Hello, I am trying to control the speed of an 18Vdc motor with the IRF740 mosfet. Originally I only had the opto-couplers and then added the BJT's when it didn't work. But it still doesn't work after I added them. It is a high side switch and yes, I have Drain connected to Vin and source to the positive of the motor. I have checked the internal diode and it is fine.
Here is the schematic.
When the arduino puts out 5V on digital pin 9 I get 7V between gate and source, the mosfet turns on. 18V at output.
When the arduino puts out 0V on digital pin 9 I get 1,25V between gate and source and the mosfet is still on but only slightly, 16V at output. Vgs(th) is 2V minimum so it should't be on. Could it be the 1,25V that is left on gate that holds it on? How do I decrease that to fully turn it off?

Mosfet, http://www.vishay.com/docs/91054/91054.pdf
BJT's, http://datasheet.octopart.com/BD233-Inchange-Semiconductor-datasheet-15980023.pdf

-Anton

 

To control the ON\OFF state of the MOSFET, you want to control the voltage of the gate-source terminals. You control the voltage of the drain-gate terminals. Nothing forces the Vgs to zero. This is often done by putting a resistor from gate to source so when it is not driven, it will go to zero.

Here is an explanation and circuit example for High Side Driver:

http://tahmidmc.blogspot.com/2013/02/n-channel-mosfet-high-side-drive-when.html?m=1

 

You need base emitter resistors across Q2, 3 of 1K, and a 1K across the fet Q1 G/ and ground terminals.

 

I got it to work, thank you StayatHomeElectronics!

 

I got it to work, thank you StayatHomeElectronics!

You can switch a MOSFET on by charging the gate capacitance with static charge by touching it with your finger, the gate capacitance should hold that charge more or less indefinitely.

As everyone else has stated - you need a bleed resistor to discharge it when drive is removed.