Components for DC motor speed control

Hi all,

I salvaged a DC motor from a cordless drill. No I would like to do speed control with PWM from an arduino.
The motor might draw up to 20A (14.4 V / 0.8 Ohm)
I'm thinking about the circuit below, will this work ? << EDIT: Nope, see below

And if so, I'm not sure what capacitor and diode to use.

Thanks !

No, drop the cap.

But do add the diode and add a huge heatsink... Or swap for a better mosfet. With 16mOhm like this one it's 6,4W of power in the heatsink.

And how did you determined the current draw?

Also note, the speed depends on the load. You can control the speed but it will vary if you load it.

Ok, thanks.

So what would be a better mosfet that would not need a heatsink ?

As for the current draw, 14.4 Volts / 0.8 Ohm = 18A peak at switch on? And the switch in the drill was rated for 20A. So my conclusion was max 20A. I'll be using the drill, well, as a drill :slight_smile: In a stand that is. I won't be doing heavy drilling so the motor is not going to be "loaded" that much I think.

Thanks again

what would be a better mosfet that would not need a heatsink ?

You need a heatsink regardless. If that's too much trouble then abort the project.

N-CHANNEL 30A MOSFET

First get the circuit right, you've completely mixed up all the black wires.

The MOSFET drain (middle lead) goes to the motor negative terminal (not ground!)
MOSFET source goes to ground.
12V supply negative goes to ground.

The capacitor across the motor should be 100pF to 1nF tops, ceramic, not electrolytic - its
only function is reducing EMI from the sparking commutator.

If you put a large electrolytic across the motor terminals and you greatly increase the stress on
the MOSFET as it has to drive large current spikes as well as a motor. 1nF tops, ceramic,
right on the motor terminals (or there's little point).

The free-wheel diode should be rated for the forward current of the motor in normal use and have a
pulse rating as high as the stall current. A 10A rectifier ought to be reasonable option.

You might want to think about taking the strain off the Arduino pin by adding 150 ohms
series resistance to the gate. Alternatively use a MOSFET gate driver chip to drive the MOSFET gate,
a much more capable way.

MarkT:
First get the circuit right, you've completely mixed up all the black wires.

Mm, yeah, I messed that because of the Fritzing breadboard mess...

And yeah, it's always going to give you heat and you need some sort of heat sink. But if you go for <10mOhm it's already a lot less.

And yeah, for a larger MOSFET a 150Ohm might be a good idea :slight_smile: But it will add a bit of heat in the MOSFET but that's better then blowing the Arduino :smiley:

Thanks for the replies, so I hope I'm going in the right direction with this then:

Full size image

And about the MOSFET, the IRLB8721 is 30V/60A . Raschemmel, you recommend 60V/30A.
Why would that be a better choice ?

I also have a BUZ11 (50V30A) could I use that ?
Edit: This is not logic level so no ?

Thanks

The IRLB8721 is fine. There is very little difference in the RDSON

Did you do the math ?
E = IR
P= I
E
Let R = 0.078 Ohms
Let I = 30A
E = 0.078 ohms * 30A = 2.34V (D-S)

P = I*E
Let I = 30A
Let E = 2.34V
P = 30A * 2.34V = 70.2W

How long can you hold your hand on a 70W lightbulb ?
Now imagine all that heat compressed to the size of the mosfet.
Did you try holding the mosfet while running the drill motor at full speed ?
You still think you don't need a heatsink ?

@raschemmel, interesting notation with E for voltage. I'm used to U (or V if it's American).

That's the textbook format.
It stands for "Electromotive Force".
It's just a habit from school (DeVry).
I use "V" at work.
Robert

It's sort of a side effect of a memory trick I used in school:
"The penalty for forgetting Ohm's Law is
a PIE (P=IE) in the EIR (E= IR)."
I know it's corny but it works.

Thanks for the math raschemmel :slight_smile:

Adding a heatsink is fine if that's required. I've never worked with MOSFETs at high loads so it's kinda new to me.

Would you want to be that mosfet with nothing to disipate the heat ?