MOSFET for controlling a DC motor

Other Hardware Motors, Mechanics, Power and CNC
1

Hi,

I am controlling the speed of a hair dryer fan motor with the attached circuit.
The MOSFET I am using is a ST 16NF06

I am supplying the motor 24V
I measure the load to about 2A when it's running on full speed.

http://www.st.com/content/ccc/resource/technical/document/datasheet/f3/e2/b6/5c/fc/9c/41/16/CD00002501.pdf/files/CD00002501.pdf/jcr:content/translations/en.CD00002501.pdf

It is working fine apparently, but after reading some more about MOSFETs this is maybe not the best one for this purpose?

Could someone please suggest a better one?

The only reason I used it is because I had some from before.

Some places they suggest a "Logic Level" MOSFET that will provide the max current at 4,5V. Is that necessary for my case?
I have looked at the diagrams in the datasheet, but I don't think I understand them properly.

I only need to control the speed in one direction, so I thought a MOSFET and PWM was the best way, but if you have a better suggestion, please suggest.

Cheers, Hal

motor.png
motor.png1175×643 40.5 KB

1 Like
2

eptheca:
It is working fine apparently, but after reading some more about MOSFETs this is maybe not the best one for this purpose?

If you want, you could try IRF3708

3

Southpark:
I'm thinking.... if it works fine, then what's your concern about it?

I have only tested it for a short while. Used a pot to adjust the speed.
But I want to run it for a long time, and after reading about it I got concerned about the current and heat, and I would like to optimize the circuit and understand more.

4

Always look for logic level MOSFETs. The one you are currently using is not, and is normally switched on by 10V on the gate. To see why, study the graph below (taken from the data sheet).

This one will work well.

mosfet.png

mosfet.png

5

jremington:
Always look for logic level MOSFETs. The one you are currently using is not, and is normally switched on by 10V on the gate. To see why, study the graph below (taken from the data sheet).

This one will work well.

mosfet.png

OK, thanks:

Looking at the graph from the datasheet, it looks like when I feed the gate 5V, and the load is 3V it still will provide 7A of current, is that right?

So in my case, where I only need about 2A it will still be fine, or?

6

The Arduino output is always a lower voltage that the power supply, so it is not "5V".

Even if it works, it is a waste of power to use that MOSFET.

7

The IRF3708 mentioned above is rated down to 2.8V gate drive so will work with any 3.3V or 5V logic
circuit. Many "logic level" MOSFETs are only good for 5V.

The specification to look for is the value(s) of Vgs quoted alongside the Rds(on) rating.

Most MOSFET datasheets start with the absolute maximum ratings, then have a table of static
values including the Rds(on).

Don't be fooled into thinking the Vthr (threshold voltage) is relevant, it's simply the point below
which the device is non-conducting. To be properly on the device has to be way above this threshold,
and the Rds(on) spec tells you the gate drive voltages that work reliably.

8

Thank you guys.
I now understand more about MOSFETs and I'll be getting some IRF3708.

Cheers, Hal

9

MarkT:
The IRF3708 mentioned above is rated down to 2.8V gate drive so will work with any 3.3V or 5V logic
circuit.

I had always assumed that International Rectifier used the IRLxxxx convention for logic level mosfets, and IRFxxxx for others. Seems I was wrong on that: live and learn.

10

If you are looking at typical curves for a MOSFET be aware that device spread is large in MOSFETs, and
normally you'd only need to do this for analog operation, not using the device as a switch - you only
need the Rds(on) and gate capacitange/charge data to select a device for switching.