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'[EE]:: Running bulbs on ACDC'
2008\03\20@104144 by Apptech

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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



2008\03\20@112840 by Martin

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Apptech wrote:
{Quote hidden}

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

2008\03\20@115253 by Harold Hallikainen

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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


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2008\03\20@120217 by Vasile Surducan

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On 3/20/08, Apptech <spam_OUTapptechTakeThisOuTspamparadise.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)

2008\03\20@131208 by Spehro Pefhany

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Quoting Apptech <.....apptechKILLspamspam@spam@paradise.net.nz>:

{Quote hidden}

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
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2008\03\20@131723 by Forrest Christian

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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}

2008\03\20@140424 by Dr Skip

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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}

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