# MOSFET, resistor question

Hi,

http://www.eng.auburn.edu/~niuguof/2210labdev/html/lab6.html

In the image under MOSFET as a switch, they show a resistor between the supply and the motor..

I thought I wouldn't need this in my design, am I wrong?

My design takes 12VDC to the motor, then through the motor to the mosfet, then to ground, no resistors. The signal to the mosfet given by a 4N35 opto, and the opto driven by an arduino.

A resistor in the path to the motor would limit the motor from going to full on.. Why would I want one?

Maybe that design needed to limit current for that particular motor, but I think you don't need it.

The resistor in series with the motor is for current limiting; and therefore torque limiting. I am not clear on what you mean by full on. A dc motor's speed is control by voltage, and its torque is controlled by current. The motor's resistance should be in the single digits so most of the voltage drop will be across the motor. You should use a large resistor, say 1Mohm, between the gate and source to prevent latch up - due to gate capacitance.

Or maybe equivalent series R of the motor. It appears to be a theoretical circuit.

Weedpharma

And here is the danger of confusing totally random snippets from the Internet, with engineering principles.

That is a college lab tutorial about FETs - it clearly has nothing whatsoever to do with controlling motors. The resistor is just as likely "idiot proofing".

Actually, it is not - it is to limit the inrush current of a stalled motor. OK, there may be a design point here, if you have a FET controlling a motor, you need to consider whether it is rated to start the motor from stall. Including a resistor is not going to be the way of doing it, but suffices for a lab exercise.

That's where I got thinking, fault conditions.

Open Circuit: The motor is not drawing current, and so there is no danger to the FET.
Short Circuit: Protection needed.
Stalled motor: Protection needed.

I'll put a fuse on the supply side.

Well its a poor tutorial since they show an inductive load without a free-wheel diode or
snubber network, so they may be blowing up MOSFETs...

They included the resistor because one of the possible loads was an LED, but
still its a poor effort.

Hi All,

Well its a poor tutorial since they show an inductive load without a free-wheel diode or
snubber network, so they may be blowing up MOSFETs...

I too spotted this, that can't be a serious practical circuit!!

Regards

Mel.

I notice the motor is drawn in a different colour. Perhaps the author took this diagram from elsewhere and scribbled over the top of an LED?