Electronic – Why have a small neon lamp in power supply input circuitry

power supplyprotectionvacuum-tube

A couple of times when taking apart older bits of electronics, I've seen a small neon lamp about the size of a fuse (but its definitely not a fuse) positioned near the power supply circuitry. What is its purpose?

Is it used as some kind of input protection? Does it illuminate under fault conditions? Why not use a MOV or some other purpose designed component?

Best Answer

It is used as discharger for overvoltage conditions - in case of overvoltage a discharge starts through the lamp and that protects the main circuit from overcurrent.

A neon lamp is used because it is relatively cheap, very reliable and there's zero current through the lamp until the discharge actually starts.