Mosfet selection for DC motor Control

[apprentice_nerd]

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.

 

[Terry Pinnell]

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'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--||<-| Suitable NMOS
micro ||__| MOSFET
|
|
|
0V

Unlike the narrow band for bipolars, turn on voltage (Vgs) apparently
varies considerably depending on load current. But with the assumption
made above I'd suggest that to be sure of getting reliable switching
from 3.5/5V logic circuitry, you focus on those for which the
'Vgs(threshold)(max)' is 2.5V or lower. IOW, such MOSFETS are
guaranteed to turn on if you apply 2.5V between their gate and their
source ('lower') connections. BTW, some of these (not all) seem to be
explicitly described as 'Low threshold' in their datasheets.

A couple of examples of NMOS types I've found so far are: TN0104N3
(4A), RFP15N06L (15A). But I've literally just started looking myself.
In fact, my initial approach is to see which types I've actually *got*
in my shed/workshop and start googling for their datasheets. The only
one I've checked out so far is the BUZ11, as I bought a few a year ago
and so far haven't go t around to playing with them. The datasheet
here
http://www.fairchildsemi.com/ds/BU/BUZ11.pdf
says
Min Typical Max
2.1V 3V 4V

I'm not into PICs/microcontrollers, so I don't know what voltage you
will deliver to the MOSFET. But if it's >4V then that BUZ11 (a common
type I believe) should be OK, and a particular specimen could prove
even less fussy.

Best way of finding out of course, providing you have a spare or two
as I do, is to try it <g>.

 

[Luhan Monat]

[email protected] (apprentice_nerd) wrote:

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.

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.

 

[Jamie Morken]

Hi,

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.

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:
http://eagleflyer.sourceforge.net

email me if you have any questions.

cheers,
Jamie

 

[Jamie Morken]

Hi,

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.

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:
http://eagleflyer.sourceforge.net

email me if you have any questions.

cheers,
Jamie