Simple fail proof overtemp protection

[gijim]

I've got an AC circuit that draws about 2A. What I need to do is cut the power to the device should a transistors heat sink overheat (defined as >100C). Preferably until the user unplugs the unit, or at least until it cools down some amount (20C?). It should be resettable, not permanent.

Simple solutions are usually the best, but unfortunately digikey only carries a to-220 thermal switch that can handle 1A, not the 2A required.

So I was thinking of either finding a way to boost the current capacity of it, or building a comparator with a NTC/PTC resistor and a voltage divider to set the trigger point. Maybe controlling a triac? I don't remember how triacs work; never even used one.

Any "battle tested" suggestions would be most helpful.

 

[ScottyUK]

A 'Klixon' type thermal switch might meet the requirement. Simple and robust. Depending on how high your voltage is the Raychem Polyswitch product range might be appropriate too. The second link is the 2005 Polyswitch shortform.

http://www.sensata.com/about/klixon.htm

http://www.farnell.com/datasheets/95815.pdf

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I don't have an attitude problem. You have a perception problem...

 

[gijim]

Thanks. Ideally if a thermal switch is used, it should be PCB mount (like the to-220 one)

I think the polyswitch won't work; I haven't seen one that can handle both the voltage and the current (1-2A@90 up to 240Vac)

 

[xnuke]

Use the TO-220 to switch a relay that has the appropriate interrupting ratings.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BobM3]

Look at

http://www.Cantherm.com.

Their encapsulated thermostats have NC contact ratings of 2 amps(Model F11-E06).

 

[gijim]

BobM3 - thank you! They have one rated at 6 amps that should work well if epoxied to the transistor and/or heat sink. I will try to find a distributor.

xnuke - thanks! That's certainly another option to look into.

 

[itsmoked]

Why don't you do it like everyone else?
You control the transistor's base/drive NOT the main switched power.

Does this thing have a micro controller involved?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

If this is a last line protection function I would not let a uP near it. Simple = effective in shutdown systems, although I suspect the shutdown systems I work with involve slightly larger amounts of energy!


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[itsmoked]

Scotty; I know what you're thinking but you still contrl the base if things are getting hot.

Last defence is what fuses are for.

Having a temp switch that maybe turns your problem ON and OFF randomly is not a solution.

If you need last defence protection you need a thermal fuse.

They make transistor based circuit breakers now.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gijim]

If the transistor gets shorted on though, disconnecting the base won't do anything.

 

[itsmoked]

If the transistor is shorted ON you need a fuse!!! Not a temperature switch! Otherwise you have a temperature switch running the shorted transistor.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com