>What is the best way to control the dimming of a 12V bulb?  (I have a 12VDC
>halogen bulb rated at 20W)

Depends on the environment.  I recently had call to set up a dimmer
for 5 lightbulbs in parallel, and to do so I used a 100k pot tied to the
base of a TIP123 (IIRC) and connected the pot and collector to +V
and the lightbulbs between the emitter and ground.  It does get
warm, but not actually hot.

>I saw circuits on the web using a digital pot to control a LM317 (variable
>voltage regulator).
>I also saw circuits on the web using PWM to control the bulb via a
>transistor.

PWM is the most enjoyable method, for a PIC type, anyway.  It
didn't work in my case because the lights were inside a Faraday
cage which was their to keep EMI out!

>I tried the second method...using PWM from my microcontroller to control a
>MJE3055T.  12 V supply is connected to one lead of the bulb, the other lead
>of the bulb is connected to the collector.  the drain is connected to GND.

Are we talking about a MOSFET or a BJT?  BJT's don't have drains...

>The transistor got _really_ hot and the bulb was hardly lit...it just
>glowed a bit.  Do I just need a transistor with a higher amp

Define really hot.  Were you unable to touch, unable to hold it for
5 seconds, 10 seconds, what?  Did you put a heat sink on it?

That's pretty important.  Even if it's rated to handle a certain
current/voltage (from which you can calculate the wasted power),
it is still going to heat up.  You CAN'T drop a volt or two across
the CE and be switching a fairly high current (for a 12V, 30W bulb,
you're talking 4.8 Ohms, so 10/4.8 = more than 2 amps) without
generating some waste heat.

I'd say your simplest solution is to use a BJT with a pot connected
to it.  My dimmer used 3 components:  the BJT, a 1/2 W pot, and
a .25 W resistor tied to the pot to limit the current a bit.

Mike H.

>James Tu

_________________________________________________________________
Our best dial-up offer is back.  Get MSN Dial-up Internet Service for 6
months @ $9.95/month now! http://join.msn.com/?page=dept/dialup

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.