At 03:10 PM 2/10/2004 -0300, you wrote:
>You could use a simple N-channel mosfet for driving each lamp. No need for
>zener or transistor this way. Gate is connected to PIC (maybe through a
>470ohm resistor), Source is to ground, and lamp is between +12V and the
>Drain.

Maybe, though in some applications (automotive and similar) having one
side of the load grounded is a requirement (and it saves a wire), hence
the popularity of (more complex and expensive) high-side drivers.
You also need a LOGIC level MOSFET to drive a low-side switch directly
from a PIC (ignoring the possibility of using, and protecting, the open
drain output, if present).

>Note that few mosfets contain a protection diode that will directly
>polarised when the supply is inverted, so all the lamps will be lit.

Do you know of any that don't? It's not a protection diode, but an
inherent part of the MOSFET structure (called a "body diode"), except
for rare beasts that have the substrate brought out independently (4 pins).

Note that you can use a MOSFET as a polarity protector. This application
uses the MOSFET in reverse (the body diode is shunted by Rds(on)). The
following circuit is covered in a US patent (6,043,965 ), assigned
to General Motors:

http://www.speff.com/60439565.gif

If you have a serious application of this, you might want to look for
prior art, it's pretty obvious to those "persons skilled in the art".

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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