help identify SMD inductor

Thread Starter

bdtprez

Joined Feb 24, 2018
2
I have a device that i do not have a schematic for. It is an automobile OBDII scanner that had a cable with a short in it.

I have checked all fuses and the only thing i see phsically damaged is what appears to be an SMD inductor (L500) that reads shorted on all legs (no resistance on lowest scale between all 4 contact points). From the peices i have been able to scavange, i can read the numbers "100" and underneath "M84". I cannot find a datasheet or cross reference. If anyone can help I would greatly appreciate it.


 

Tonyr1084

Joined Sep 24, 2015
3,578
First, it would have been helpful if you'd have identified where L500 was. I found it, but there was a lot of circuitry to look through. I wasn't looking for something that large. Nevertheless, an inductor is just a coiled wire. Resistance will be very very low, if your meter can read resistances that low. And coils don't typically go bad unless broken to the point where their cores or their windings are compromised. Also, testing a coil in circuit would likely be very difficult if not impossible. I would agree your coil is damaged. Without the surrounding ferrite material, its value probably has changed.

Unfortunately my skills with coils is almost nonexistent. I would imagine that you might have to remove it from the circuit and test it using some sort of tester, or some home built circuit to test whether it's good or not. And just because you have continuity on all four points - doesn't mean its shorted. It COULD be but I'd think it's not likely. If your OBDII reader has failed then it's possibly due to some other mode of failure. If the coil DOES prove to be damaged to where its rating has been changed then you'll likely have to get the schematic along with a bill of materials (BOM) to find a suitable replacement.
 

ebp

Joined Feb 8, 2018
2,332
It is almost certainly just a 10 µH simple inductor and the resistance is expected to be very low. The bit of ferrite broken off the top will only have a small effect on in.

Inductors like that are usually marked in microhenries in standard two digits and exponent fashion: 100 means 10 x 10^0 (100 µH would be marked 101 - 10 x 10^1, 220 µH would be marked 221) Inductors of less than 10 µH might be marked with 2 digits in microhenries or 3 in nanohenries. M84 is probably just a cryptic date code.

The winding is just a fairly short length of copper wire, sometimes 2 or more strands in parallel, so the resistance is low. The flanges on the ends of the "bobbin" core don't contribute a great deal to the inductance. These are open magnetic path inductors with a huge "air gap" between the bottom end of the core and the top end. The air gap dominates.

It will be part of a DC-DC switchmode converter, probably converting 12 V down to 5 V or 3.3 V for the scanner's internal circuitry.
 

MrChips

Joined Oct 2, 2009
19,273
It is very unlikely that inductor L500 is at fault.
How did you conclude that there was a short in the OBDII cable?
 

Thread Starter

bdtprez

Joined Feb 24, 2018
2
It is very unlikely that inductor L500 is at fault.
How did you conclude that there was a short in the OBDII cable?
I did not see it because it was tossed, but i was told that the wires were frayed and when they were kinked a certain way thats when the device stopped working. They have a brand new cable for it now.
 
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