componentsidentification
When opening up a car alarm system I found this strange-looking component. I thought that it could be something which is light dependent, but the entire board is enclosed in a black plastic box and the alarm should trigger before the thief reaches the depths of the electric system and thinks of opening it.
Here are some photos:
What component is this?
This is a Hongzhi Enterprises (HEL) brand metal-oxide varistor.
The website doesn't mention pending safety approvals, but this part has both UL Recognized component and (what looks like) CSA marks on it. It's not surprising that the website is a little out of date, I've seen it quite often with offshore smaller suppliers.
I've never used this brand of part, so I cannot speak for its robustness.
Specs:
I agree with others that this is most likely intended as a transient clamp meant to suppress the inductive kickback of a de-energized relay coil.
They are Panasonic's "Coil Type EMI Filter" as given in Armandas' answer. I just wanted to add some extra info.
They are mostly T type LC filters:
simulate this circuit – Schematic created using CircuitLab
The numbers indicate the capacitance inside the module. So, "222" means 2200pF. The module acts as 3rd order LPF and each model has a different cutoff frequency:
Panasonic has stopped manufacturing those, so the whole series became obsolete. But if you want to replace it (unlikely) you may be able to find at Digikey.
Best Answer
It's a piezoelectric disc buzzer with a weight. It's being used as a vibration sensor, not a sound generator.
The weight amplifies the strain produced by vibration.