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Old 09-14-2018, 08:35 AM   #1
Dannyx
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Question Modifying laptop adapter for 12v

Good day folks. Out of pure boredom and also to learn stuff, I'm trying to hack a laptop power brick to output 12v instead of 19v. Now before you people start yelling at your screens for not going with an off the shelf 12v supply, that's not the point - the point is to hack stuff and also to put something something unused to good...use

So, I know how SMPS feeback works and I DID this once before with great results, but this one is a bit different: I was expecting to see a TL431 on there with a resistor divider on its REF pin to drive the feedback optoisolator, but this one uses an IC (this DAS001 which is TWO comparators in one. I put together a rough schematic and also some shots of the secondary of the board with the IC removed so you can see the traces beneath.

I BELIEVE I need to tweak those resistors on the left side of the schematic (just barely encircled there) which correspond to the ones to the left of the IC in the pictures, since I tried to draw it close to the original layout. The way I see it, by lowering the value of the one going to VCC, the inverting input will be biased towards VCC more, thus turning its output more "negative" and make the LED inside the optocoupler shine brighter, telling the switching IC in the primary (not pictured) to reduce the switching frequency and lower the output....at least that's the theory and how I did it with the TL431 version. Now I don't really understand why there's TWO comparators used here, for what purpose ?
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Old 09-14-2018, 10:11 AM   #2
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Default Re: Modifying laptop adapter for 12v

hmm.. yes, resistive divider on pin 2.
not sure why it has a snubber-type link to pin one though - some type of damping/soft-start maybe?

second comparator may be "just incase it's usefull".
like if your monitoring current or something.

Last edited by stj; 09-14-2018 at 10:13 AM..
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Old 09-14-2018, 10:28 AM   #3
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Default Re: Modifying laptop adapter for 12v

Ooh right, current sensing. It could be for that...it's pretty packed with components on the other side and you can't really see it well, but I don't see a current sense resistor on there....or is it ? :| I'll dig deeper, but my money is on that left set of resistors.
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Old 09-14-2018, 10:37 AM   #4
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Default Re: Modifying laptop adapter for 12v

Can we see good clear pictures of the top side of the board and full view of the bottom side of the board?
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Old 09-14-2018, 10:44 AM   #5
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Default Re: Modifying laptop adapter for 12v

Starting Monday - it's at the shop. I put the IC back in the meantime.
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Old 09-14-2018, 01:14 PM   #6
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Default Re: Modifying laptop adapter for 12v

If you're sure you diagram is drawn correctly, I agree that it's the resistor divider connected to pin 2 that you want to change. You can either change the resistor that's between pin 2 and ground (increase its value) or change the resistor between Vcc and pin 2 (decrease its value).

That said, one thing I learned from uN1qU3's PSU modding threads (at least PSUs with TL494 chips) is that changing the lower resistor in the divider might be preferable to do, as it doesn't mess with the circuit's compensation.

Quote:
Originally Posted by stj
hmm.. yes, resistive divider on pin 2.
not sure why it has a snubber-type link to pin one though - some type of damping/soft-start maybe?
Feedback loop compensation.
A bit over my head how to calculate stuff for that network or how to explain it... but basically, at different loads, the PWM changes and that can mess with the feedback circuit interpreting the voltage (even though it is filtered). To avoid oscillation, you have to dampen certain frequencies and boost others on the feedback. So that's what the compensation does. In this case with the RC-network, it attenuates the high frequency gain of the loop.

Last edited by momaka; 09-14-2018 at 01:17 PM..
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Old 09-14-2018, 01:37 PM   #7
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Thumbs up Re: Modifying laptop adapter for 12v

Quote:
Originally Posted by momaka View Post
changing the lower resistor in the divider might be preferable to do, as it doesn't mess with the circuit's compensation.
By "lower resistor" you're referring to the one going to GND ? I was concerned that lowering the resistance between that pin and GND will cause more current draw, so I thought I'd lower the VCC one instead, though now that I think about it, it's silly really, since an op-amp has very little (ideally zero) current going into its pins, so it shouldn't be an issue even if shorted to GND directly (in which case, theoretically speaking, the output would drop to the minimum possible value of around 2.5v which is the reference).

CORRECTION: with the inverting input tied directly to GND, the output would shoot up to the maximum possible value...whatever that might be, because now the NON-inverting input is always going to be higher than the inverting input (which is grounded) and never allow the output of the op-amp to swing low to turn on the LED, so the primary would try pumping even harder to reach equilibrium.

Last edited by Dannyx; 09-14-2018 at 01:41 PM..
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Old 09-14-2018, 02:25 PM   #8
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Default Re: Modifying laptop adapter for 12v

Quote:
Originally Posted by Dannyx View Post
By "lower resistor" you're referring to the one going to GND ?
Correct.

