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.
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 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.
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.
The IRLB8721 is fine. There is very little difference in the RDSON
Did you do the math ?
E = IR
P= IE
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 ?
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.