overheating MOSFET?

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Hi All,
       My clock worked great for several weeks, but then stopped working-- no numbers displayed at all.  I put my voltmeter on the test point and discovered that there's no high voltage.  I noticed that while it was working the MOSFET got quite warm, and I have it in a very small enclosure with no airflow.  I haven't performed extensive troubleshooting, but has anyone else had an issue with the MOSFET overheating and failing?

Thanks!

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Further testing:

I desoldered the MOSFET from the board and it tested good-- I ran some LEDs through the drain to source, and was able to toggle them off and on by touching the gate to +5V and ground.  I soldered the MOSFET back onto the board and I'm now getting high voltage at the test point-- mysterious!

I think the MOSFET got hot enough to partially melt the solder, and so it eventually worked itself into a point where it was effectively cold soldered to the board, and not making a good enough electrical connection to operate properly.

So the remedy is to find a way to get a heatsink on it, but I can't see doing that with my current board configuration.

By the way, I'm using the Arduinix circuit design but a custom board layout, as the Arduinix would have been too wide to fit in my enclosure:
http://blog.wingedvictorydesign.com/2010/10/13/my-nixie-clock-build/

But I would be curious as to what temperature everyone else's MOSFET is-- warm to the touch?  Scalding?  If so, learn from my mistake and engineer some airflow into your enclosure.

[nonentity]

Beautiful project!  Great build log!

I'll make sure to post about needing to install the 10 ohm resistors on the anodes.

Thanks!