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Thanks everyone for the wonderful advice.

I think I will go using PWM with a logic-level FET.

-James
P.S.  a drain in a BJT?  It was late...I think there was a drain in the=20
B-R-A-I-N.  :)


At 03:34 AM 12/5/2003 -0800, you wrote:
>What is the best way to control the dimming of a 12V bulb?  (I have a=20
>12VDC 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=20
>lead 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=20
>be best.  Can someone a circuit for this and also recommend the transistor=
=20
>to use?
>
>Thanks!
>
>James Tu
>james@2-bit-toys.com
>
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James Tu
james@2-bit-toys.com

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