Choosing components for logic level MOSFET motor driver?

Projects General Guidance
1

Hey everyone. I posted on here a few days ago about me getting started on a project involving having a motor spin at a certain speed. After initially feeling discouraged and resorting to having someone do it for me, it turns out I simply needed a whole night of research to figure out all I needed to pursue the project on my own.

Anyway! Since my motor will only need to run in one direction, I decided to go with a logic level MOSFET driver like a member here suggested.

mosfet motor driver
mosfet motor driver840×654 43.7 KB

I have picked the motor I will be using after figuring out my project would not require that much torque. A mere 0.3kg.cm will do. I will therefore be using that motor from Solarbotics.

It has a stall current of 400mA, and does 40RPM at 3V. I will be powering it with an Arduino Pro mini, that is itself being powered by 3 AA batteries in series, so about 4.5V. I will control the speed with PWM to have it spin within the ballpark of 30 to 35RPM.

I need to pick the correct MOSFET for my purpose now, but I'm a little lost here... There a lots of values being thrown at me when I look online on Mouser. Is this one good for my purpose? It says "Maximum DC Collector Current: 500 mA", so, enough for the motor's stall current, and "Emitter Base Voltage: 4V". The motor is rated from 3 to 6V.

Any help here will be greatly appreciated.

2

That is not a MOSFET it is a BJT transistor.
IRLZ44N are commonly available

3

As mentioned that transistor is a BJT, i.e. it is not a MOSFET.

These examples will give good results.

1 Like
4

Oh... Off to a great start aha. Thanks for your answers!

So... It looks like I can use 'any' MOSFET I want?

I was told, quote, "Choose the MOSFET based on motor supply voltage and stall current. You must pick a motor driver based on motor supply voltage and the motor stall current. Stall current can be many times the running current. The motor will draw stall current, briefly, every time that it starts. Until you know the stall current you cannot, intelligently, choose a driver."

So it sounded to me like the MOSFET had to be very closely matched to my specs, for matters of efficiency, or regulation?

5

What is the motor’s resistance ?

6

No. The one I selected will work and is commonly available

7

Assume power supply is 4.5V and stall current is 400mA.

4.5v / .4A ≈ 12Ω

Any of the MOSFETs the PDF will work.

8

Great, thanks, that makes it a lot clearer. Since my project has to be as efficient as possible, I was wondering, does using a slightly mismatched MOSFET results in loss of efficiency?

9

What is this mismatch you refer to ? :thinking:

10

"Matching" is not the problem. You simply pick a logic level MOSFET that can easily handle more than the motor stall current, and it will work fine.

For very best performance with high power motors, then choose from a selection the MOSFET with the lowest internal resistance Rds(on).

1 Like
11

That makes a lot sense, exactly the answer I was looking for and couldn't find anywhere. Thanks!

12

One last question regarding the schematics of the MOSFET driver.

mosfet motor driver
mosfet motor driver840×654 43.7 KB

I understand the wiring of the bottom part.

Digital out controlling the MOSFET for PMWing the motor, Arduino ground connected to MOSFET ground, two resistors.

What I don't get is the upper part. D1 is a flyback diode, this I understand, but what's the optional C1? Why is it optional? And what does that greek letter inside the circle inside the rectangle represents? Is it the motor?

13

That is the letter M on its side :wink:

The capacitor can help reduce high frequency electrical noise form the motor brushes, i.e. a capacitor is acting as a fast acting filter.

14

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.