No, actually there are plenty of high current diodes out there in TOP-3
packages which would suit nicely too, but you're right, they're big but
not as cumbersome as the stud mount diodes.

Adios,
LarZ

---------------  TAMA - The Strongest Name in Drums  ---------------

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Gary Neal
Sent: Tuesday, 9 October 2001 10:58
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Mosfet speed control of 12V DC motor - Partially
Solved

Guys,

         Thanks for the help.  Flyback Diodes!  Daaaa!  Completely
forgot
that.  I put a 1N4004 (only 1A, but the biggest one I had lying around)
on
the motor and it worked like a charm.  I'm running at about 20khz.  This
diode only gets hot when the duty cycle is less than about 80%.
         I don't have a whole lot of experience sizing these flyback
diodes.  Someone said it should be rated for as much as the full current
of
the motor (~30A).  A 30A diode is HUGE!  These big diodes are all Stud
mount types it seems.  This doesn't seem very compatible to mounting to
the
leads of a motor.
         Can anyone give me some insight on what to use?  Potential
sources
for the product?  I'd like to keep it as cheap as possible.  Don't want
to
spend $20 on a single diode when I've only got about a total of $10 in
all
the other parts combined.
         Also, any other methods to help this reverse current from the
motor?  Slower PWM frequency (audible noise doesn't really matter to
me)?  Some configuration of caps?  I'm all ears when it comes to you
experts  :-)

Thanks,

Gary Neal


At 08:32 AM 10/8/01 -0700, Bob Blick wrote:
>Hi Gary,
>
>I notice you didn't say you were using freewheel diodes(in parallel
with
>motor).
>
>They should be fast rectifiers rated for at least as much current as
your
>MOSFETs. The heat dissipation will be as high or higher than your
MOSFETs,
>so you must heatsink them. With the low voltages you are using, you can
>probably use schottky rectifiers and that will cut the heat in half,
but
>it will still be substantial.
>
>You must have them. That's why parts are dying now.
>
>Cheers,
>
>Bob
>
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