Msg: 2121 *Conference*

03-24-92 09:33:54

From: DREW BLANCHAR

To : CORY HAWKINS

Subj: ELECTRONIC WIZ

Cory,
  You didn't say how smoothly you wanted to dim your battery operated light nor
how much money and room you want to invest.
  If the lamp is flurescent, forget about dimming.... trust me, you don't want
to spend that much time & money.
  If the lamp is incandescent, then you just have to add some resistance in
line between the batteries or from battery to lamp. The old rule of thumb says
you need 5 times the resistance of the lamp (v^2/watts) to dim it out enough to
call it black.  If you just want a half-bightness stage, just add a resistor
equal to the lamp resistance (v^2/w) and add a switch that will short out the
resistor for full bightness.  You can see that you could add a multiple contact
switch and put in or take out any number of resistors to dim the lamp.
  
  The trouble with using a rheostat or potentiometer is the power the pot has
to disipate when you're at very low resistance (near full brightness).  Since
the current will be near that of the lamp alone, you might be putting 1 amp
(assuming the lamp draws 1 amp) across a winding or carbon area of 1 ohm thus
heating it to  1 watt power which is too much for the very small area to handle
without vaporizing.
  
 If your lamps are higher power and you're using 12 volts, you might look into
the dash board light dimmer for Toyotas or Datsuns at your local auto wrecking
yard.  I know some model railroad owners who use these for their train speed
control units.
  
  Beyond the above, we're talking some fun electronics and a schematic via land
mail.
  Hope to read you again soon and good luck.
  -dB