anyone good with circuits?

[justDIY]

I'd like to build some sort of failsafe circuit for my MH light, seeing as how it's made from wood ...

if should the fan fail, temperatures could build up to the point of being unsafe.

any ideas?

I don't think you have anything to worry about. the hole for the fan will allow heat to escape in the time it would take for you to be able to fix it. I ran a MH 175 watt for a year using aluminum foil coverd cardboard. Now talk about a fire hazard, it never burned through it though. I had it about 6" above it.

You could find a reverse thermostat at an electrical supply outlet that will cut the power if it gets above 250*...

[RogerMcAllen]

I agree, a cheap thermostat would do the trick.

[loganj]

Maybe a high limit switch (cheap thermostat basically ) for a furnace? I'm not really sure what temp range you need. That's what you'll have to determine first. Then it will turn into a search for the thermostat or limit that falls into that range. I wonder if a stat for a hot water heater would do it? That way, you wouldn't have to use a contactor as the stat would be able to carry the line voltage. Bear in mind that many of the stats and limits are designed to work with a relay or contactor and will not handle line voltage.

[BillyZ]

As the wood surface temperature rises beyond 212° F to about 450° F, major gases abundant in creosote are produced: carbon dioxide, carbon monoxide and acetic and formic acids. <<<SNIP>>> The process by which gases are released from wood and burned is called primary combustion. Primary combustion begins at about 540° F, continues toward 900° F

source: http://www.montana.edu/wwwpb/pubs/mt8405.html

so, all moisture driven out at 212, gases released at 450 and ignition at 540. I'd say you would want to keep things below 400 as a "killing point"

If you have a thermometer that reads high enough, just leave the probe taped to the wood above the bulb and see how high it gets with the fan off. It may not be an issue.

or it may be an issue... at which point the above posts make good suggestions.