Mosfet getting hot and motor vibrating using a 3.6v motor.

I am very new at this so forgive my lack of knowledge. I've researched beforehand but everything I find doesn't quite make sense.

I'm using the arduino uno starter kit. I set up the circuit shown in project 9 of the starter kit, the motorised pinwheel. Which works fine, my problem is that I attempted to modify the circuit by using the battery and motor from an old electric screwdriver, which is a 3.6v motor powered by essentially 3 AA batteries. Reason for this being that I want to combine the knock knock lock and the motor and gearbox from the screwdriver to open a door on my TV stand.

My problem is that when all hooked up, the mosfet gets hot after 10 - 15 seconds and the motor does not turn, only vibrates (or that's what it feels like)

The mosfet I'm using is that supplied with the starter kit.

I'm sure it's a fairly simple answer but I'm struggling. Any help would be appreciated.

Can you show us your schematic? Either hand drawn or picture will do.

Yeah of course…

The only difference between that and mine is the battery and motor, my battery is three AAs and the motor is 3.6v

Either the MOSFET is backwards or the diode is backwards. Does it still get hot when not connected to the arduino?

The circuit how I have it now did used to work with the 9v battery and motor that comes with the starter kit. It’s only since I changed the motor and battery that this has happened.

Here’s some pictures it might help…

A PP3 9V battery is a very low current power source so presumably the motor designed to work with it only draws a tiny current. A bigger motor would draw more current, and a lower voltage motor would also typically draw more current (for the equivalent power). It may be that the motor is simply trying to draw more current than your MOSFET can provide.

Have you calculated / looked up the current needed to turn the motor, and compared that to the spec of the MOSFET?

are you using a logic-level MOSFET? which one are you using?

Thanks for the help so far, I am going to get this working but I'm working tonight so can't check the info on the motor.

However, I've just looked up the mosfet and its an IRF520NPbF which I think from looking at the datasheet is capable of 9.6A?

What voltage is the ARduino using to control the MOSFET?
I may be wrong, but if I’m remembering my transistors right, if your Vds < Vgs, then you can get some weird results that you do not want when trying to use a MOSFET in a digital system. Your problem may be that you are driving the MOSFET at 5V, and your Vds is only 4.5V (3x 1.5V Batteries). You need more voltage (like the 9V battery). You could possibly use the 9V battery by adding a series resistor, though it might get hot.

Someone better at semiconductors could explain this better.

Ok think that may be the problem.... I can't find the exact model of my motor on the net but the one I can find looks exactly the same and has only a slightly different model number.

The one I can find has:

Rated voltage 3.6VDC No load: Current(A) 2.012

At Max Efficiency: Current(A) 8.067

At stall: Current(A) 28.83

As I think the MOSFET is 9.6A could this be the reason for it not working? If so is it as simple as getting a MOSFET with a higher amp rating?

Yes I think it's using 5v. Would it be possible to use the 3.3v power supply on the arduino to connect to the breadboard then have two MOSFETs, one at 5v powered by the actual arduino which switches the second MOSFET connected to the 3.3v from the breadboard?

Hi, did you say you are trying to run the bigger motor off 3 x AA batteries, and that the NOLOAD current is 2.012A?
There’s your problem, no way will AA batteries run a 2A load.
Measure the AA batteries voltage while trying to run the motor with the arduino.
Try running the motor directly connected to the battery.
I think you will find if it does run, it will be slow and not for very long.
Tom :slight_smile:

The battery and motor are taken from an electric screwdriver. To me the battery looks like three AAs but it might not be, there's a picture above of them. I've tried connecting the motor directly to the battery and it works fine. Could this mean the info I have on the motor is wrong?

The motor looks fine, I've used many of those in the past, but that battery does look a little too small to supply the necessary current needed for this to work correctly. Try to get a 9V, and see if it makes a difference. If anything, you will rule out the power source and if that still doesn't solve the problem, then it could just be a bad Mosfet.

Keep us posted.

Bugzio89:
As I think the MOSFET is 9.6A could this be the reason for it not working? If so is it as simple as getting a MOSFET with a higher amp rating?

At 9A it ought to be supplying enough current to generate a considerable amount of torque.

When you have the Arduino notionally turning the transistor fully on and the motor doing its juddering thing and the MOSFET getting hot, what is the voltage across the motor and what is the voltage across the battery?

Hi, change the layout on the protoboard, where you have the 3.3V battery connected is a long way from the motor through small conductors.

Move the positive battery connection up to next to the positive motor wire, and the negative battery to the source lead of the MOSFET. This way you minimize the length of fine gauge conductors under the board.

Do you have digital multimeter to measure around the circuit?

Tom... :)

TomGeorge: Hi, change the layout on the protoboard, where you have the 3.3V battery connected is a long way from the motor through small conductors.

Move the positive battery connection up to next to the positive motor wire, and the negative battery to the source lead of the MOSFET. This way you minimize the length of fine gauge conductors under the board.

Do you have digital multimeter to measure around the circuit?

Tom... :)

It's 4 inches away! It's not going to make a difference. The problem is current.

Hi, remove the MOSFET and place a link from where drain and source pins used to be, see if motor runs at proper speed. If it does then check MOSFET, if it doesn't fix problem try my solution.

Tom.... :) 2 amps through the protoboard is in my opinion not good.

have you tried measuring the voltage across the MOSFET it may be that it's not switching fully on