On Mon, 18 Feb 2002, Dave Dilatush wrote:

> Dale Botkin wrote...
>
> >My question relates to how to switch the 12VAC to the lights on and off.
> >I'm thinking a sensitive-gate (logic) TRIAC...
>
> TRIACs.  Ugh.  Voltage drop.  Lost power.  Unclean.

Do I sense you don't like TRIACs?  8-)

> No one ever said you can't use MOSFETs on AC; you just have to be a
> little clever, is all.  Usually, that means you end up having to use two
> of them in series-- one for each half-cycle-- because of the intrinsic
> body diode that shunts the drain and source.
>
> The circuit below uses no voltage-dropping, power-wasting TRIAC, no
> likewise bridge rectifier, no relay^H^H^H^H^H "electromechanical logic",
> and allows you to control your lamps either ON/OFF with a logic level
> from a PIC (might want to boost that up to a little higher than 5V,
> unless you use logic level FETs), or control lamp brightness with the
> PIC's PWM.
>
>            0--------------------------0
>
>         FROM 12V                  TO LAMP
>         XFORMER                    STRING
>
>            0------+           +-------0
>                   |           |
>                   |           |
>                   |           |
>                   |           |
>                   D           D
>               Q1    G---+---G    Q2
>              Nch  S     |     S  Nch
>                   |     |     |
>  GATE DRV  0------------+     |
>                   |           |
>                   |           |
>  PIC GND   0------+-----------+
>
> Voila...

Voila indeed.  Thanks, Dave, this is an awesome idea.  I had thought of
anti-parallel MOSFETs, but not anything nearly as elegant.  I'm so used to
using bridge rectifiers on a transformer, I just didn't think of making
one side ground.  As a bonus, it looks like I could use one more pin and
turn on just one MOSFET to drive the lights with half-wave rectified DC.
Hmmm.

I took a quick look at Digikey - 2SK2614 looks like a winner.  50V/20A,
less than .08 Ohm Rds(on), 4V gate drive and a tad over a buck each.  I
can drive the gates with RA4 and a pull-up.

Dale

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