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

I saw circuits on the web using a digital pot to control a LM317 (variable=
=20
voltage regulator).
I also saw circuits on the web using PWM to control the bulb via a=
 transistor.

I tried the second method...using PWM from my microcontroller to control a=
=20
MJE3055T.  12 V supply is connected to one lead of the bulb, the other lead=
=20
of the bulb is connected to the collector.  the drain is connected to GND.

The transistor got _really_ hot and the bulb was hardly lit...it just=20
glowed a bit.  Do I just need a transistor with a higher amp rating?
Here are the specs for it...
VCEO Collector-Emitter Voltage (IB =3D 0) 60 V
VCBO Collector-Base Voltage (IE =3D 0) 70 V
VEBO Emitter-Base Voltage (IC =3D 0) 5 V
IC Collector Current 10 A
IB Base Current 6 A
Ptot Total Power Dissipation at Tcase =A3 25 oC   75 W

It seems like it should be fine.  60V and 10A.  What am I doing wrong such=
=20
that the transistor is heating up so much?


Or should I just try the LM317 approach?

The least parts the better...it seems like the transistor approach would be=
=20
best.  Can someone a circuit for this and also recommend the transistor to=
 use?

Thanks!

James Tu
james@2-bit-toys.com

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