> > >            0--------------------------0
> > >
> > >         FROM 12V                  TO LAMP
> > >         XFORMER                    STRING
> > >
> > >            0------+           +-------0
> > >                   |           |
> > >                   |           |
> > >                   |           |
> > >                   |           |
> > >                   D           D
> > >               Q1    G---+---G    Q2
> > >              Nch  S     |     S  Nch
> > >                   |     |     |
...

That circuit works just fine. Also the body diode is shorted by the FET
and the voltage drop is to be calculated wrt. the Rdson of the FET (FETs
conduct backwards just fine when turned on - not just through the diode).
So the circuit will dissipate 2*I^2*Rdson when on. At 10A and 45E-3R this
is 2E2*45E-3 = 9W and it will loose about 0.9V on the open devices. This
is comparable with a triac however (1.2-1.5V loss). Primary switching with
a SSR that will only be rated 2A at mains looks better than ever (about $8
in 1's last time I checked and will need no heatsink) ...

You could also use 10 VFETs in parallel (5+5) and reduce Rdson to 9E-3R
which will cut dissipation to about 2W (total) so the devices could run
w/o any heatsink (each would dissipate 0.125W). The voltage drop would
become 0.2V. You would need a push-pull driver to turn them on properly
however. I often use a 3-transistor push-pull dc amplifier (class B) set
to gain = 3. This serves as level converter from CMOS logic (PIC) and
drives many (many) FET gates in parallel without problems. The low gain
makes it suitable for PWM and switching up to 100kHz in despite of using
cheap transistors (1A,20MHz finals and a 0.5A,80MHz driver). A small trick
is used to avoid the forbidden state (output never sits at 1/2 Vcc).

Peter

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