File puptcrit/puptcrit.0606, message 287

Date: Thu, 15 Jun 2006 21:44:16 -0400
Subject: Re: [Puptcrit] lighting questions for shadow puppetry

At 09:07 PM 6/15/2006, you wrote:
 >Thanks for these tips - the "two dimmers flipped" idea is what I've
 >done; it worked great with normal bulbs - and today I tried it with to
 >OP's and it worked well.


The use of a household dimmer obviously worked in your instance, but 
it may not in some overhead projectors.  Let me explain the issues;

Many projector bulbs are not 120 volts, but are as low as 12 volts 
(they do that because they can make the filament smaller if the 
voltage is lower).  Note that Wattage is a separate issue from voltage.

For the above reason, many projector bulbs are powered by a power 
adapter to bring the voltage down. The older style heavy transformer 
is now often replaced with a lighter weight electronic version.  The 
problem with these electronic versions is that they will actually try 
to maintain the same voltage to the bulb as you try to dim the power 
to them.  Though, at some point it can't compensate and the bulb will 
dim... but the electronics in the power adapter and your dimmer may 
be strained.  But, even on the older heavy transformer type, the 
dimmer gets strained by this type of load (called an inductive load).

I've know one person to regularly dim low voltage lighting using a 
household dimmer.  The bottom line is that he gets away with it, but 
still has to replace a dimmer occasionally.   Being careful to have 
more wattage capacity in your dimmer than you need is probably a big 
help. Your mileage may vary.

As for a mechanical shutter, one method of making it electric is to 
use the servos used in radio control cars. There are electronic kids 
(around $50) which will allow you to directly control them with a 
potentiometer knob. Then, you would use it just the same as you would 
the dimmer.

Joe Dunfee
Gordonville, Pennsylvania, U.S.A. 

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