Quote:
Originally Posted by Dannyx View Post
I was concerned that lowering the resistance between that pin and GND will cause more current draw, so I thought I'd lower the VCC one instead
You misread my above post.

If changing resistor between pin 2 and ground, _increase_ the resistance. (This will cause a higher voltage readout at pin 2, and hence more often pulling of the output to ground).

If changing resistor between pin 2 and Vcc, _decrease_ the resistance. (This will cause a higher voltage readout at pin 2, and hence more often pulling of the output to ground).

Quote:
Originally Posted by Dannyx View Post
CORRECTION: with the inverting input tied directly to GND...
Well, if you are trying to have any kind of regulation, you never tie any of the op-amp pins to ground.
And if you tie pin 2 to Vcc, your PSU will output the reference voltage set by the resistors on the lower-right side of your diagram (the ones that connect Vcc, pin 5, and ground).

Last edited by momaka; 09-14-2018 at 02:31 PM..
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Old 09-14-2018, 03:18 PM   #9
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Talking Re: Modifying laptop adapter for 12v

Actually THIS is what I'm going to use it for: please have a look at that scribble in MS Paint I threw together: I got my hands on a car headunit, MP3 whatever and it's got AUX in...sounds pretty good and I want to install it in my shop (don't judge guys ). Power to the outlets gets cut when I close the shop, which would cause it to lose its settings permanently, so I found a UPS battery in my underpants which I'm going to use in that configuration. When AC is available, it will be turned to DC 15v via this adapter I'm trying to hack, close the relay, power the headunit and also (this is the part I'm not sure about), charge the battery via the diode. When power gets cut, the coil has no power (since DC from the battery can't keep it on thanks to the diode) and it prevents the headunit from running on JUST the battery (so you don't forget it on). Meanwhile, though this is not figured, there's a second "VCC" wire coming from the wire loom that will be permanently connected to the + terminal of the battery to preserve its settings, so the wire I'm actually switching is the ACC "accessory" wire that gets toggled on/off with the ignition - not the main power to the unit itself....what do you think ? That's Danny for you...
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Old 09-15-2018, 07:15 AM   #10
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Default Re: Modifying laptop adapter for 12v

^ That should work in theory. However, if the battery is low enough in charge, it could try to pull too much current from your adapter and trip it. The diodes will do nothing to limit the current. You will also want to check the voltage on the battery with the 15V adapter. A regular diode will typically start with a 0.6-0.7V drop and then go higher under load. So that should keep the battery charged up to about 14.4V. But be careful! You don't want to do is keep the battery voltage too high, as that will kill the battery over time.

Lastly, I suggest checking how much power the radio unit is pulling in "standby" mode (i.e. when the accessory signal gets toggled off to indicate there's no key). If it's just a few mA or less, maybe put something like 3x-4x 300-470 Ohm resistors in parallel between the diode output and the battery. That way, even if the battery is discharged, it won't trip your adapter, because the charge current will be limited by those resistors (to around 190 mA or less and each resistor will be dissipating about 0.9W or less).

Last edited by momaka; 09-15-2018 at 07:17 AM..
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Old 09-15-2018, 12:34 PM   #11
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Default Re: Modifying laptop adapter for 12v

Well in THEORY, the battery's label says "initial current" less than 2.5A....which is TECHNICALLY the adapter's maximum current - of course, it's silly to think those numbers will always be this perfect, so I too was worried about current. I'll "tune" the adapter so it's 13-14v to keep it safe and also drop a low value resistor in series with the battery just in case - it should not require more than 1-200mA in standby....I shall do some testing.

I just remembered that I FLATTENED the hell out of that battery: I put a small lightbulb on it to check if it can hold a moderate load....and TBH I forgot about it It will have to endure another day before I can finally release it from its misery and charge it...it could be that's it's already done for since lead acid batteries release some sort of gasses if overly discharged....sure hope I don't find a nice spew of battery guts over my shop floor on Monday morning
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Old 09-17-2018, 01:48 AM   #12
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Talking Re: Modifying laptop adapter for 12v

Here's some more pics of this supply. It's pretty packed in there so I took some close-ups of the output stage as well. That definitely looks like a current sense resistor in that piece of heatshrink tube there, so there is some current sensing going on too.

The one going to VCC is that 24kOhm (2432 resistor). I botched a thru-hole 20k one in its place instead and it works: the voltage dropped to 13,5v, so that's the one....I was lucky to get it just about right from the first time, though I think I'll go slightly higher at around 21-22k, since 13.5 is just slightly too low for charging a lead-acid battery, especially with that diode I'm trying to use which will insert some Vdrop of its own.....
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Last edited by Dannyx; 09-17-2018 at 02:11 AM..
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Old 09-17-2018, 03:29 AM   #13
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Default Re: Modifying laptop adapter for 12v

You can leave the 24 KOhm in place and just put another resistor in parallel on top of it. 240-470 kOhms should give you 21.4 to 22.8 kOhm range.

