Re: TDC DC

[P3D Michael Kersenbrock <michaelk@xxxxxxxxxxxxxxxxxx>]

> 1. Use DC. (you guessed that!).  This reduces flicker if you use a full
> wave rectification system on AC.  Of course, real DC is better!  This
> improves life.
> 
> 2. Use a capacitor in series with the bulb, or other source of slow turn
> on to avoid stressing the filament at turn on.  A 30 second time
> constant is fine.  This measure prolongs bulb life by avoiding the turn
> on surge.  Don't forget to surge limit the capacitor by current limiting
> in the power supply or an external series dropping resistor.  (At turn
> on, the capacitor is essentially a short circuit, some power supplies
> without current limiting will blow the fuse, the safer and more
> preferrable form of current limiting compared to frying the power
> supply, house wiring, etc.)

I'm a little confused.  If one uses #1's "DC" and puts a capacitor in series,
it seems that one gets no current though the bulb at all (except at
turn-on).  Capacitor charges up, and that's it.  Might the capacitor
been in parallel?

Correct me if I'm wrong, I think your #2 comments are for the situation of
a controlled-voltage DC power supply, not the full-wave rectified line
voltage as talked about in #1.  

Straight full-wave rectification plus a capacitor would yield about 160 VDC.
A  suitable dropping resistor between the rectifiers and the (parallel) 
capacitor would yield 120VDC (w/o flicker) and low bulb-surge voltage, 
but might be rather sensitive to bulb-to-bulb variation without adjustment
when bulbs are changed.

There are surge-protection thermistors that might be used for
surge protection.

Similar sorts of surge "protection" have been done for the last half
century by ham radio folk ("I are one" or used to be) for the filaments
of their power amplifiers.  Put a power resistor in series with the
bulb and a relay's winding accross the bulb.  Have the contacts of the
relay be across the resistor.  With suitable relay and resistor selection,
powering the projector lamp on  has limited current flow because
of the resistor -- but when the bulb heats up enough to have significant
voltage across it, the relay kicks on and brings up the voltage up the
rest of the way.  This "trick" would be of the same age as the stereo
projectors we use, so perhaps it's more "authentic" to do it this way.  :-)
This method has zero-drop (unlike the thermistor method) because the 
resistance is evenutally shorted out by the relay.  There are variations on
this scheme (most popular is a fixed-time delay on the shorting relay).

> 3. Alternate the polarity with each power on.  This improves life.

This sounds like a good way to get around the metal-migration problem I mentioned
earlier.  This might be "automated" by using a solonoid type latching relay 
(or whatever they're called, haven't seen one in years) that increments it's 
toothed wheel each time lamps are powered on.  

> 4. Regulate the current or voltage based on sensing at the bulb,
> preferably the sense wiring is soldered to the bulb.  Make sure that the
> controlled current or voltage is at or just slightly under the bulb's
> rating.  This is both a stability and long life improvement.

That sounds good.

Mike K.

P.S. - Not to sound like a party pooper, but another way is to just have
       a spare set of bulbs.  They don't burn out *that* quick do they?  :-)


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