mosfet turning motor on/off

[hunt4it44]

i use a 16f628 to control a 12V motor. the motor only runs a couple times a day in short bursts of 10 to 30 seconds. there is no PWM, just on/off.

I control the motor by using a pin on the micro, outputing 5v, through a 2.2K resistor to a mosfet (RFD14N05L). I have used this arrangement on 6V motors with no problem. However, on 12V motor's, the output pin on my chip gets zapped. I do have a flyback diode across the leads on my motor. the 12V motor pulls about 5A. would adding a diodie in between the pic and the mosfet be suffecient to protect the pic or are there better alternatives?

this is probably a very novice question but I could not find the answer in a search of previous posts on this forum. Any help is appreciated.

 

[Hero999]

It sounds like the 12V power supply is being capacitively coupled to the PIC's IO pin. Adding a diode between the PIC's IO and the MOSFET's gate should solve the problem.

 

[hunt4it44]

thanks Hero. I will give it a try.

 

[Hero999]

Sorry I think I've poorly worded my previous post.

You should connect the doide from the PIC's VDD to the IO pin with the cathode going to VDD and the anode going to the IO pin. You might need to use a small schottky diode as the voltage drop on an ordinary silicone diode might be too high to protect the PIC.