> On 6/28/06, Spehro Pefhany <speff@interlog.com> wrote:
>> At 08:29 AM 6/28/2006 -0700, you wrote:
>> > > I replaced one of of the (series string) bulbs with a (carefully
>> > > insulated) 1N4007 to knock the voltage down to 70% of line and get
>> several
>> > > years lifetime 24/7.
>> > >
>> >
>> >
>> >Which reminds me... Some projectors use 70V lamps. I assume they get
>> the
>> >70V by half wave rectifying the 120VAC line.
>>
>> That would give you ~85VAC. It also might be prohibited because of the
>> unpleasant input waveform. Maybe they regulate the voltage fed to those
>> bulbs.
>>
>
> Are you referring to film projectors?  They use a transformer for the
> bulb voltage. I believe this is to keep noise out of the audio
> preamplifier.  I found this out when I made an SCR dimmer for a
> projector.
>


Yes, it's film projectors. How would the lower voltage keep noise out of
the audio preamp? I DO remember seeing old 16mm projectors that used a
high frequency oscillator to run the sound exciter lamp. Never could
figure out why they didn't just use DC.

Harold

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