PWM-Mosfet-Motor?

Hi Guys,

I will be working on a motor control project a bit later this summer. The motor I will be driving is 6 Volts @ 8 Amps. I only need it to run in one direction but I would like it to be variable speed. So, I figure I could use PWM output from a PICAXE and feed that into a logic level N-Channel Mosfet to switch the motor on and off.

As I don't know much about electronics, I am not sure if there is anything else I need in the circuit. I suspect I need a diode to route EMF back to the Positive, but beyond that????

Any helps or suggestions would be great, even a link to a motor driver circuit somewhere would be great as well!

Thanks a bunch

Chris

A 10k to 33k resistor from your output to
the MOSFET gate may save your picaxe in case
of a total MOSFET failure.
And a 1M resistor from the gate to neg is
usually a good idea also.
And the diode is a must.

I agree that a resistor should be used between PICAXE and gate but I would go for a much lower value such as 220R. MOSFETs have a fairly high capacitance on their gates which needs to be charged/discharged each time they are turned on/off. When used as a switch, this is not usually a problem. However, when used for PWM, the switching is much more frequent and the MOSFET will spend a lot more time in it's linear mode which will produce a lot of heat when driving a heavy load such as a motor.

Thanks guys!

Chris

That's useful advise concerning connecting the gate to ground with a 1M ohm resistor. I though that I had damaged the mosfet because it stayed switched on all the time once I applied a voltage to the gate. It took me days and the purchase of extra mosfets to realise the problem. I suggest that this info be available in the interfacing section of the manual.

Regards
dermot

There are also function specific low level to mosfet driver ICs which push up and pull down mosfet gates with more vigor - sounds like an excerise program... however they work!

There are also function specific low level to mosfet driver ICs which push up and pull down mosfet gates with more vigor - sounds like an excerise program... however they work!

a complimentary NPN/PNP transistor pair can also be used to charge/discharge the gate. The capacitance value can be found on the mosfet spec sheet. The diode across the motor MUST be a fast device, the common 400X devices are no where near fast enough AND WILL NOT DO THE JOB.
cheers BobR

Assuming a logic level mosfet, add a 4v7 zenor between SOURCE and GATE. This along with the 220R resistor helps prevent latch-up of the Picaxe. Latch-up occurs when the capacitive coupling backfeeds during flyback.


Evan

WOw, thanks for the ongoing guys, much appreciated.

Chris

Chris,
Sounds like you may have the same thoughts as I do about motors and PWM except this fun project of mine uses a DC motor rated for 24 vdc @ 16 amps. Currently it's being pulsed at about 500Hz. It's a tiny tiny bit noisy as you can hear the 500 hz. PWM buzz!!! But it's fun experimenting

http://www.members.westnet.com.au/page3/

Hope the link works.

Regards,
HamRadioAddict

Chris,
Sounds like you may have the same thoughts as I do about motors and PWM except this fun project of mine uses a DC motor rated for 24 vdc @ 16 amps. Currently it's being pulsed at about 500Hz. It's a tiny tiny bit noisy as you can hear the 500 hz. PWM buzz!!! But it's fun experimenting

http://www.members.westnet.com.au/page3/

Hope the link works.

Regards,
HamRadioAddict

Slightly off topic -

bobrayner, you mentioned that the 1N400X series diodes are too slow, I'd been using a 1N400X series for years thinking it was ok (I did fry and A/D two weeks ago, but I don't know if that was back EMF or my computer PSU going out of spec under heavy load - a heafty motor attached). What should I be using and is there any easy way to test it's effeciency without a scope?

Matthewriley
MUR1560 or similar is a good choice. Rated 600V at a foreward current of 15A. Described as a fast recovery switchmode rectifier and has a quoted recovery time of 60ns (pretty fast). TO220 package. Whichever device you use it has to be capable of the same current as required to operate the motor (the collapsing field will generate this current briefly at mosfet turn off). Hope this helps.
cheers BobR

Matthewriley
Bit of a PS. It is usually the switching (mosfet?) device that suffers when the reverse voltage spike produced by the collapsing field exceeds the switching device ratings. If you are using a dedicated driver IC it is quite possible the "flywheel" diodes are built in. On the other hand if you are driving a mosfet without a driver IC or a transistor pair the switch on / off time would be quite slow and a 1N400X diode may be OK. Not a good idea though as the switching device would get pretty hot. The name of the game is to get a mosfet on and off in the quickest possible time so it does not spend half of its life trying to get rid of power generated heat.
cheers BobR

Thanks Bob. I'll keep your recommendations handy for any future projects - I have some IRF530's and IRF9530's waiting to be used, just not sure what at the moment.