Mosfet selection for DC motor Control

Hello

I am attempting to control a small DC motor (on/off one direction only) with an avr microcontroller. After extensive googling it was obvious that a mosfet was needed + pwm control.(relays and solid state relays were excluded and triacs alternistors scrs and quadracs were too exotic to consider) Unfortunately the number of mosfets is enormous and most datasheets do not contain the necessary information (for the uninitiated) to make a decision on which mosfet to use. Could anyone please indicate any book www-site application note or news posting that provides information on how a mosfet for an application is selected? Moreover assuming that the load is a light bulb and we dont want any kind of protection for the mosfet ,is it possible to drive it using just a microcontroller (ie mcu pin connects to mosfet pin and the rest of mosfet pins connect to load/ground/Voltage without other special components)? I have seen elaborated circuits with all kinds of protection for the mosfet and mcu and mass production quality however i just want to learn how to control a small load and understand how it was done.I am sure this simple first step would be a great incentive to try to understand the more complex matters that are involved in power electronics. Copying what you cant understand is no fun at all and its dangerous also.

Best regards.

I would never use a mosfet unless a plain transistor or Darlington would not work. Mosfets are much less rugged, and prone to failure when handled. Also, its hard to find ones that work great on 5 volts of drive (but I'm sure they exist). Transistors have no problem with this.

Also, driving the low-side (ground end) is usually simpler all around.

--
Luhan Monat: luhanis(at)yahoo(dot)com
http://members.cox.net/berniekm
"Any sufficiently advanced magick is
indistinguishable from technology."

I'll be interested in the replies from the experts, as I too started looking for MOSFETs to play with PWM motor control about an hour ago! As you say, there are lots available, and I don't think either of us will have any serious problem finding some to suit. You don't say what load current you anticipate, but I'll assume it's in the 4A-10A range until you specify it. (In my case, my ex-screwdriver motor with inbuilt gears takes over 10A when stalled, so I'm looking at MOSFETS with > 15A continuous drain current capability.)

I was surprised at Luhan's reply. OK, they do need handling with care over static (as do CMOS ICs), but that's no big deal. Just grab something that's earthed before you handle it. But from my little reading I understand that power MOSFETS are otherwise superior to bipolar transistors, particularly for use as switches. This is for two main reasons: they have (effectively infinite) input resistance at their gates; and they don't have any risk of the thermal runaway and 'second breakdown' problems that bipolars do.

You don't say what voltage supply Vcc your load will need, but I'll assume it's not the same low voltage of your micro. So, as I understand it, that implies you need a negative (NMOS) type, with the load between its 'upper' connection (counter-intuitively called 'drain' IMO) and Vcc. The gate could be driven directly, but I would play safe and insert a protective resistor of 1k.

12V DC | (MOTOR) | __| || From -----1k resistor--||

All quite good information, but I still like darlingtons for under 1 amp and like MOSFETs for larger currents. Also, considering the question comes from someone without much experience, I like to advise more 'sure fire' rather than 'optimal' designs.

In actual practice, I once replaced the 'standard' MOSFET motor drive circuit with darlingtons. With 8 volt motors, drawing under one amp from a 12 volt supply, the solution proved vastly more reliable both to produce (low production volume) and to field service.

--
Luhan Monat: luhanis(at)yahoo(dot)com
http://members.cox.net/berniekm
"Any sufficiently advanced magick is
indistinguishable from technology."

check out this mosfet: NDP603AL

If you buy it from digikey the digikey part# is NDP603AL-ND

it is in a TO-220 package and should work well for what you are doing. I used this mosfet with an AVR a couple years ago and the AVR code and board schematic are posted here:

email me if you have any questions.

cheers, Jamie

check out this mosfet: NDP603AL

If you buy it from digikey the digikey part# is NDP603AL-ND

it is in a TO-220 package and should work well for what you are doing. I used this mosfet with an AVR a couple years ago and the AVR code and board schematic are posted here:

email me if you have any questions.

cheers, Jamie