Laptop question - Page 1
Forum section: electronics / beginners

author: Kerkana
author_rank: Contributor
timestamp: August 19, 2023, 08:51:40 am
content: Hi,On an old PC when turning it on it consumed 0.4A then cut off, on an input mosfet there was a weird behavior, 0V on the source and 19V on the drain, I don't know how it's possible, 19V on the shunt resistor, so to try I removed it and made a bridge, without disconnecting the lab power supply, and there I have a short on the 19V line with a short on the processor, but there already had a short on coils not far before trying anything, nothing was heating up on the thermal camera, do you think the manipulation fried the processor? I would tend to say no and that it already was, since then there is the 3v/5v chip, but in doubt...

author: Shonky
author_rank: Frequent Contributor
timestamp: August 19, 2023, 08:57:09 am
content: Very hard to understand your problem. Maybe give step by step of what you see in separate sentences.If you had a short and then you bridged a MOSFET that probably was on anyway it shouldn't make much difference. Find the short you think you see.

author: Kerkana
author_rank: Contributor
timestamp: August 19, 2023, 10:29:23 am
content: - Before disassembling it, I plugged it into the lab power supply, by pressing the button, it consumed 0.4A, then cut off, we could see that it was trying to start because the screen was changing contrast- I tried to remove the mosfets near the 19V input by doing a bridge to see if they weren't broken, but when I reconnected the lab power supply there was a short circuit on the 19V, the processor was heating up but there was already a short circuit on the coils circled in the photo- I reassembled the mosfets, there is a different behavior when I make a bridge and when there are the mosfets- With the bridge on the coils of the processor there is 1.7V if I remember correctly, with ~ 5ohms on the coils not far away- If I keep the mosfets, the superio seems to cut everything since there is 0V on the coils of the processorI have a short circuit on the circled coils, with the thermal camera I see nothing, by injecting current there is a consumption of 0.5A

author: amyk
author_rank: Super Contributor
timestamp: August 20, 2023, 05:41:06 am
content: Make and model, or part number(s) on mobo? Perhaps a schematic can be found to better answer your questions.

author: Shonky
author_rank: Frequent Contributor
timestamp: August 20, 2023, 06:14:27 am
content: What do you mean by a "short circuit on the coils"? Inductors will essentially look like a short circuit when measured with a multimeter.You're really seeing 0.5A at 19V that's 9.5W going somewhere. That should be fairly easy to find with a thermal camera. However if you're injecting 19V with a 0.5A current limited supply and there really is a short, then the supply voltage will drop significantly and you won't be dump 9.5W into the board.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: August 20, 2023, 10:58:49 am
content: Modern laptops with lithium batteries will incorporate supply protection, usually with MOSFETS and current sense resistors. To see the way this works look up the LTC4368 data sheet by Analog Devices. That IC is designed to provide pretty foolproof protection. Your laptop may have something similar.SJ