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Stiv2k
Rm -Rf As Root

join:2002-07-22
Orlando, FL

Trying to repair laptop charging circuit

Click for full size
My board. Notice blown open 4925b

Other user's board from this thread: d=46024&p=2
Hello friends,

I received this netbook from my mom and it no longer charges from the AC adapter, but it works fine from the battery. I have a few other dead motherboards laying around with the same exact problem so it appears that this is a common issue among laptops these days. Well she told me that the netbook was plugged into the wall and the electricity in the house started fluctuating (lights flicker, clocks reset, etc...power in FL sucks). Next she said that the laptop stopped charging so I suspected a blown motherboard component.

When I opened up the Asus Eee PC 900, I saw a MOSFET that had been blown wide open, with a matching black spot underneath the keyboard cover. I did a little research and looked on Asus Eee forums and identified this blown MOSFET to be a "4925b" - Dual P-Ch Enhancement Mode Mosfet. I went ahead and ordered 2 of them from Mouser electronics to attempt the repair, but it is on backorder so I have to wait until at least April (»www.mouser.com/ProductDetail/Vis ··· H97Bk%3d).

While I am waiting for these replacement ICs to come in, I am starting to wonder the likelihood that other components on the board are blown and how to go about determining it. Close inspection with magnifying glass shows no physical sign of burning to any other components except the 4925b. Also as I mentioned before the machine works fine when powered by battery. I understand this is a multilayer board and not much can be done without a circuit diagram, but being that it is a relatively simple circuit, smaller board with fewer components than a normal laptop, I'm hoping a few experienced folks here could give me some pointers.

Finally, I have attached 2 photos. The first one is the actual photo of my Eee motherboard. The second is from the Asus Eee forums, of a 701 where you can see a blown diode. The user here has partially traced the charging circuit and you can see the same 4925b that is blown on my board. Maybe this partial diagram can help me diagnose other possibly blown parts.

Thank you
--
- Steve Bularca

irc.neoturbine.net - NeoturbineNET IRC


cowboyro
Premium
join:2000-10-11
Shelton, CT

1 edit
First check the power adapter. The last thing you want is blowing another one because the adapter outputs more than it should.
Check if any diode nearby is shorted. There are few transistors nearby, w/o knowing whether they are bipolar or MOSFET you can only check for an obvious short. Then replace the toasted circuit and cross your fingers...
Edit: looks like »www.mouser.com/ProductDetail/Vis ··· y0PBo%3d could be a good replacement and is pin-to-pin compatible... also in stock.


Stiv2k
Rm -Rf As Root

join:2002-07-22
Orlando, FL

1 edit
I checked the adapter, it puts out 12V as specified, so I think it's OK. Damn, I wish I hadn't already ordered a part on backorder... I might order one of those also just in case. Maybe I can get mouser to cancel my original order?

BTW, how did you find that part? I searched for 4925b and I couldn't find any part in stock.

Also, Using the diode tester across the DC jack, my multimeter shows O.L. in one orientation and ~0.300 (300 ohms?) in the other direction. Does this mean a short?

EDIT: Actually, there is a diode connected across the DC jack right on the board, so I guess it must be there for a reason and functioning properly. Perhaps to protect against reverse polarity?

Thanks
--
- Steve Bularca

irc.neoturbine.net - NeoturbineNET IRC


cowboyro
Premium
join:2000-10-11
Shelton, CT
reply to Stiv2k
Looked for a part with similar or better characteristics

public

join:2002-01-19
Santa Clara, CA
said by cowboyro:

Looked for a part with similar or better characteristics
»pdf1.alldatasheet.com/datasheet- ··· BDY.html
any higher voltage or current would work. There must be a reason why it was destroyed. Unless it is found you will burn more parts.


DrStrange
Technically feasible
Premium
join:2001-07-23
West Hartford, CT
kudos:1
reply to Stiv2k
said by Stiv2k:

EDIT: Actually, there is a diode connected across the DC jack right on the board, so I guess it must be there for a reason and functioning properly. Perhaps to protect against reverse polarity?

Thanks


Correct. I lost count of how many 1N4003s I replaced due to people not watching which way they connected the DC, back in the day. I suppose it was better [not to mention less expensive] than replacing the parts the 1N4003s were protecting.


