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Why have a small neon lamp in power supply input circuitry?

up vote 14 down vote favorite

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?

2 Answers

up vote 4 down vote accept

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.

up vote 3 down vote

On some old electronics (especially during vacuum tube era) the neon bulb was put in parallel with a fuse and used as a blown fuse indicator. Other times it was merely a power applied indicator (pilot lamp).


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Why have a small neon lamp in power supply input circuitry?

up vote 14 down vote

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?


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up vote 9 down vote

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.

edit

+the only downside to Neon lamps is the relatively high input capacitance (when open) and the high steady state voltage. MOVs and TVses go down to tens or single-digit picofarad ratings, and when they clamp, they clamp to a lower voltage with lower impedance. That is why eventually MOVs and TVSes won out. Otherwise a Ne lamp is functionally the same as an MOV. - user36129 May 22 at 15:22

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