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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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  • \$\begingroup\$ was it in the primary winding circuit or any high voltage secondary ? \$\endgroup\$ – greg May 27 '14 at 14:00
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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.

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    \$\begingroup\$ +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. \$\endgroup\$ – user36129 May 22 '14 at 15:22
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    \$\begingroup\$ You will also encounter these on old network-interfaces (for coax networks). For exactly the same reason. To discharge over-voltage that can buildup in the coax cable by itself or is caused by static discharge from people touching the connectors. \$\endgroup\$ – Tonny May 22 '14 at 15:23
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Another perspective would be for safety during troubleshooting. From the venerable Bob Pease:

So, whenever I start work on a high-voltage circuit, I tack in a neon lamp in series with a 100k resistor across the high-voltage busses. Then when I see the neon's glow, I'm graphically reminded that this really is a high-voltage circuit, and that the power is still ON (I don't care what the power switch says) and I should revert to the mode of High-Voltage Cautions. If I grab onto a really hot wire, the shock might not injure me, but I might convulse and jerk backwards. That's not a good idea if I'm standing on top of a ladder, for instance. So, looking for the glow of a neon lamp is a way to remind me to be serious, and I recommend it for you, too.

From http://electronicdesign.com/electromechanical/whats-all-reflex-response-stuff-anyhow

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  • \$\begingroup\$ Glad to hear Bob Pease quoted. +1 just for old times. Another +1 because he knew how reality connected. Any young timers, do a search on this venerable old man. And the answer is quite correct. \$\endgroup\$ – Marla May 23 '14 at 0:46
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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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