This power adapter looks pretty decent, especially with those Rubycon caps. Also appears to use synchronous rectification. And yes, looks like it does have over-current protection.
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Old 09-17-2018, 03:41 AM   #14
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Default Re: Modifying laptop adapter for 12v

I was also being a dipshit in my original post: the original voltage wasn't 19v, but 16v, since IT'S A SONY ! I was a bit surprised at how much the voltage dropped with such little change in resistance, but it didn't drop from 19v, but from 16 which was pretty close to begin with. I went with a 20k and a 1k one in series for 14.4 output which is perfect. Charging away quite nicely with no complaints...remember I told you that battery was completely flat after running that lightbulb all weekend

The radio draws less than half an amp with just the VCC connected (iginition terminal disconnected) and around 500mA when running, though it had no speakers connected. I'm sure the adapter can run it quite happily and charge the battery at the same time. The maximum allowed voltage on that radio is 16v according to the manual, so I could've gone slightly higher in volts on that adapter, but I figured 14 is enough for "cyclic" use.
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Old 09-17-2018, 04:00 AM   #15
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Default Re: Modifying laptop adapter for 12v

Quote:
Originally Posted by Dannyx View Post
the original voltage wasn't 19v, but 16v, since IT'S A SONY !
Ah. That's good, though. Power adapters with lower output voltage typically have higher output current (at least for a given standard power output... i.e. 40W, 65W, 90W, etc.) Should help a bit if running 4-Ohm speakers or lower.

Quote:
Originally Posted by Dannyx View Post
The radio draws less than half an amp with just the VCC connected (iginition terminal disconnected) and around 500mA when running, though it had no speakers connected.

Half an amp is equal to 500 mA as far as I know.
So your radio is drawing the same current in standby and when turned on? That sounds goofy. If correct, though, are you sure the radio will lose settings if the Vcc is disconnected? I don't see a car radio drawing 500 mA constantly in stand-by. That will flatten any car's battery rather quickly over time. And all I can tell you is that when I disconnect my car's battery, the only thing that my (factory) radio looses is the clock. Programmed stations and bass/treble settings hold okay.

Last but not least, there shouldn't be a difference whether you have the speakers connected or not if the radio is not playing anything. Power draw will stat to jump up only when you crank up the music. With that said, I suggest you connect a pair of 8-Ohm floor speakers to the radio. They will sound pretty good and also have lower current draw... that is if you have any.
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Old 09-17-2018, 04:12 AM   #16
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Default Re: Modifying laptop adapter for 12v

I said LESS THAN half an amp, if that. I think it's like 0.003A, so yeah, nevermind half an amp....WAY less than half an amp. Yes, it loses its settings since I tried it on the bench when I got it. The Pioneer one I got in my car also does this sadly - not too top of the line but wasn't cheap either, so it's not a price point issue AFAIK. Had to replace my battery a couple of years ago and had to set it up again after that. That's one thing that crossed my mind: why don't (the majority of, as you pointed out) these headunits store their settings somehow ? I can understand this one since it's rather old, but newer ones don't do this either. The Chinese got it right once again with a cheapo desktop/thermometer clock I got at home: it's got a CMOS battery inside and even remembers its clock when power goes out and I LOVE IT

I have a full set of surround speakers for that matter, two which are blasting right now in my shop - I'll add another 2 plus the sub when I get my AUX cable for this thing, since it uses a nonstandard plug and had to order it off Ali....I could've botched some wires there but I'll probably want to sell it if I ever leave this place and pack up all my sh!t and the new shop doesn't allow "customizations"
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Old 09-21-2018, 05:06 AM   #17
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Talking Re: Modifying laptop adapter for 12v

Well, just for those curious how the end product looks like, here's the "charge unit" I concocted The diode is a STSP3045...zip tied that to the relay...nice and neat. Adapter VCC to blue wire, ground to blue wire, battery + to red wire. Zener diode on that relay just in case, since I wasn't sure how well it's play with 14v for long periods of time, though I'm sure I'm just paranoid. Also a 1n4007 diode to "catch" the coil kickback when the relay turns off.

When the adapter is "available", it sends VCC to the "ignition" terminal (not connected yet in the pics) of the CD player and also closes the relay which allows VCC to also go through the diode into the battery. When the adapter is disconnected, the ignition terminal is switched off, the relay opens and the diode prevents the battery from keeping the relay on with its own power. Meanwhile, the yellow "power" wire for the CD player is always connected to the now-charged battery to keep its settings and stuff (also not pictured) and it all works great
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