Stiv2k
Rm -Rf As Root

join:2002-07-22
Orlando, FL
reply to public
said by public:

said by cowboyro:

Looked for a part with similar or better characteristics
There must be a reason why it was destroyed. Unless it is found you will burn more parts.
Well the best reason for why it blew up was because of poor electricity coming from the wall. My mom explained that the lights flickered in the house at the time that this happened. I suppose it was either a voltage spike or power brownout? Not sure how those can effect electronics, but that's my best guess.
--
- Steve Bularca

irc.neoturbine.net - NeoturbineNET IRC


IllIlIlllIll
EliteData
Premium
join:2003-07-06
Hampton Bays, NY
kudos:7

1 edit
said by Stiv2k:

said by public:

said by cowboyro:

Looked for a part with similar or better characteristics
There must be a reason why it was destroyed. Unless it is found you will burn more parts.
Well the best reason for why it blew up was because of poor electricity coming from the wall. My mom explained that the lights flickered in the house at the time that this happened. I suppose it was either a voltage spike or power brownout? Not sure how those can effect electronics, but that's my best guess.
the adapter is a switched mode power supply.
it can handle a range between 80VAC and possibly 240VAC without affecting the 12VDC output.
fluctuations on the input side shouldnt affect the output side.
not unless the output regulation in the adapter is faulty, which is a strong possibility.
perhaps a faulty capacitor, solder connection inside the adapter.
the other possibility is the jack connection on the laptop.
if it has an intermittent connection, the spikes caused by it drawing current intermittently, would be enough to destroy the mosfet.
hopefully, the mosfet that shorted, is shorted to ground which would trigger the safety in the adapter to shut itself off, preventing 12VDC from flowing past the mosfet causing more damage.
--


shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA
Reviews:
·EarthLink
·Comcast
·Atlantic Nexus
reply to Stiv2k
Switch-mode supplies do not handle fast changes in the input voltage well. That is one reason they are often paired with a LDO regulator. They can also overshoot with sudden changes in load.

As the part is only 30V, the supply probably had a spike that exceeded the voltage causing the transistor to break down.

Used to live in Florida, lots of surges and power transitions. Had many switching power supplies fail or burn out downstream devices. Worst is when power cuts in/out several times over a few seconds. The supplier are running at a high PWM duty cycle to maintain the output voltage with a low input voltage. Then the input jumps and the output spikes. How much depends on the design of the supply (current or voltage mode design.)
--
Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder


Stiv2k
Rm -Rf As Root

join:2002-07-22
Orlando, FL

1 edit
reply to IllIlIlllIll
Well I already tested the adapter and it puts out a solid 12V... although it was tested while not under load. I'll try to find a way to test the DC jack's continuity to see if it's intermittent or not, I have to do it w/o actually applying power and burning the mosfet even more.

EDIT: Should I buy a new genuine adapter to be safe? Is there any way to determine if this is absolutely necessary?

--
- Steve Bularca

irc.neoturbine.net - NeoturbineNET IRC


aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1
Reviews:
·Frontier Communi..
reply to Stiv2k
said by Stiv2k:

My mom explained that the lights flickered in the house at the time that this happened.
I think the upstream switching supply had an overshoot during that time. It would've been a prudent design if that DC input had a 24V - 27V zener to provide some protection against voltage spikes.
--
And the winner is:


Stiv2k
Rm -Rf As Root

join:2002-07-22
Orlando, FL

1 recommendation

reply to Stiv2k
Click for full size
Click for full size
Click for full size
UPDATE:

The Eee LIVES!!!

The MOSFETs I ordered as per cowboyro See Profile 's suggestion came in today, so I broke out my hot air station and began the component swap. After accidentally blowing off 2 microscopic capacitors (directly above and below the 4973 mosfet) and getting a cold solder the first time, I finally manage to get everything back on and remove all the solder bridges. After checking for shorts and giving the OK, I plugged in the original (risky) adapter and the machine powered right up. The battery seems to be weak, as it's been sitting on the shelf for 6 months, but everything seems to be working fine so far.

Thanks everyone for your help, I feel somewhat stunned that this was the only faulty component on the board, LOL
--
- Steve Bularca

irc.neoturbine.net - NeoturbineNET IRC

TheMG
Premium
join:2007-09-04
Canada
kudos:3
Reviews:
·NorthWest Tel
reply to Stiv2k
Tip: use a smaller soldering iron tip and/or smaller diameter solder.

That's a LOT of solder you've got on that component there.

Anyways, good job bringing the computer back to life!

Mustafa

join:2006-10-04

1 recommendation

reply to Stiv2k
You might want to read through this.

" Ceramic Input Capacitors Can Cause Overvoltage Transients"

excerpt

"To illustrate the problem, a typical 24V wall adapter used in notebook computer applications was connected to the input of a typical notebook computer DC/DC converter.
The DC/DC converter used was a synchronous buck
converter that generates 3.3V from a 24V input."

»www.linear.com/pc/downloadDocume ··· id=24956

Quickly turning on and off an SMPS can be a brutal test. If you have a scope cycle the input power to the board and view the input waveform, once you get it to work.


Stiv2k
Rm -Rf As Root

join:2002-07-22
Orlando, FL
Hi,

an interesting read, but I do not currently have a scope so I cannot perform that test. I have, however, been giving some serious thought in purchasing one for learning purposes. So far I have been using the netbook for about 12 hours without any prolems, including the original A/C adapter so unless i experience another electricity fail I think I'll be OK.