Some advice on short circuit - Page 1
Forum section: electronics / beginners

author: PeterSpie
author_rank: Contributor
timestamp: November 04, 2022, 12:50:15 pm
content: Hello,I have watched a lot of videos and have some experience in reparing stuff.But this new to me:My boss has connected a device with the wrong adaptor. In stead of using 12 volts he connected 48 volts to this device.I am measuring and there is a dead short between + and ground.My first suspect is this cap (see picture), because of the different colors. Or is the color of C77 normal?Should I just remove the cap and do some measurements to see if the sort is gone?And what about D5, it also looks not that great.Or is the device not savable anymore.By the way: the device is a Netgear LB2120 4G routerRegards, Peter

author: Grandchuck
author_rank: Super Contributor
timestamp: November 04, 2022, 12:58:46 pm
content: Welcome PeterI think the diode is more likely to be shorted.

author: ledtester
author_rank: Super Contributor
timestamp: November 04, 2022, 01:01:09 pm
content: Concerning color this answer from Quora says:Quote

author: PeterSpie
author_rank: Contributor
timestamp: November 04, 2022, 01:03:16 pm
content: Thanks ledtester,learning every day. I Like it.Regards, Peter

author: chilternview
author_rank: Frequent Contributor
timestamp: November 04, 2022, 01:04:35 pm
content: Post a schematic. We might then be able to speculate what the possible failure mechanisms are - I doubt they're even in this photo.

author: PeterSpie
author_rank: Contributor
timestamp: November 04, 2022, 01:06:05 pm
content: Hi Grandchuck,Thanks for your reply.When I remove it, do I need to replace it? I think it is for protection if you connect the adapter the wrong way?But, if I need to replace it, how do I know the value? I do'nt have a schematic.By the way: the device is a Netgear LB2120 4G router.Regards, Peter

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 04, 2022, 01:08:59 pm
content: Inject 12V 500mA current-limited, find the part heating up, replace, done.

author: PeterSpie
author_rank: Contributor
timestamp: November 04, 2022, 01:09:58 pm
content: Sadly, I don't have a schematic.The only thing I found was this topic:https://www.eevblog.com/forum/repair/unidentified-switching-regulator-from-netgear-lb2120/msg2976514/#msg2976514

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 04, 2022, 01:24:24 pm
content: Those boards are pretty simple, no schematics needed.

author: PeterSpie
author_rank: Contributor
timestamp: November 04, 2022, 02:24:46 pm
content: But when I find the defect component, how do I know (for example the diode) which diode I need to order?

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 04, 2022, 03:28:01 pm
content: That diode seems to provide only reverse polarity protection, not overvoltage, althought I could be wrong.I say that because I *highly* doubt it would look so unharmed after dumping 48V on it.Likely the short is either caused by a capacitor or the voltage regulator.

author: PeterSpie
author_rank: Contributor
timestamp: November 04, 2022, 04:18:32 pm
content: I have removed the diode, and the short is still there.So, i will continue to look at the capacitors.The voltage regulator looks ok to me.

author: Grandchuck
author_rank: Super Contributor
timestamp: November 04, 2022, 04:25:01 pm
content: Do you have a current limited supply? If so, DavidAlfa gave good advice.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 04, 2022, 05:39:59 pm
content: Remove the IC, probably that's it.Very easy to remove, apply some solder so it bridges all the pins, quickly apply the tip to both sides few times and should come off.

author: PeterSpie
author_rank: Contributor
timestamp: November 05, 2022, 07:23:11 pm
content: Thanks for all the hints!I don't have a goor power supply jet, but ordered one today.When I inject the voltage, how can I tell which part is defect|? I don't have a thermal camera. Can I use flux or IPA for this? Put it on and try to see where it "melts" first?Thanks @DavidAlfa, I will try that also.Regards, Peter

author: kripton2035
author_rank: Super Contributor
timestamp: November 05, 2022, 07:52:23 pm
content: build yourself a shorty-with-display device. it will always serve you to find shorts on a pcb...http://kripton2035.free.fr/Projects/shorty-display.html

author: PeterSpie
author_rank: Contributor
timestamp: November 05, 2022, 08:13:36 pm
content: Hello kripton2035,this makes live so much easier! Great, I would love to buid this.On your site it says PCB coming soon. Can you tell me when? Or can I buy this "Shorty" complete?Regards, Peter

author: kripton2035
author_rank: Super Contributor
timestamp: November 05, 2022, 09:35:08 pm
content: I began working on the pcb, but got caught on other projects, and now it's been long time and I'm not sure to make a (working) pcb one day...it's anyway quite easy to build it on a breadboard like I did the prototype. other people did it too.also it's become so difficult to make and sell small electronic devices from france (or europe) with so many norms, patents, taxes, that I could not sell anything like this on my own. you need a big company and this small $20 device would have to sell at more than $100 to even exist...