Laptop PSUs - sensing circuit...

[tmcw]

I was wondering if anyone has any experience of laptop PSUs and the sensing circuit that is in some of them?

I have a PSU for a Dell, PA-12 family (65W), it's a no-name replacement for the original, and has worked well for the past couple of years, but a few days ago it stopped charging the battery, and when the PC starts up, the BIOS complains that I should plug in a 65W PSU, but the laptop will run as it is, but performance will be compromised. And it is as it says, the battery won't charge, and the laptop runs ok.

The voltage coming off it is still ok (19.5V), and a bit of googling hinted that the cable from the brick to the laptop might be bad, that the signal through the centre pin isn't getting through. But I cracked the brick open yesterday, and the continuity is fine on all 3 wires from the brick to the laptop.

There were a couple of 1000uF/25V caps near the bricks output (KW brand, looked dodgy) that I replaced with 35V FMs, but the problem persisted.

I also tried a couple of other batterys, and also tried the PSU in a couple of other laptops, but it's got to be the PSU thats at fault. It powered up all laptops, but wouldn't charge any battery, in any combination.

So, a replacement is still cheap, but this is badcaps afterall, so if anyone has any ideas from any similar experience, I'd be happy to hear your suggestions (even if just to say don't waste my time, and buy another adaptor!). Do you guys even bother messing with these PSUs? Maybe OEMs, but what about no-name replacements?

[rogfanther]

Re: Laptop PSUs - sensing circuit...

The problem should be in a small chip, looking like a small transistor, near the output of the psu. It is a 1-wire small memory ( 512 - 1024 bytes, from what I remember ) that stores the identification of the psu and is read by the notebook through the center pin.

Other than taking it from a working supply, or buying and reflashing one, I don´t think there is another easy way to solve it.

[tron]

Re: Laptop PSUs - sensing circuit...

resolder the small sense wire to the smps, and check for continuity

post #7 http://www.edaboard.com/thread234475.html

[tmcw]

Re: Laptop PSUs - sensing circuit...

Thanks for the info and links...

I had noticed the "transistor", with only two legs soldered to the board, wasn't sure if it could be read with a multimeter, it was reading open anyway. I did resolder the points the output cable wires are attached to, and rechecked continuity, and all 3 are fine.

Plenty of info out there when you know what to look for. In addition to the page tron linked, I found another page that was an interesting read:

http://www.laptop-junction.com/toast...stery-revealed

The component on mine is marked 2501 like the other sources I've read (DS2501), so I'll have a look and see if I can get some replacements, and do the 1-wire programming, looks like an interesting exercise.

I've also uploaded a couple of photos of the board, for whoever is interested, top side is pretty sparsely populated compared to other laptop bricks I've cracked open. Bottom side doesn't look like the tidiest or cleanest (looks like it was left in a puddle for a few days), but it still outputs 19.5V.

Wish I'd kept the original PSU, as I recall, the cable got badly damaged. Might have been useful for the component I need, or even for a cable substitution if the insides of the original hadn't been fried when the cable was damaged. And I'm angry with myself as only last week I threw an original PA-12 back into an electronics dumpster because the jack end was a little damaged on the outside, and the power lead end had some weird connection (was like a figure of 8 connection, only it had 3 pins in a line).

[b700029]

Re: Laptop PSUs - sensing circuit...

It's normal for the 2501 to look open, because it's not really a transistor. The 1-wire protocol is really interesting, essentially the device is powered by the same signal that it communicates on, with an internal capacitor as a power source during the low periods.

Once you've built a 1-wire adapter you can try reading the existing one, to see if it's only partially corrupted. If you get a replacement 1-wire EEPROM (they don't sell 2501s to individuals) here is a description of the data that's supposed to be in it:
http://www.howtofixcomputers.com/for...in-108608.html

To guard against future problems one workaround is to relocate it inside the laptop, permanently connected, and disconnect the center pin from the input socket.

My personal preference is for laptops that don't do any adapter sensing and just take +/- DC, or those that look for just a resistance between the sense pin and ground (like some of the Thinkpads).

[rogfanther]

Re: Laptop PSUs - sensing circuit...

Other times, I have just found complex circuits for 1-wire writing , with components hard to find around here. If twmc finds some simple one, please post it here !
I´ve yet to try the solution here http://www.howtofixcomputers.com/for...-108608-3.html , but it seems to me it could be easier to build than the 1-wire .