Help identifying SMD component for my car tail light - Page 1
Forum section: electronics / repair

author: jilted
author_rank: Newbie
timestamp: January 27, 2018, 02:38:37 am
content: Hello all,One of my tail lights on my car has a crack in the glass and after some heavy rain it killed the PCB inside. So after disassembly of both tail lights and some comparison between the working and the broken I narrowed it down the three components marked RED in the attached picture. The pads below are voltage IN, 12V.I meassured the resistance on a working one with my multimeter (both in circuit and a desoldered one) and they all gave me 0.35 OHM's. What's confusing me is the look of it, it does not look like a regular SMD resistor, so some input from the pros. would be much appreciated.Thanks in advance,Michael

author: stj
author_rank: Super Contributor
timestamp: January 27, 2018, 03:44:14 am
content: they look like inductors to me.

author: arteoq
author_rank: Newbie
timestamp: January 27, 2018, 04:27:19 am
content: I would desolder one of this component (the one looks still working of course), and check it with LC meter. And perhaps below the component u could find the designator letter on the pcb

author: Armadillo
author_rank: Super Contributor
timestamp: January 27, 2018, 05:58:08 am
content: Inductor in series to a pair of parallel filter capacitor.

author: AndyC_772
author_rank: Super Contributor
timestamp: January 27, 2018, 02:16:52 pm
content: How exactly did you narrow the fault down to these parts?I'd agree that they look like ferrite beads, and there's really nothing in them to fail unless they're subjected to a massive overload. I wouldn't expect them to be a part that dies, except as a result of a short caused by something else dying first, and a resistance of 0.35 Ohms is entirely normal.By all means swap them between the working and failed lights if your soldering skills are up to the job, but personally I'd be looking elsewhere.

author: jilted
author_rank: Newbie
timestamp: January 27, 2018, 02:56:46 pm
content: They don't give any reading in continuity-, resistance- or capacitance mode. The working ones do, 0,35ohms in resistance mode. I'll give it a try to swap them and see what happens...Thanks for all the help so far!

author: zvonex66x
author_rank: Contributor
timestamp: January 27, 2018, 03:30:29 pm
content: Quote from: jilted on January 27, 2018, 02:56:46 pmThis is inductors...

author: jilted
author_rank: Newbie
timestamp: January 27, 2018, 03:38:41 pm
content: So, I did the swap and it actually worked!So, now I just need to know how to identify the value of the inductor... Any tips?

author: Armadillo
author_rank: Super Contributor
timestamp: January 27, 2018, 03:47:43 pm
content: Just jumper the inductor with a wire making 1 loop. Not critical circuit anyway.ORTypical vehicle power line EMI suppression filter;BLM21PG for size 0805BLM31PG for size 1206BLM41PG for size 1806

author: jilted
author_rank: Newbie
timestamp: January 27, 2018, 04:33:08 pm
content: IT WORKED!Me sooo happyYour help saved me €1200!THANKS ALL!

author: jilted
author_rank: Newbie
timestamp: January 27, 2018, 04:34:18 pm
content: Quote from: Armadillo on January 27, 2018, 03:47:43 pmThanks! It worked with the loop solution, but will order some 0805 filters. Thanks again!!

author: pinkman
author_rank: Regular Contributor
timestamp: January 28, 2018, 05:59:36 pm
content: Quote from: Armadillo on January 27, 2018, 03:47:43 pmI totally disagree with this statement - If you know anything about automotive EMC then you will too. The input filtering components are generally critical to mitigating conducted emissions which will travel back into the power harness and wreak havoc elsewhere. The only time I can think of that this would not be true is when they are being used as surge current limiters. Eliminate these ferrites, and depending on the switching frequency and input current ripple on the CC driver front end, you end up with intermittent active key issues/inop, or tire pressure sensor communication issues, or GPS/DAB/Ecall module issues, etc - I would strongly recommend that you try to replace these with similar components, and not shorts.I agree that these will almost certainly be EMI suppression beads from murata. Since these devices dissipate the damped frequencies as heat, it's concievable that the manufacturer under-rated these components. Porsche is obviously low volume, and in this low volume case there is generally a much lower concern for BOM cost, so maybe there is some unanticipated issue as the result of another problem elsewhere on the board. If you have one good bead left, I would recommend getting this on a VNA or even a spectrum analyzer with a sweep generator, and try to find a BLM series bead that is similar. I would also measure the peak current during the enable sequence at max brightness and pick a bead that is rated appropriately, taking into consideration the temperature derating in the datasheet + a guesstimate of the temperature rise of the component at max current operation (DC current + some noise suppression... Sorry can't help for the noise guesstimate). Worst case, you might need to go up a case size but you can probably get an 0805 on 0603 pads. Might not be able to get 1206 on 0805 pads unless you have the luxury of extra adjacent copper + scrape some soldermask. Put a little glue down if you are worried.Anyway, this is obviously some paranoia, which is what these automotive designs operate on, but in that car you have active key, TPMS, likely GPS, DAB, I'm guessing E-call and who knows what other wireless devices in the system. EMC is critical.