Comments on: Shine on, little bulb

Question: Shine on, little bulb

amicamentis — Tue, 30 Mar 2010 13:52:55 -0800

Is the light bulb in the microwave different than the ones in our lamps?

We've had our microwave for years. We leave the door open more than I'd like to admit. Yet the light still burns steady.

Our lamps, however, burn out like crazy after a few months or so.

What is the different? Is it because the microwave is a much lower wattage? Is it a different kind of bulb? If so, why aren't regular light bulbs made the same way? How does one replace a burned out microwave light bulb?

And how many hours does the typical microwave light live for?

By: MesoFilter

MesoFilter — Tue, 30 Mar 2010 13:56:12 -0800

A light that burns at a lower wattage will burn longer. I remember stories of 'the worlds longest lasting light bulb' - but they had less current than what would be needed to get it to 1 candle power brightness.

What about the light in your oven or refrigerator? They don't burn out that often either.

By: Mwongozi

Mwongozi — Tue, 30 Mar 2010 14:02:14 -0800

The light that burns twice as... *cough*

Yes, you can intentionally under-volt bulbs in order to make then last essentially forever, since the relationship is logarithmic.

Before the invention of the LED, this is what manufacturers used to do for indicator lamps, so that they would very rarely need to be replaced.

By: Mwongozi

Mwongozi — Tue, 30 Mar 2010 14:03:14 -0800

(Although I should add that the lamp in my microwave burnt out after a few years. It turns out that replacement lamps are unreasonably expensive. I now let my food cook in the dark.)

By: JackFlash

JackFlash — Tue, 30 Mar 2010 17:18:06 -0800

A typical 120 volt 100 watt household bulb is rated around 1000 hours. Microwave bulbs range from 1000 hours to 2000 hours depending on construction and wattage (typically 40W to 25W). Lower wattage generally means a longer life. These numbers are just averages. Individual bulbs could last half or twice as long.

To replace the bulb in the microwave you will probably have to open up the case by removing the screws on the back and lifting off the entire metal oven enclosure. There are some big capacitors inside that can give you an uncomfortable shock if you carelessly stick your fingers in the wrong place but nothing life-threatening. Just limit your touching inside to the bulb and its connections and you will be okay. The type of bulb can vary. You may be able to find a replacement on the internet using the numbers on the bulb or on a schematic inside. Or just go to an appliance repair shop and ask for what you need.

By: gjc

gjc — Tue, 30 Mar 2010 17:56:59 -0800

Minor correction- a light that is being under-driven will last virtually forever. The bulb's filament (versus the voltage applied) is what determines the wattage. Electricity is applied to it, and as it heats up the resistance increases until the bulb hits a point of equilibrium where it stops getting brighter and just sits there glowing.

So if you want to make a bulb last forever, get one that is designed for 240 volts and put it into a 120 volt socket/appliance. It will glow less brightly and more yellow, but since its filament was designed to "withstand" 240v and you are only giving it 120v, you aren't stressing it.

By: gjc

gjc — Tue, 30 Mar 2010 19:55:30 -0800

I worded that poorly- the filament combined with the supply voltage blah blah blah

Also, as JackFlash says, the bulbs are rated for many more hours and are most likely heavier duty than a 10 cent standard bulb. Combined with the percent of a day the bulb is actually lit, those 1000s of hours turn into years.

By: yohko

yohko — Tue, 30 Mar 2010 21:21:33 -0800

You used to be able to buy bulbs for automotive/shop work lights that were made with a thicker filament. I suspect that a similar type of bulb in used in a microwave.

By: hattifattener

hattifattener — Wed, 31 Mar 2010 00:31:01 -0800

Mwongozi: Actually, IIRC, the relationship is only polynomial, but it goes as the 12th power of the bulb voltage (so running a 240V lamp at 120V should make it last roughly 4000 times as long).

Ambient temperature also has a big effect; if the microwave bulb is in the airflow that cools the magnetron that might help it last longer too.

By: aimedwander

aimedwander — Wed, 31 Mar 2010 06:59:37 -0800

An under-volt bulb lasts a long time. A lamp with a sturdy filament last a long time. We like things to last a long time. Why aren't all lamps made like this? It's a trade-off between reliability and efficiency.