On Wed, 2008-02-13 at 17:10 -0800, Jim Brown K9YC wrote:
> I do several things to make these bulbs last longer and get more
> brightness from lower current (long life) bulbs. First, I put white
> tape on the inner black panel below the dial and around the bulbs
> (all the black paint that isn't visible through the glass) to better
> reflect the light up to the tuning dial. That improves brightness.
> Second, I use 756 bulbs, which draw only 80 mA at 14V and are rated
> 15K hours. (The "standard" 1815 bulb draws about 300 mA and is rated
> only 3K hours.) Third, to further improve life, I run them at
> slightly lower voltage by adding one or two forward-biased 1A diodes
> in series with the DC feed from the RCA connector.
>
> If you don't have 756 bulbs, you can still add multiple diodes in
> series with the standard 1815 bulbs. That will also reduce brightness
> a bit, and improve life quite a bit.
>
> FWIW -- I've tried multiple LED's in each opening with series R to
> limit current in one 229 tuner. LED's last OK, but I have not been
> happy with the result visually.
>
> 73,
>
> Jim K9YC
>
>
What we did at Collins with #327 (a short lived 28 volt 40 ma lamp) was
to wire a 56 ohm resistor in series with each lamp. And then I
discovered the 387 that sacrificed light for life. We still used that
56 ohm resistor. So we dropped the lamp voltage about 8%. A rule of
thumb I learned about lamps from a GE lamp data book since then is that
when you lower the lamp voltage by 5%, you double the life and cut the
light by 10%.
So at 12 volts with 300 ma lamps, the appropriate resistor would be 3.9
ohms or 15 ohms for the 80 ma lamp. I like a resistor better than a
couple diodes because the resistor will limit the inrush current to the
lamp. The resistance of tungsten changes 15 to 16 times from cold to hot
(depends on how hot is hot) so the inrush current without limitation is
15 to 16 times greater than the operating current. So that 14 volt 300
ma lamp has a hot resistance of 48 ohms and an inrush of 4 amps. That's
hard on the filament. A cold resistance of 3 ohms. Wiring 3.9 ohms in
series cuts the inrush current more than in half. Adding two diodes
drops the inrush current about 10%. I don't know data on how much effect
current limiting has, but every time an incandescent lamp fails it seems
to be the instant its turned on. That inrush is why (unless the lamp
gets dropped, hot or cold).
So the resistor lengthens lamp life by lowering the operating
temperature and by significantly reducing the cold inrush current.
73, Jerry, K0CQ
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