You can NEVER leave a fet or hexfet gate open. Its high impedance input will pick
up any stray capacitance and turn on, off, or any partial state in between. Most
times it will partially turn on and usually dissipate considerable heat when the
drain is tied to a substantial load. NEVER leave a gate open. NEVER.
Rick

James Tu wrote:

> I'm using a HEXFET IRFU3706 to control a halogen light (using PWM)  (Thanks
> PIC list!)  Everything works great!
>
> However, I experienced this once...the wire that is connected to the Gate
> of the HEXFET was free (not connected to my microcontroller) and
> accidentally touched part of my microcontroller...(I'm not using the PIC,
> I'm using a BX-24) and the HEXFET turned on.  And then the wire was free
> again, touching nothing, but the HEXFET remained on.
>
> Then something started to smoke and I quickly turned off power to my circuit.
>
> I touched the HEXFET and it was _really_ hot.  What happened?  Is it
> because the HEXFET was not totally saturated?  Is that why it got so hot?
>
> Another time, I accidentally touched free wire (while I was re-wiring my
> circuit) with my fingers and that turned on the HEXFET (I guess I just had
> enough charge on my body.)  This time the HEXFET didn't feel hot at all.  I
> tried this a few times, and sometimes the HEXFET lit the light very dimly,
> sometimes it turned the light fully on.  I touched the HEXFET and it didn't
> seem to be warm at all.  I guess during the times when it was dim, it was
> because I only had a small charge on my body and the HEXFET was not in
> saturation?
>
> James Tu
> james@2-bit-toys.com

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