I have a bike light controller that uses PWM to drive a mosfet to con=
trol a 20W
halogen bulb.  I just have 3 levels of dimming, but that's just a sof=
tware issue.
I will try to draw up the circuit after work today and post it...  I'=
ve been
meaning to document it for my friends who have the circuit anyway.  T=
he idea is
=66rom Willy Hunt (Google for Willy Hunt LVR).  I am using a PIC12F67=
5 with a LM2951
regulator and an IRF7353D1 mosfet.  During operation the FET does not=
 even get warm
to the touch.

Pete

James Tu wrote:

> What is the best way to control the dimming of a 12V bulb?  (I have=
 a 12VDC
> halogen bulb rated at 20W)
>
> I saw circuits on the web using a digital pot to control a LM317 (v=
ariable
> 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 co=
ntrol a
> MJE3055T.  12 V supply is connected to one lead of the bulb, the ot=
her 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 ju=
st
> 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 wro=
ng such
> 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
> best.  Can someone a circuit for this and also recommend the transi=
stor to use?
>
> Thanks!
>
> James Tu
> james@2-bit-toys.com
>
> --
> http://www.piclist.com hint: The PICList is archived three differen=
t
> ways.  See http://www.piclist.com/#archives for details.
>
>   -----------------------------------------------------------------=
-------
>
> ---
> Outgoing mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.532 / Virus Database: 326 - Release Date: 10/27/2003
>
> --
> http://www.piclist.com hint: The PICList is archived three differen=
t
> ways.  See http://www.piclist.com/#archives for details.

--
____________________________________________
Peter Donahue
AMTS (Code Monkey)
COM DEV R&D
P (519) 622-2300 x2439
F (519) 622-8706

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