What could this SMD component labeled J1 be?  - Page 1
Forum section: electronics / projects

author: silencer_ar
author_rank: Contributor
timestamp: August 28, 2023, 07:37:17 pm
content: Hello!I'm trying to figure out why an LCD panel from an old 386 notebook is not working. According to the datasheet, it looks like it should be getting -23v for powering. But it's not. I followed the pin from the LCD that should be getting -23v and eventually reached the power board. Looking for -23v in some place of it, I found a component labeled J1 which has -23v in one side and 2.6v in the other.Looks like it had some code but now it's gone. Using a sharp point as if looking for cavities, I can feel a little crater in the surface (visible in the picture).So I guess this component is faulty? but I have no idea what this is. Do you know?Pictures of the whole board below (J1 can be found near the center):Thanks in advance!

author: edpalmer42
author_rank: Super Contributor
timestamp: August 28, 2023, 08:18:53 pm
content: "J" is typically used to designate a jumper. The component could be nothing more than a zero ohm resistor. I see that R40 is labelled as 000 so that's consistent with the remaining part of the symbol on the component at J1.The question is why did it blow in the first place?Ed

author: silencer_ar
author_rank: Contributor
timestamp: August 28, 2023, 08:38:31 pm
content: That is a very good question! what's the point of a 0 ohm resistor? something like a fuse?

author: Benta
author_rank: Super Contributor
timestamp: August 28, 2023, 09:10:23 pm
content: Quote from: silencer_ar on August 28, 2023, 08:38:31 pmOften used for configuring options at assembly time, eg, country-specific versions or alternate functionality.

author: TimFox
author_rank: Super Contributor
timestamp: August 28, 2023, 09:57:51 pm
content: The packaged "0-ohm" resistors are used as jumpers, when needed, installed with the same automated equipment that installs the "normal" resistors in the same package.The usual joke was that some vendors actually mis-stated +/-5% tolerance on them.Less common, there are THT 0-ohm resistors that are packaged like 1/4 W real resistors.They were sometimes used on single-sided PCBs when a track had to cross another track.One design engineer (around 1978) told me that they looked better to the customer than a short insulated wire.

author: Benta
author_rank: Super Contributor
timestamp: August 28, 2023, 10:11:58 pm
content: Quote from: TimFox on August 28, 2023, 09:57:51 pmMore likely that the "cut, bend 'n place" machine didn't have the tool head to insert wire jumpers.

author: silencer_ar
author_rank: Contributor
timestamp: August 29, 2023, 03:07:30 pm
content: I can see how they look better than a simple wire jumplerApparently it was just a 0 ohm resistor. I have no idea why it broke, but jumping it made the screen work again.Thank you so much!

author: jmelson
author_rank: Super Contributor
timestamp: August 29, 2023, 04:37:35 pm
content: Quote from: silencer_ar on August 29, 2023, 03:07:30 pmOh, COOL! I didn't expect that to work, but glad it did!Jon

author: kripton2035
author_rank: Super Contributor
timestamp: August 29, 2023, 05:16:12 pm
content: it can also act as a fuse, if it seems to be a negative power supply ?

author: silencer_ar
author_rank: Contributor
timestamp: August 29, 2023, 06:01:23 pm
content: I thought about that, I'll see if I can find a new resistor to put there