6 Volt Lamp in Series with MOV

Are you sure it's not a Negative Temperature Coefficient (NTC) thermistor to protect the brake lamp from burning out when its filament is cold?

https://www.physicsforums.com/threads/ntc-thermistor-in-a-circuit.888226/

This is not an automotive application,it is on an old power supply,5Volt DC.

OK, I've never seen it, but it sounds like a good idea. A 40W bulb is a pretty large indicator light!

OT: Back in my younger years, I was stationed on an island, and in my spare time I was building an XY plotter. I was working on the stepper motor drivers and I needed some power resistors. The only place I could get what I needed was the auto parts store. Brake lamps make great power resistors that I knew could dissipate the power, and while the plotter was running, the sequencing lights were mesmerizing!

Some Amateur radios have a "lamp fuse" in the power supply.

It's a technique used in now-antique extra-low-voltage power supplies to reduce the inconvenience of a fuse blowing and needing to find and install a replacement. It still works.

This was in a rack mount power supply,so I don't think it was that old.It was in series with a MOV,not a fuse.I believe it was used to prevent damage if the MOV shorted to ground,and also to indicate a bad MOV. For a fast pulse,it was nearly invisible,having low resistance,allowing the MOV to do it's job,but in a sustained overload,the lamp would absorb the current and prevent damage to surrounding components.It would also be hard to ignore a 40 watt indicator,even with a reduced 5 volt supply.

There are rack-mounted power supplies well over 50 years old.

50 Years...1969? No vacuum tubes.All power transistors and diodes. Makes me think it is much newer than that. I will research some of the ccomponent part numbers just for S&G.

https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1962-07.pdf

How can an MOV be used to substitute for a fuse? They only conduct during the brief interval of a spike exceeding the designed voltage.Unless it is a very severe relatively long spike,fuses don't have time to react,whereas an MOV will. Please explain the application you are referring to.A schematic would be great.I appreciate your patience and instruction on this matter.

Er, um, it is the lamp that is used as the fuse replacement. See #4⇑.

I've seen an MOV placed in series with a gas discharge tube (GDT). For DC applications, I consider the GDT the nuclear option of surge suppression because once they fire they crowbar the bus to near zero until you run out of juice or pop the breaker or fuse. Maybe the lamp is there to provide a time (thermal) based tempering of the fault current?

I'm kinda surprised that the MOV was used in that low a voltage range. My preference would be for a Zener-based TVS as they tend to be a bit more 'accurate' for the hold-off voltage and clamping voltage down at those lower levels.

At higher voltages, MOV's do become attractive due to their power handing capability because you are dealing with a bulk material versus a semiconductor junction.

I agree.There are better ways to do it,but what I've got is what I've got. Just trying to figure out why it was designed as such.Maybe it was to provide a more tempered clamping of a sustained over voltage,and to reduce a dead short fault of the MOV to a manageable level by dissipating the energy slowly. A millisecond pulse would hardly heat up the filament to any degree, and not affect the MOV response time, but a shorted MOV would simply light it up and prevent further damage to surrounding components.I have seen MOV's explode and do severe damage to surrounding area.

We used to use incandescent lamps as a feedback element for an oscillator to generate a pseudo-sine wave. This would work for some applications that required a sine wave input, power supplies as I recall.