I fully agree with Jerry in that a series resistor configured to drop the
operating voltage by 5% to 10% is more effective than a diode. Inrush
current is the killer for incandescent lamps.
73
Bob, K4TAX.
----- Original Message -----
From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
To: <tentec@contesting.com>
Sent: Thursday, February 14, 2008 10:24 PM
Subject: Re: [TenTec] Better Bulb Life in TenTec 229, 238 Tuners
> 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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