Naked Science Forum
Non Life Sciences => Technology => Topic started by: thedoc on 11/03/2014 18:30:01
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SHIRLEY DUTKE asked the Naked Scientists:
DOES TURNING THE LIGHT SWITCH ON/OFF THRU THE DAY CAUSE THE LIFE OF THE LIGHT BULB TO SHORTEN OR SHOULD ONE JUST LEAVE THE LIGHT ON?
What do you think?
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Yes switching incandescent lightbulbs on causes them to fail, [ thermal shock ] ,
but the additional electricity cost of leaving the bulb on 24/7 outweighs the cost of replacing an incandescent bulb.
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I agree the surge current thru a cold filament will be at least ten times the normal operating current which causes vibration and magnetic effects, that's when your lamp is going to fail, the operator's of the ENIAC vacuum tube computer found it best not to switch off the tube heaters as during switch on that was when failure's were most likely to happen.
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If a lamp is an important safety device, as on a crane or runway marker, it's best to leave it on 24/24 so that it is 50% likely to fail in daylight, rather than 100% likely to fail at night when it is needed.
Incandescent traffic lights and some theatre lights are kept on warm standby to avoid thermal shock, or a thermistor can provide a soft start - sometimes used in avionics.
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Some LED lights can turn on and off millions of times per second; there are research groups working on "LiFi" to transmit information to portable devices by sending digital signals over the room lighting. Telecommunications lasers turn on and off billions of times per second.
However, the same can't be said for the electronics which drive the LEDs. There is an inrush current when power is first applied, as capacitors charge up. Electronic circuits start up in a somewhat unknown metastable state which draws more current.
One estimate I saw suggested that if you need an electronic circuit to turn on and off just once per day, if you plan to use it more than 8 hours per day, it is more reliable to leave it running 24 hours per day. (I am sure that this answer will be slightly different for different circuits.)
Some circuits can enter a standby state which draws very little power, but avoids the damaging effects of inrush current.
All this assumes that someone else is paying the electricity bills, or that the cost of repairing the circuit far exceeds the lifetime cost of the electricity.
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In my experience thermistors cause more trouble than they save in the Siemens computers with which I used to work the PSU's incorporated thermistors and they were invariably the reason why the PSU's failed.
They also fail in CRT type TV's causing bizarre colour effects.