# I need to energize a 25 watt incandescent bulb. When that bulb burns out another 25 watt incandescent bulb will energize

I need to provide a small amount of heat to my downstairs sink when we are away for the winter. A 25 watt incandescent bulb is fine for the job, but last year the bulb burned out at some point. I want to make a circuit that will sense when the bulb burns out and switch to another 25 watt incandescent bulb

• A better solution is to use a heat source that has a much longer life. Something like a heated floor mat, although everything I can find right now is smart, it may turn off after a power outage. Or, 2 light bulbs in series. At low voltage, light bulbs will last almost forever. Commented Oct 30, 2023 at 13:52
• Consider a 25 W wirewound resistor, which is extremely unlikely to burn out. Much simpler than the complexity of detection and switching. Commented Oct 30, 2023 at 14:14
• Any one of these power resistors screwed down onto a random piece of aluminum a few inches in area should last pretty much for ever. Commented Oct 30, 2023 at 14:45
• @jonathanjo If they want 25W of heat they don't want a 25W resistor, it should be overrated. Since it will be left unattended I'd say at least 2x rating or 50W. And usually they need to be heat sinked. Commented Oct 30, 2023 at 14:46
• XYproblem.info. Commented Oct 30, 2023 at 14:49

The lifetime of an incandescent bulb goes roughly inversely as the 14th power of the voltage (was a 'fact' I remembered from somewhere in my dark engineering past). If you use two bulbs in series, so each sees half the voltage, you can assume they will last 'for ever'.

With the bulbs lasting a very long time, the premise of the question becomes moot.

This wikipedia page gives the lifetime voltage exponent as between -12 and -16. I would imagine the range is to cover all the variations for vacuum to inert gas to halogen fill, wattage, envelope size, materials other than tungsten for the filament, etc. Whether the exponent is -12 or -16, 0.5 raised to that power is fairly big, and it doesn't matter to you whether your rated 1000 hours becomes 4096000 or 65536000 hours.

The power of a filament bulb, due to the large positive tempco of the filament, only goes as the 1.5 power of the voltage, not squared as for constant value resistors. You would therefore want two 40 watt bulbs in series to give you a total of 25 watts.

Those of you who are old enough might remember the brief appearance of '2000 hour' light bulbs on the market. They were designed to run slightly cooler, so lasted longer. They were in response to a feeling among consumers that light bulb companies were designing bulbs to fail quickly, in only 1000 hours, so that we would have to buy more bulbs. Because it ran cooler, the 2000 hour bulb was much less efficient. In terms of pure cost for light, it was cheaper to buy two 1000 hour bulbs and have them run more efficiently, the savings in electricity more than making up the cost difference. They only survived in niche applications where it was very expensive to change a bulb, for instance when the luminaire needed specialist access equipment to reach it.

This illustrates the benefit of analysing what is really needed. If you want light from a filament bulb, you need to operate it as hot as possible, until the lifetime becomes a problem. If you want heat from a filament bulb, you can run it much cooler, and it will last for ever.

• +1 for knowing what wattage bulbs to put in series to get 25W. Commented Oct 30, 2023 at 14:54
• @Seamus Isn't the whole point that they won't (or are extremely unlikely to) burn out? Commented Nov 1, 2023 at 4:11
• @Seamus If bulbs' filament burns out/through, that creates an open circuit. If bulbs' filament does not burn out/through, then current keeps flowing. My answer uses the 'does not burn out' technique. Commented Nov 1, 2023 at 8:31
• @Seamus, the low estimate for two 40-watt bulbs in series is that they'll last five centuries. On a merely human timescale, that's the same thing as "does not burn out".
– Mark
Commented Nov 2, 2023 at 2:30
• @Seamus Youneed a frame shift. Instead of thinking 'light bulb', think 'removable heating device with self-indicating function'. You can run this device at 240 V, when it will last for 6 weeks, and deal with the predicted failure, or run it at 120 V, when it will outlast your great great great grandchildren. However, this device is not rated for 120 V operation, because people usually want the self-indicating side effect rather than the heat, or have done in the past. Commented Nov 2, 2023 at 7:50

Rather than a lightbulb, there are better solutions. They're probably also cheaper than the extra parts you'd need to build a suitable mains-rated circuit.

Low wattage heaters: There are various low-power heaters designed for this application. I know them as tube/tubular heaters. They're also sold for use inside pianos and greenhouses. The latter have a higher power rating, but also have a thermostat so they won't run more than necessary.

Trace heat tape: There's also trace heat tape which is wrapped around the pipes at risk (typically in attics and outbuildings). It delivers the heat exactly where it's needed, and some types can be applied under insulation, but it's not so useful if you have tanks, taps, or drain traps to consider.

Trickery with lighting products: If you really want to build something, I'd connect the 2nd bulb to a dusk-to-dawn sensor illuminated by the first bulb, in a darkened room: when the first goes dark, the 2nd comes on. If the room can't be darkened enough, restricting the light that can fall on the sensor would help.

• As no one else had given the frame challenge answer, I though I would - but a way of doing what the OP actually asked for as a bonus, if the room can be kept dark. Commented Oct 31, 2023 at 13:11
• Another option for buying low wattage heaters is small heating pads for reptiles. They're available in flexible mat form for \$10-20, often with a thermostat. Commented Nov 1, 2023 at 16:03
• I hadn't even known about tubular heaters that use a lamp socket. Thanks for this recommendation! It sounds ideal for OP, and a good way to keep my bathroom a bit warmer throughout the winter. Cheers! Commented Nov 1, 2023 at 17:30
• @lucas I've never seen them with lamp connectors in the UK but do have an ancient (and probably illegal these days) connector I'd be happy to use for suitable currents. Commented Nov 2, 2023 at 6:29
• The trace heat tape is the correct "smart" solution. The intelligence is in the termperature coefficient of the metallic conductors, that regulates power such that it mainly produces heat when the temperature is low. We don't just worry about your bulb burning out in winter, but also about wasting energy which in most cases is linked to CO2 emissions. Besides, if you are familiar with the heroic efforts of Edison cs to build a lamp with a usable service time, it is a shame to use light bulbs just to produce heat. A piece of cheap iron wire will do for you, just add some isolation Commented Nov 2, 2023 at 16:59

You can of course build a current-detection with a relay to switch over, but I see some challenges. What happens when the second also burns out?

An alternative solution can be to put 3 10W lamps in parallel, when one blows you still have 20W, till you check it.