Thank you for your answers. I think I found an appropriate solution to
my problem: an IRF630, which is rated 200V, 9A and pulsed current 36A so
I think it's ok for a siren. No I have to manage to find a circuit to
control it from a PIC pin, because I guess it's not correct to drive the
MOSFET directly.
Am I right ? What protection do I have to add ? The FET already has in
it's internal structure a diode, so I could only put a resistor in
series with the pic pin ?

Lucian

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Herbert Graf
Sent: 17 aprilie 2004 18:41
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:]Driving a siren

> Dear Piclisters,
>
> I have yet another question for you. I am still a newbie in
electronics
> and PIC's and I feel a little bit shamed that I only come to ask
> questions on this list and rarely know how to answer to other
questions,
> but hope that someday I will be able to be more active here.

        Please, do NOT feel ashamed. EVERYONE here was, at one time, a
"newbie",
and did what you do (it's called lurking), in fact many PICLIST members
still mostly lurk, and there is NOTHING wrong with that, it's why the
list
is here IMHO.

> My questions is: I want to drive a siren from a microcontroller pin.
The
> siren is an alarm one, and consumes 12V and 3A at maximum. Which would
> be the best way to control it (on/off) ? A power MOSFET, a triac
maybe,
> a relay ?
> Thank you for your answers.

        Well the "best" way depends on you and your situation. IMHO the
EASIEST way
is a relay, a simple jelly bean transistor (say a 2N2222 or 2N2904)
driving
a relay. The reason I say it's "easiest" is it completely electrically
isolates you from a devices that may do very ugly things on it's power
lines
(sirens can be very inductive causing huge power spikes on the lines)

        The relay solution however isn't the most elegant, uses more
space then it
has to and wastes power. The most efficient way would be a MOSFET, use a
logic level MOSFET that can handle at least 5A (I'd say get one that
does
10A, the siren is speced for 3A but I'm willing to bet it's surge
current is
higher). Check out the company International Rectifier for good parts,
their
IRFZ line should be what your looking for, if memory serves.

        TTYL

----------------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

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