MOSFET getting stuck open?

[octovert]

I suppose it's more accurate to say "stuck closed", not an engineer here. I'm using an ATTiny chip to drive an n-channel mosfet which in turn is controlling a motor. Here's a pic of the schematic.

This circuit works exactly as designed if I use a motor that draws about half an amp. Once I use the motor I actually like (which draws about 1.5-2 amps) it's like the mosfet gets stuck open, and the motor never shuts off. The mosfet in question is this one: https://www.sparkfun.com/products/10213

I've tried heat sinking the mosfet, which changes nothing. What am I missing? I built this part of the circuit using the tutorial found here:

http://bildr.org/2012/03/rfp30n06le-arduino/

Also, this is an older schematic that used an 8V reg on the motor - I've since moved on to a different motor and have removed the regulator altogether, so that isn't it.

 

[(*steve*)]

What is happening on the gate of the mosfet when it's "stuck on"?

What happens if you disconnect the gate from the microcontroller (leaving the resistor in place)?

 

[octovert]

Alright, tried both of those. When the mosfet gets stuck on, the gate pin is almost continually receiving voltage, and the output pin on the microcontroller is also putting it out when it apparently shouldn't be. Pulling the microcontroller from the circuit does shut everything off, so the MOSFET doesn't appear to be latching on.

Gonna try a new microcontroller - maybe I fried this one. Anything else to look at?

 

[(*steve*)]

What you may have is noise on the power supply which is causing the microcontroller to behave strangely. If this is true, replacing it won't help -- you have to fix the problem.

Have you decoupled the power supply to the uC at all?

The simplest thing is to place a 0.1uF capacitor across its power pins as close as possible to the device. If this doesn't work, you may need more effective measures.

You could google "microcontroler power supply decoupling"

 

[octovert]

I soldered the 0.1uF cap directly to the power and ground pins on the microcontroller (as you mentioned distance was an issue) and everything worked perfectly! Thanks so much! Revising the schematic as we speak.