Troubleshooting Buzzing Incandescent Bulbs - What Causes Them and How to Fix It

[shoestring]

Occasionally I've noticed ordinary incandescent bulbs give off a buzzing sound before dying. Usually I've turned it off and tried to turn it on again, which was all it took to finally break it. Last time it happened I decided to leave it on, and it took around 15 minutes before it gave up on it's own. The sound is similar in character to the buzzing sound that can be heard from fluorescent lamps, but not as loud as some fluorescent lamps can be.

Is it possible that when the thin filament begins to burn out, it becomes so thin at some point, before finally breaking, that a small, local corona discharge or something like that could develop, or is there a better explanation?

 

[turbo]

Occasionally I've noticed ordinary incandescent bulbs give off a buzzing sound before dying. Usually I've turned it off and tried to turn it on again, which was all it took to finally break it. Last time it happened I decided to leave it on, and it took around 15 minutes before it gave up on it's own. The sound is similar in character to the buzzing sound that can be heard from fluorescent lamps, but not as loud as some fluorescent lamps can be.

Is it possible that when the thin filament begins to burn out, it becomes so thin at some point, before finally breaking, that a small, local corona discharge or something like that could develop, or is there a better explanation?

If you have an incandescent lamp on a circuit that contains a dimmer, you can often get a buzzing sound, since dimmers often chop up feeds into square-waves. That kills lamps.

 

[Bob S]

I have worked around ac-powered incadescent lamps near dc magnetic fields. The Lorentz I x B force makes the filament vibrate. Maybe the Earth's magnetic field interacting with the ac current in the filament has something to do with your situation.

Bob S

 

[vk6kro]

When the filament gets really thin, it is also very flexible at that point and this let's the filament oscillate due to magnetic forces caused by the flow of current.

It tries to oscillate at 120 Hz (or 100 Hz) because of the mains sinewave input.

When it does this, it hits the filament supports and this produces the sound you hear.

Have a look for this oscillation in the filament. It can be as much as a millimeter or so.

This is more vigorous with more current, which is why lamps tend to destroy themselves at switch-on, when a large current flows for a few cycles.

 

[shoestring]

Thanks for the answers! I don't have a bar magnet here at the moment, but perhaps the effect can be created by holding one close to a bulb?

 

[vk6kro]

It may be worth a try if you have a very strong magnet.

I have seen and heard that effect but it is quite rare. Mostly a light bulb just stops working at switch-on.

 

[Adjuster]

I have read that sometimes when a filament fails, the current will continue flowing for a short time, due to an arc bridging the broken section. I'm not sure how convincing this is, but perhaps it has actually been witnessed in lamps with clear envelopes?

 

[vk6kro]

I guess it depends on the construction of the lamp. If the filament was well supported, the ends may stay near each other and there could be some arcing.

I have seen lamps from cars where the gap in a broken filament was barely visible because the rigid filament ends had stayed in place.
Our 230 volt incandescent lamps hold the filament at widely separated points, so there is no chance of the filament staying in place once it had broken.

I even heard of an odd way of using dual filament lamps where one of the filaments had blown out (such as the brake/rear lights of a car).
You apply power to the good filament and the other filament behaves like the anode of a vacuum tube diode, so you can use it like a rectifier. Not a very good one, but a bit of a novelty.
It is possible you could get a similar effect while the filament of a lamp that had just blown was cooling down, but only briefly.