I have a motor that draws just under 7A @ 12V and I’m driving it via a
MOSFET that I’m PWMing. What size flyback diode should I be using?
Same as the motor rating, i.e 7A?
Alan Burlison
I have a motor that draws just under 7A @ 12V and I’m driving it via a
MOSFET that I’m PWMing. What size flyback diode should I be using?
Same as the motor rating, i.e 7A?
I would say at-least the same as the motor involved.
Also, you’ll need to be aware that MOSFETs have implicit diodes
in their drain-source path so you may end up getting some conduction
via that path.
Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com
I concur with my colleague Benjamin
If the motor is rated at 7A then a 1N4007 is the diode to fit.
Cheers
AlexOn 28 May 2012 13:11, Ben Dooks bjdooks@googlemail.com wrote:
On Sun, May 27, 2012 at 12:04 AM, Alan Burlison alan.burlison@gmail.com wrote:
I have a motor that draws just under 7A @ 12V and I’m driving it via a
MOSFET that I’m PWMing. What size flyback diode should I be using?
Same as the motor rating, i.e 7A?I would say at-least the same as the motor involved.
Also, you’ll need to be aware that MOSFETs have implicit diodes
in their drain-source path so you may end up getting some conduction
via that path.–
Ben Dooks, http://www.fluff.org/ben/ bjdooks@googlemail.com–
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I would say at-least the same as the motor involved.
The biggest I could get hold of quickly over the weekend was a 6A
rectifier diode, the actual motor rating is 6.7A so I’m hoping I’ll
get away with that - I’ve put the diode and the MOSFET directly in the
fan airstream - with previous testing (with a smaller diode) the
MOSFET wasn’t getting warm at all, although I didn’t check the diode -
I’ll do that when I turn it on again. I’m hoping that as I’m only
(deliberately) PWMing at audio frequencies (< 3Khz) I’m not going to
get horrendous heating effects. If the diode does get warm I’ll
double up with two of them.
Also, you’ll need to be aware that MOSFETs have implicit diodes
in their drain-source path so you may end up getting some conduction
via that path.
From what I read [1] they don’t generally play much of a part because
they are relatively slow and have a high breakdown voltage. The
external one I’ve used is wired directly across the terminals of the
motor.
[1] http://www.physicsforums.com/showthread.php?p=2945567#post2945567
Alan Burlison
If the motor is rated at 7A then a 1N4007 is the diode to fit.
That’s only rated at 1A, the ‘7’ refers to the voltage (1000V) [1].
I’ve used a P600A [[2], 6A @ 50V.
There seems to be a widespread belief that you need a diode with a
fast reverse recovery time for use as a flyback diode, i.e. a
schottky. However someone has done a rather nice writeup that shows
that’s not true [3] - when active the diode is forward biased so it’s
the forward recovery time that matters, and with cheapo rectifier
diodes it’s just a few nanoseconds.
I’ve attached a couple of pictures of the build - it’s upside-down so
the motor can be seen, when in use it’s the other way up and blows a
load of thin coloured plastic streamers up into the air to give a sort
of ‘flame’ effect.
[1] http://www.fairchildsemi.com/ds/1N%2F1N4001.pdf
[2] http://www.vishay.com/docs/88692/p600a.pdf
[3] http://www.cliftonlaboratories.com/diode_turn-on_time.htm
Alan Burlison
I’ve used a P600A [[2], 6A @ 50V.
Tested it and neither the diode not the MOSFET get even remotely warm,
the thing that gets the warmest is the motor, and then only when it’s at
low speed (10%), probably because it’s not shifting as much air past
itself. Spent half an hour playing badly tuned scales on the motor and
randomly tapping on the piezo to make the fan whizz up 
Alan Burlison