I managed to find one just as I received your message!!!

The IRF740 from Fairchild is a logic level MOSFET capable of handling 400V
at 10A! Two or three of these and I am sorted! (I think) Seeing that they
are logic level can I just use a opto isolator in between the PIC and the
MOSFET or not?

Something else bothers me a bit. I have just read through the spec of the
motor and I don't think this motor can handle more than 220V DC. Our mains
supply is 220V Ac. Putting that through a bridge rectifier and a cap will
probably push it to 320 V DC!! which is miles to much. What can I do to get
this down to around 200V DC again?

Thanks,

Kevin

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Bob Blick
Sent: 13 April 2002 08:54 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]:High power DC switching..


> I forgot to ask you what the freewheel diode is for...

Hi Kevin,

When your transistor turns off, there is energy stored as inductance in
the circuit(primarily in the motor) that must be directed somewhere.
Otherwise it will turn into a huge voltage and kill your transistor. So
the freewheel diode conducts that energy back into the motor.

There are plenty of MOSFETs capable of 400 volts, you might need two or
three of them in parallel. Digi-Key is a good source of IR parts in the
US, don't know what you have there but there are other manufacturers.

You will need something (simple) to drive them from a PIC.

Big electric car motor controllers typically use a dozen or so MOSFETs in
parallel, also a dozen or so freewheel diodes. And a few Big capacitors
across the supply, not optional!

Cheers,

Bob

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