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?

Insufficient base drive. You need to give it around a tenth of the collector
current to get it really saturated. The Darlington configuration is an
inefficient way of doing this (it raises the saturation voltage by one
Vbe drop, which means the losses go up several-fold).

A Sziklai pair is one way but you'll be drawing 200mA from the 5V supply for
a 2A (24W) load. Or just use a garden variety power n-channel MOSFET (but=
 make
sure to either do the PWM slow or provide a proper gate driver or you'll=20
reduce
the efficiency due to the large  gate capacitance.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the=
 reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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