Post by thread:[EE]:: Running bulbs on ACDC

Any thoughts on the longevity of tungsten bulbs running on - AC supply - Same supply but full wave rectified. - Same supply but full wave rectified and filtered to DC. The bulb is in fact two 12V bulbs in series running from a nominal 16 VAC supply. In the second case the filament has a constant polarity bias but is exposed to cycles of heating and cooling which intuitively (to me) feel the same as for AC. In the third case the DC voltage will tend to be higher than the RMS AC voltage so the bulb will operate at a somewhat higher wattage. Russell

Apptech wrote: {Quote hidden}> Any thoughts on the longevity of tungsten bulbs running on > > - AC supply > - Same supply but full wave rectified. > - Same supply but full wave rectified and filtered to DC. > > The bulb is in fact two 12V bulbs in series running from a > nominal 16 VAC supply. > > In the second case the filament has a constant polarity bias > but is exposed to cycles of heating and cooling which > intuitively (to me) feel the same as for AC. > > In the third case the DC voltage will tend to be higher than > the RMS AC voltage so the bulb will operate at a somewhat > higher wattage. > > > > Russell > > > Surely you mean AC/DC? =) .. The logic I've heard is that when run from DC, the metal on the filament evaporates and redeposits on the + side of the filament, reducing it's lifetime. I'm not sure how much I believe that.. Rectified seems as though it would be the same as DC regarding this supposed redeposition action. AC was (is?) considered the best. Of course higher wattage would reduce the bulb life regardless of other factors. - Martin

I believe the filament evaporation being uneven (as mentioned earlier on this thread) shortens the life of the filament when running on DC. On the "rectified and filtered" choice, it depends on what the "and filtered" means. If it's just a capacitor across a full wave rectifier output, the capacitor charges to the peak voltage, yielding a DC voltage that is sqrt(2) times the RMS voltage. The lamp will last a shorter period of time with higher voltage. I don't think the filament cools enough between peaks on AC to stress the filament from repeated heating and cooling. However, inrush current from starting the lamp with full voltage when the filament is cold causes significant current and significant magnetic fields that can destroy the lamp filament. A "soft start" of the lamp can significantly increase the lamp life. I believe lamp manufacturers have graphs of lamp life versus applied voltage and lamp "efficiency" (lumens per watt) versus applied voltage. As the voltage drops, life increases and efficiency deceases. The rated voltage is a compromise between these two. Note that lumens per watt, in my opinion, tells us how much of the input power is converted to optical power in the visible spectrum (weighted for frequency response of the eye). There is a fair amount of optical radiation outside the visible range, especially in the infrared. As lamp voltage is decreased, more of the input power shows up in the infrared. Harold -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available!

On 3/20/08, Apptech <spam_OUTapptechTakeThisOuTparadise.net.nz> wrote: > Any thoughts on the longevity of tungsten bulbs running on > > - AC supply > - Same supply but full wave rectified. > - Same supply but full wave rectified and filtered to DC. > > The bulb is in fact two 12V bulbs in series running from a > nominal 16 VAC supply. > > In the second case the filament has a constant polarity bias > but is exposed to cycles of heating and cooling which > intuitively (to me) feel the same as for AC. > > In the third case the DC voltage will tend to be higher than > the RMS AC voltage so the bulb will operate at a somewhat > higher wattage. Depends who are the manufacturer of the bulbs. AC seems a good option with power on from zero to nominal current in 30 seconds or more. Many options for this feature, I've seen the triac version.Turning OFF untill the filament becomes dark red and keep it warm. Most of the filaments (for well supplied bulbs and not huge spikes on power ON caused by aged switches) are crashing due a mechanical stress (cold to hot or hot-cold-hot too fast)

Quoting Apptech <.....apptechKILLspam@spam@paradise.net.nz>: {Quote hidden}> Any thoughts on the longevity of tungsten bulbs running on > > - AC supply > - Same supply but full wave rectified. > - Same supply but full wave rectified and filtered to DC. > > The bulb is in fact two 12V bulbs in series running from a > nominal 16 VAC supply. > > In the second case the filament has a constant polarity bias > but is exposed to cycles of heating and cooling which > intuitively (to me) feel the same as for AC. > > In the third case the DC voltage will tend to be higher than > the RMS AC voltage so the bulb will operate at a somewhat > higher wattage. > > > > Russell Life should be better with the same RMS voltage AC, particularly if the filament is relatively thin. DC is known to cause "notching" (there's your search term) on tungsten filaments, and there are modified tungsten filaments that have better life on DC. Eg: http://www.harison.co.jp/english/p_pro1_info.html So, I predict average life will be longest for a), shortest for c) with b) in the middle. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" s...KILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

Others have dealt with your three cases, but there is an additional one: - Same supply, but half-wave rectified. My understanding is that this (perhaps significantly) increases the longevity of at least household-grade bulbs, at the expense of at least some lumens. There were products on the market which basically fit in the light socket and contained a diode internally. Not sure how well this actually worked, but worth a shot. I will say that running bulbs even slightly below their rating significantly improves their lifetime. Most long-life bulbs are actually 130V bulbs being run at 120V. -forrest Apptech wrote: {Quote hidden}> Any thoughts on the longevity of tungsten bulbs running on > > - AC supply > - Same supply but full wave rectified. > - Same supply but full wave rectified and filtered to DC. > > The bulb is in fact two 12V bulbs in series running from a > nominal 16 VAC supply. > > In the second case the filament has a constant polarity bias > but is exposed to cycles of heating and cooling which > intuitively (to me) feel the same as for AC. > > In the third case the DC voltage will tend to be higher than > the RMS AC voltage so the bulb will operate at a somewhat > higher wattage. > > > > Russell > > > >

Yes, I've done that. They'll outlive me.... Plan on using the next bigger size to get near the original lumens, and having a warmer color temp. Forrest Christian wrote: {Quote hidden}> Others have dealt with your three cases, but there is an additional one: > > - Same supply, but half-wave rectified. > > My understanding is that this (perhaps significantly) increases the > longevity of at least household-grade bulbs, at the expense of at least > some lumens. There were products on the market which basically fit in > the light socket and contained a diode internally. Not sure how well > this actually worked, but worth a shot. > > I will say that running bulbs even slightly below their rating > significantly improves their lifetime. Most long-life bulbs are > actually 130V bulbs being run at 120V. > > -forrest > > Apptech wrote: >