An SMD inductor fell of the board. It is marked L12. I have another working board and the are no markings on it, it's tiny and black. I touched the two points with a screwdriver for a second and the board LEDs lit up.

The device is an Extralis Vesda VLF-250.

How can I find what value to replace it with or is there a generic value to use?

I can't find a schematic.

Does anyone have a solution?

  • \$\begingroup\$ Putting a screwdriver across the pads is not advised. There’s a fair chance you’ve caused overvoltage to the circuit. l12 is probably 12uH. \$\endgroup\$
    – Kartman
    Apr 12 at 11:54
  • 3
    \$\begingroup\$ You could measure it with a multimeter. But given that there are no schematics, let the manufacturer repair it, since it is a smoke detector for business critical environments. You don't want to end up having a dodgy smoke detector in a business critical environment. \$\endgroup\$
    – Justme
    Apr 12 at 12:03
  • \$\begingroup\$ "I touched the two points with a screwdriver for a second and the board LEDs lit up." Good God! takes of glasses and stares \$\endgroup\$
    – winny
    Apr 12 at 12:06
  • \$\begingroup\$ Inductance can be measured. \$\endgroup\$
    – user253751
    Apr 12 at 12:07
  • \$\begingroup\$ Gordon Johnston - Hi, Is "L12" marked on the PCB near to the component (which is what I expect you mean) or is this a marking on the component itself (which is how some other people are interpreting your question)? Please clarify (and, if possible, please edit your question and add a photo of the component and that area of the PCB). Thanks. \$\endgroup\$
    – SamGibson
    Apr 12 at 12:28

2 Answers 2


Most likely L12 is just a designator. It gives zero useful information about the characteristics of the inductor or ferrite bead unless you have the bill of materials for the particular circuit board in question. SMT inductors (except relatively large ones) are seldom marked.

If it's used as a filter (perhaps it is a ferrite bead), there are generic values that would probably work okay. If it's part of a switching power supply, replacing it randomly could well cause a lot of other damage.

A clear photo of the part and the general area of the PCB from which it was removed might allow more specific advice.


If it's L12 on the PCB, then impossible to tell the inductance. If you measure the size of it and compare to similar or identical ones, you can probably get a good estimation of saturation current capability of it and select a similar one.

To tell for sure, get yourself an LCR meter, unsolder the other one and measure the inductance. Choose one with same size and your chance of it not saturating and working should be high.


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