Reverse voltage error on laptop - Page 1
Forum section: electronics / repair

author: pork_belly
author_rank: Newbie
timestamp: May 11, 2021, 02:29:23 pm
content: Hello forum,I had to repair the power supply of my laptop (Toshiba Satellite C855), but I confused the two poles which finally led to a reverse voltage error. The laptop immediately turned off, but I could restart it again and load the OS (it ran about 2 min.) until it turned off a second time. After that I couldn't start the laptop anymore. I couldn't find any notable damage on the circuit board and couldn't smell any kind of burnt plastic. The loading status LED didn't glow when the laptop was connected to the power supply. What could be broken here and could the laptop still be repaired? Thanks for any help.

author: theHWcave
author_rank: Regular Contributor
timestamp: May 11, 2021, 05:25:55 pm
content: Could you add a few more details on what happened pleaseI am guessing here: so you reversed polarity in the DC output of the PSU and plugged it in: was the laptop running on battery at that time and "immediately turned off"?Did you leave the (wrong polarity) DC input plugged in and tried to restart the laptop and it ran for 2min?Now, with DC polarity correct it does not boot and on battery it does not boot either?I am wondering if a (destroyed) battery / battery protection circuit prevents booting. Have you tried removing the battery pack completely and see if it runs on (correct polarity!) DC input?

author: pork_belly
author_rank: Newbie
timestamp: May 11, 2021, 07:54:00 pm
content: After the laptop, which ran on battery before, turned off by itself I noticed that something was wrong and that the misrepaired power supply was the culprit, so I borrowed the power supply from my neighbour and started the laptop again. I could load the OS and open some folders, but after a while it turned off again. When I used it solely with PSU and without battery, nothing happened, not even the LEDs glowed. The fuses Q and Y on the photos are not blown. My layman's thought was that the damage couldn't be too severe, because otherwise the laptop wouldn't have been able to restart after the reverse voltage error occured. I attached the circuit diagram and some photos of the mainboard.

author: theHWcave
author_rank: Regular Contributor
timestamp: May 11, 2021, 11:29:30 pm
content: Does it draw any current on the DC input? It should at least draw a little because in the schematics, pg 5 there is a voltage divider of R6002 and R6003 right across the power rail (to derive the AC-detected signal). If FUSE6000 is ok, there should be at least some current flow because of that.Does the battery pack still produce a voltage on its terminals? If so have you tried running it on battery alone?I am a bit confused by your pictures. According to the schematics there should be a Littlefuse 451-series 12A fuse in the connection to the adapter socket and similar a Littlefuse 451-series 15A in the connection to the battery pack. According to the Littlefuse data sheet for the 451-453 series, these fuses should be marked brand amp-rating, e.g. LF 12A or LF 15A. Are you sure you have identified these fuses?Since you have the schematics, your best bet is to use it to find what voltages should be present at various component pins, then identify the components on the PCB and check with a voltmeter. A tedious process but it should help to narrow-down the problem.With respect to what has gone wrong, it could be anything polarised like diodes or capacitors that got damaged by reverse polarity and after the correct polarity was restored, heated up over 2 minutes until they failed completely. My iniitial suspect was the battery but since you say that the PC doesn't work even without battery pack, its probably something else.

author: aargee
author_rank: Frequent Contributor
timestamp: May 12, 2021, 04:33:54 am
content: My thoughts:The power supply circuit has something blown or some component on the power input on the laptop motherboard. Possibly protection diode, etc. So nothing coming from a power supply connection, not even to charge the battery.The battery flat, can only power the system for a few minutes at best.

author: pork_belly
author_rank: Newbie
timestamp: May 12, 2021, 08:08:41 pm
content: Thanks. Yes, the laptop draws current from the DC input, but the battery seems to be almost discharged,so the laptop doesn't run on battery alone. I have read that most modern laptops have some MOSFETs to protect the main circuits from reverse currents, therefore it is very likely that some MOSFETs or capacitors might be blown, but I don't know how I can systematically delimit the error and retrace the current. Are my coloured marks correct? Green represents capacitors, yellow fuses, blue coils and red MOSFETs.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 12, 2021, 08:12:13 pm
content: Does it start and run with the battery out?

author: pork_belly
author_rank: Newbie
timestamp: May 13, 2021, 11:55:48 am
content: No, not even a LED lights up.Do almost all modern laptops also run with removed battery or do some of them need the battery as a buffer? How can I find out which part of the circuit is corresponding to a certain part on the circuit diagram? I'm asking, because I found a tiny ceramic capacitor that doesn't show any electrical flow and seems to be short-circuited.

author: SilverSolder
author_rank: Super Contributor
timestamp: May 13, 2021, 01:28:04 pm
content: I have never encountered a laptop that won't run on AC without a battery inserted, but I don't know your specific model.If it won't work on AC without the battery, you can likely eliminate the battery from the list of suspects.If there are any tantalum or electrolytic capacitors in the circuit... these are sensitive to polarity and could have been damaged - you may be on the right track.Check if it draws current from the AC adaptor with the battery out. See if you can determine where the current is going...