24v 350w pwm controlled motor

Hi guys and girls,

I’m looking to control a large motor (24v, 350w) it only need to turn in one direction so was going to use a n-channel mosfet (specifically the adafruit 30v 60a).

Is that mosfet suitable?
From what I’ve read I’ll need something like a 1n4001 diode in parallel with the motor to prevent feedback spikes.

Will I need a heat sink for the fet? And if so what do I use?

Also guessing the current would be too high for a breadboard so would I be best to just solder it all together?

Sorry for so many questions I’m a bit of a newbie when it come to this.

TIA
Dackie

Hi,

Dackie:
I'm looking to control a large motor (24v, 350w) it only need to turn in one direction so was going to use a n-channel mosfet (specifically the adafruit 30v 60a).

Is that mosfet suitable?

Your motor will need at least 15A (15A * 24V = 360W), but may easily take several times that on startup if your power supply could handle it.

Check the datasheet for the IRLB8721 MOSFET...

                                             min  typ  max      Conditions
RDS(on) Static Drain-to-Source On-Resistance –––   6.5  8.7 mΩ   VGS= 10V, ID=31A
                                             ---  13.1 16        VGS= 4.5V, ID=25A

If you try to drive this directly from an I/O pin at 5v, at 15A it is going to dissipate 3.6 watts so it'll get hot without a good heatsink.

If you use some intermediate transistor to switch the mosfet with VGS = 10V then it will be dissipate less (about 2 watts at 15A), I'd still go with a small heatsink like HEAT SINK, TO-220 8.6°C/W or better (i.e. a lower °C/W figure)

From what I've read I'll need something like a 1n4001 diode in parallel with the motor to prevent feedback spikes.

Yes that's right.

Also guessing the current would be too high for a breadboard so would I be best to just solder it all together?

Definitely solder it together, using a plug-in breadboard would involve funny burning smells etc.

Strip board will be ok as long as you solder on nice thick copper wires for the high current lines.

You may see circuits like this:
Diagram:
MOSFET_drive.png

...but this has a couple of problems in this case: a) at 24v the MOSFET gate voltage would be too high and b) if the Arduino is turned off, the motor runs, which you might not want with a 360W motor.

Maybe a better, but a bit more complicated, solution would be like:
Diagram:

Yours,
TonyWilk

MOSFET_drive.png

The 1n4001 is inadequate to protect the system. You should be looking at a diode rated nearer the motor current since, at the moment of switching off the motor, the diode will have to be capable of 'dumping' the equivalent reverse current.

jackrae:
The 1n4001 is inadequate to protect the system. You should be looking at a diode rated nearer the motor current since, at the moment of switching off the motor, the diode will have to be capable of 'dumping' the equivalent reverse current.

Oops... I did get that completely wrong :frowning:

You'd need something like a 40EPS12 (e.g. Mouser: VS-40EPS12PBF )

Thanks jackrae.

TonyWilk

Thanks for your help guys
Couple of questions (gonna sound like a total noob now)
the 1r6 resistor (R1) is that 16 or 160 ohm?
What's the "drive" connect too?
I guess I would have 2 separate grounds? So the low power side of the opto is basicly a seperate circuit from the high power?

TIA
Dackie

Dackie:
the 1r6 resistor (R1) is that 16 or 160 ohm?

Ah, that's just there to represent the motor (didn't have a 'motor' symbol)
the value of 1.6ohms was just to simulate a load of 15Amps at 24V

I guess I would have 2 separate grounds? So the low power side of the opto is basicly a seperate circuit from the high power?

Yes, the two sides can be completely isolated.

TIA
Dackie
[/quote]

Just had a thought what wattage resistors am I going to need?

Dackie:
Just had a thought what wattage resistors am I going to need?

The small 1/8w resistors are fine for this.

all of 'em in that diagram will be dissipating less than 0.1W, ( "R1" is the motor of course )

Yours,
TonyWilk

A 24V 350W motor will have about 100 to 150A stall current, ensure the switching device can handle spikes
of that magnitude, and steady currents as large as the power supply can provide. Actively driving the MOSFET
gate with a MOSFET gate driver chip is a very smart idea for high power operation and essential for PWM
operation.

Your MOSFET should be rated at least 50V for 24V supply operation. The other key spec is the on-resistance,
which would be good be 10 miiliohms or less for these high currents.

You realize a single switching device means you'll have no active braking?