12v, the 129 resistor in series with the LED (in reality R51 on the board) is too low for the LED in the optocoupler to turn on, causing the primary to boost the output, but as this happens, the optocoupler suddenly turns on and the primary cuts back again, then the cycle repeats, hence why I'm getting this "semi-regulated" output - it doesn't overshoot, but it's not stable either. The LED ITSELF could be playing a part in this: with V+ at 12v, the drop across it is now too low

Solution: remove and jump the LED OR replace R51 with a lower value.....which should also help keep the LED on during operation, which would make the end result more professional. Any thoughts based on my doodles there ? Thank you.
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Old 06-16-2020, 10:58 AM   #2
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Default Re: Modifying power brick output voltage question

Usually going that far off, if the psu wasn't designed to go there, will result in instability or poor behavior. I'm still trying to get the characteristics of my 60VDC to 120VDC homemade boost PSU, it behaves poorly at 40V and I suspect my series dissipative regulator will not like it if I go to 80V input, at 60V it's pretty wasteful already.
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Old 06-16-2020, 11:06 AM   #3
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Default Re: Modifying power brick output voltage question

I am watching this post to see if you have any progress with type of mod
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These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

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Old 06-16-2020, 11:08 AM   #4
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Default Re: Modifying power brick output voltage question

I had some limited success: I dropped it down, but it's not stable, so I was curious if it's down to that LED somehow. Sure, it's a useless thing to do overall, since an off-the-shelf power brick is the way to go, but it's all for a bit of fun, so I too would like to learn more.
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Old 06-16-2020, 11:44 AM   #5
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Default Re: Modifying power brick output voltage question

Usually the amount of mods needed to change the voltage significantly is higher than the cost of buying another mass produced wall wart... Same reason why those "ATX PSU into bench PSU mod" end up requiring buying another psu module to install in it...
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Old 06-16-2020, 11:51 AM   #6
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Red face Re: Modifying power brick output voltage question

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Originally Posted by eccerr0r View Post
Usually the amount of mods needed to change the voltage significantly is higher than the cost of buying another mass produced wall wart... Same reason why those "ATX PSU into bench PSU mod" end up requiring buying another psu module to install in it...
Yeah, but in some cases it's just a simple resistor, though I'm guessing the expected voltage difference is the limiting factor in my case, unless someone has a clever idea, or I do some testing on my own.

The Vaio brick I mentioned (which is HERE BTW), had two op-amps as the feedback control, since I tried referring back to that thread, but it's the same thing: changing a resistor and hoping for the best....
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Old 06-16-2020, 01:35 PM   #7
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Red face Re: Modifying power brick output voltage question

CORRECTION: R51 reads 621, not 192 like I wrote on the piece of paper, so it's 620ohms. I'll start by replacing that with a trimmer set to 620 ohms, which I'll then slowly turn to lower the resistance to see if increasing the current across the optocoupler's LED improves regulation. I imagine worst case scenario would be burning out the LED in the optocoupler, followed by the output overshooting past the rating of the diodes and output caps and something going poof, at which point it's....
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Old 06-16-2020, 05:41 PM   #8
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Default Re: Modifying power brick output voltage question

Well again, you can usually change the voltage a few percent with no problem, but if you're trying to double or halve the voltage, unless there was a lot of tolerance in the design to begin with, you will end up with unstable output or worst case, magic smoke leaks.

The largest percentage I've hacked a psu is a 5.1V LG phone charger to about 8.5V to power my DMM that requires 6 "C" sized batteries (8.5V instead of 9V so that I have more of a margin of the 10V caps in the unit). Fortunately the design of this wall wart was lenient enough to do this.
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Old 06-17-2020, 01:48 AM   #9
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Default Re: Modifying power brick output voltage question

Ok, here's what I've been messing around with: I replaced that 620 ohm resistor (R51) between the cathode of the LED and the anode of the optocoupler with my trimmer set at 620ohms. I turned the power on and began lowering turning it to lower the value - no change: the output was still fluctuating and the LED didn't stay on even after turning it all the way down, so essentially creating a wire in place of R51, so that's not it. No funny side effects, but not functional either...

Then I jumped the LED and left the trimmer in the same position, in place of R51: no dice: no improvement whatsoever even when shorted....then the brick quit on me My probe accidentally slipped and touched the two pins of the optocoupler together - this killed the AS431 for some reason. I replaced it with a KIA431 and had another go, this time messing around with the two resistors going to GND, rather than the one going to V+. I WAS able to bring the output to 16v (as high as the trimmer would go in place of the 912 9.1kOhm resistor), but the same thing happened: no current at all: even a PC fan brought the output to its knees at around 7v, so it looks like massive jumps like these are impossible, at least not without massively modifying the primary as well, since I imagine the transformer itself is tuned to the output and can only go as high/low....

