Speed Control Test...

Hey everyone,

I'm testing an N-channel MOSFET to be used for the post-PWM part of a DC-Geared Motor speed control.

Before I try the PWM (which will be simply implemented by mcu ) I'm testing this MOSFET by connecting a potentiometer to manipulate voltage intended for opening the
gate of the MOSFET. Thing is, the motor won't turn unless I touch the MOSFET (??) so apparently something's missing; so what's wrong?

btw the MOSFET is IRF620 (enhancement mode Power MOSFET) , potentiometer is 1kΩ, the voltage applied to the pot is 5v.

thx in advance!
regards

Since you didn't post a circuit diagram, I can only guess at the problem, which is likely an incorrect connection of the pot to the transistor.

But why are you testing in with a pot? For PWM the transistor needs to be switched fully on and off. According to its data sheet, the IRF620 transistor requires 10V to fully turn on. 5V will barely turn it on. That voltage needs to be applied between the gate and source terminals of the transistor.

How much current does the motor require?

True...

Hi again,

Indeed the IRF620 has a gate threshold VGS(th) = 10 V, but here's what happened:

- In my first test I connected the motor to the Source terminal of the MOSFET, which resulted in my first problem (the MOSFET did not switch fully on).

- In a second test I just connected the motor to the drain terminal and voila, the MOSFET switched fully on on 5V!!

I'm gonna post again 'cause I've just started the project.

By the way I'm using a 12V DC geared motor of 30 rpm.

regards.

Mr CCE said:

Indeed the IRF620 has a gate threshold VGS(th) = 10 V, ...

Click to expand...

The VGS(th) is 3V typical. The voltage to fully turn it on is 10V.

Mr CCE said:

- In my first test I connected the motor to the Source terminal of the MOSFET, which resulted in my first problem (the MOSFET did not switch fully on).

Click to expand...

Yes, in a source follower mode you need to have the gate voltage 10V above the motor voltage to fully turn on the MOSFET.

Mr CCE said:

- In a second test I just connected the motor to the drain terminal and voila, the MOSFET switched fully on on 5V!!

Click to expand...

Are you sure it's fully turned on? What was the drain-source voltage when it was on?

Mr CCE said:

By the way I'm using a 12V DC geared motor of 30 rpm.

Click to expand...

That doesn't tell me how much current it takes.