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'[EE:]Driving a siren]'
2004\04\17@131648 by Andrew Kilpatrick

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I've made high current drive for 12V halogen lamps up to 100W with
an IRF530 MOSFET. You will find that you need a voltage converter
between the PIC and the MOSFET. The way I did it was with a 2N2222A
transistor. This is wired something like this:


                         +12V          +12V
                         |              |
                         |              -------- LOAD
                         <              -------- LOAD
                         >  10K         |
                         <              | d
                         |___________|--
               1K        | c      g  |--   IRF530
PIC PIN ------/\/\/\----|/               | s
                    b  |\  2N2222       |
                         | e            |
                         |              |
                        GND            GND


This seemed to work very well. The trick is that the MOSFET must
have its gate switch between fully on (+12V) and fully off (0V)
otherwise it will get very hot.


Andrew



On Sat, Apr 17, 2004 at 07:31:31PM +0300, Lucian wrote:
{Quote hidden}

> {Original Message removed}

2004\04\18@113620 by Lucian

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Thank you all who answered to my siren question. Now I got it and hope
it will work :)

Lucian

-----Original Message-----
From: pic microcontroller discussion list
[spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU] On Behalf Of Andrew Kilpatrick
Sent: 17 aprilie 2004 20:16
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: Re: [EE:]Driving a siren]

I've made high current drive for 12V halogen lamps up to 100W with
an IRF530 MOSFET. You will find that you need a voltage converter
between the PIC and the MOSFET. The way I did it was with a 2N2222A
transistor. This is wired something like this:


                         +12V          +12V
                         |              |
                         |              -------- LOAD
                         <              -------- LOAD
                         >  10K         |
                         <              | d
                         |___________|--
               1K        | c      g  |--   IRF530
PIC PIN ------/\/\/\----|/               | s
                    b  |\  2N2222       |
                         | e            |
                         |              |
                        GND            GND


This seemed to work very well. The trick is that the MOSFET must
have its gate switch between fully on (+12V) and fully off (0V)
otherwise it will get very hot.


Andrew



On Sat, Apr 17, 2004 at 07:31:31PM +0300, Lucian wrote:
> 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
{Quote hidden}

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
{Quote hidden}

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2004\04\18@162658 by Russell McMahon

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> Thank you all who answered to my siren question. Now I got it and hope
> it will work :)

One extra caution.

In the circuit you attached to your answer (below) you MUST use a "catch
diode" across the load. This will "catch" any inductive current which is
being conducted by the load when the FET turns off and allow it to circulate
until the inductive energy is dissipated. Without this the voltage on the
FET drain will rise to a VERY high value and can destroy the FET and/or
cause noise spikes which get into the microprocessor circuit and cause it to
operate very strangely.

Select a diode which is rated for the FET maximum load current and which
operates OK at the switching frequency concerned. For many purposes cheap
low frequency power diodes such as IN400x (1 amp rated) or or 1N54xx family
(3 amp rated) work fine. You can use several diodes in parallel to get extra
current rating but it is best to use a single properly rated diode if
possible.

Connect the diode across the load such that it does NOT conduct when the FET
is on - that is, diode's anode to the FET drain and the diode's cathode to
the power supply positive.

Even though you are using a motor (siren) here rather thana pure inductor,
failure to use a diode as above will cause you endless problems.



           Russell McMahon



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