Last edited by Dannyx; 06-17-2020 at 02:37 AM..
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Old 06-17-2020, 05:49 AM   #10
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Default Re: Modifying power brick output voltage question

i would say unless you want to start unwinding the secondary of the transformer - forget it.
the pwm range of the control i.c. is unlikely to let you go down 70%
even if you did, the mark/space ratio would hit the output caps like a hammer - you would have to hugely up the uf and ripple current!
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Old 06-17-2020, 05:56 AM   #11
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Default Re: Modifying power brick output voltage question

Yeah, I gave up: I soldered back the original resistors to at least leave it as it was, so there's that...
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Old 06-17-2020, 07:07 AM   #12
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Default Re: Modifying power brick output voltage question

Are there any off shelf adjustable switching power supply that do not cost an arm and a leg to buy
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Old 06-17-2020, 07:11 AM   #13
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Default Re: Modifying power brick output voltage question

I have a adjustable switch power supply but I paid almost $100.00 for it works petty good but it has a 10 amp output but it adjustable current is not as fine as I would like so I do not use it very much

I thought about mod it but the inside of the face has to much electronics already there and it not very wide so there is not much room to any thing to it

Last edited by sam_sam_sam; 06-17-2020 at 07:14 AM..
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Old 06-17-2020, 07:20 AM   #14
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Default Re: Modifying power brick output voltage question

Are there any off shelf adjustable switching power supply that do not cost an arm and a leg to buy

I know you can buy power supply that have a adjustable voltage pot but those type of switching power supply have a limited how much you can adjust it

Could this type of power supply be mod it for this type of use
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Old 06-17-2020, 07:44 AM   #15
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Thumbs up Re: Modifying power brick output voltage question

Well "universal" laptop bricks are so-called because they're technically adjustable, but they only have fixed values - can't go in between them, though I'd imagine you wouldn't need 13v anywhere and either 12 or 15 would do....

Back at my old shop where we had virtually everything we needed, we had a handful of Hama brand adapters and I was very pleased by them - would definitely stick with this brand if I ever need an off-the shelf adjustable brick (not counting original ones for a particular brand, of course). Naturally, a service center shouldn't have to struggle when the client forgets to bring their adapter, so we got an universal one and after observing how well it plays with all kinds of brands, some of which can be "picky", we got another one and another one, plus all kinds of plugs for them off Ali, so laptop adapters were no longer an issue to us

A more "industrial" approach would be something like this MeanWell which they say can be adjusted across a decent margin, but depending on your budget, it could be pricey...not sure how the site displays in other regions, despite me always having it set to English (some translations suck), so you may need to adjust the language at the top-right....

Last edited by Dannyx; 06-17-2020 at 07:45 AM..
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Old 06-17-2020, 09:29 AM   #16
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Default Re: Modifying power brick output voltage question

Quote:
Originally Posted by Dannyx View Post
Well "universal" laptop bricks are so-called because they're technically adjustable, but they only have fixed values - can't go in between them
that's not always true,
i'v seen some with a rotary selector that has click-stops but inside it's just going onto a pot.
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Old 06-17-2020, 09:21 PM   #17
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Default Re: Modifying power brick output voltage question

Quote:
Originally Posted by stj View Post
even if you did, the mark/space ratio would hit the output caps like a hammer - you would have to hugely up the uf and ripple current!
^ This.

Dropping from 36V to 12V is a really huge step. Remember, the primary and secondary voltages at the transformer output are always the same, regardless of the PWM value.

For 36V output, the secondary transformer winding is probably pulsing with a 45-50V square wave (could be more or could be less, depending on how the PSU was designed - narrower output pulses with higher voltage will generally yield a more efficient PSU with more provision for overloading, but will also stress the output caps more and usually require more output filtering as well... i.e. bigger output caps.) It's the output caps' job then to smooth those 45-50V square wave pulses into an average voltage of 36V. So trying to go down to 12V with any significant load means the PWM pulses will have to get really, really low. This means, the caps will charge for very small amount of time, but then be expected to hold the average output voltage steady for a very long time. Going down in voltage a little is typically not a problem, because the capacity of the output caps is selected for the given output load range of the adapter (and with some room for under/over-loading and component degradation).

So like stj said, you would really need to increase the capacitance to make the PSU stable. And even then, that is not guaranteed, because the compensation loop on the feedback may not be designed for such a large change in output capacity.

Of course, if you want to experiment further for educational purposes, you can try increasing the output capacitance 3 to 5 times of what it is right now. Not only that, but the additional caps you install will need to be placed very close to the output of the PSU, just like where the original caps are. Otherwise, this may not work.

That said, even with the increased capacitance, there is no guarantee the PSU will work. The second design parameter that will affect how much you can adjust the PSU is the compensation network. Typically, this consists of a capacitor and/or resistor-capacitor network(s) placed across the top resistor in the feedback loop. There may also be an additional cap and/or cap-resistor network between the feedback and reference pin. The point of the compensation circuit, as the name implies, is to compensate for the response of the feedback loop near its limits (i.e. at minimum and maximum load the PSU was intended for) so that the PSU is stable and provides a consistent voltage across the entire output current range.

Messing with the compensation isn't exactly trivial and requires understanding of Bode plots (something I remember we barely touched in university in EE classes, but remember absolutely no details about how to do it anymore. )

However, you are on the right track when you did this:
Quote:
Originally Posted by Dannyx View Post
I replaced it with a KIA431 and had another go, this time messing around with the two resistors going to GND, rather than the one going to V+.
This is indeed the proper way to fine-tune or adjust the output voltage for any PSU or motherboard/VGA VRM circuit.

As mentioned above, the upper resistor typically has a compensation network in parallel with it. So changing the value of the upper resistor in the feedback network is not recommended. On the other hand, the lower resistor usually does not have a compensation network and can be adjusted to any value desired... at least until you hit a point where you are clearly asking the PSU to go way too far beyond what it was designed for without changing anything else in the PSU.

Last edited by momaka; 06-17-2020 at 09:27 PM..
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