I'm a bit confused about which rating of fuse I need.

I need it to trip when the ambient temperature reaches a certain point. The load is around 750ma and generates heat itself, as well as heat coming from the circuitry. Its all contained within a closed case with a temperature controlled cooling fan regulating the ambient temperature via 2 small inlet and outlet vents. The fuse is there to trip the load if a) the vents are blocked or b) the fan fails, I need it to trip around 60-70C.

Having studied the data sheets I thought a fuse with 650ma hold current (1300ma trip current @ 20C) would be a good place to start as the temperature derating chart showed that it should trip around about where I want it to and yet it trips far too early, the cooling fan does not even kick in.

So where am I going wrong?

Now I'm typing this I think it may be glaringly obvious that the hold current should match or be more than the load but for some reason I assumed it could float in between hold and trip?

 

So where am I going wrong?

By your choice of device. A poly fuse trips when the current gets too large. You want a thermal cutoff switch. These come in both resetable and non-resetable versions, which work similarly to your polyfuse or a standard fuse, respectively.

Seems you want a self-resettable type where once the temperature goes back to normal the device starts to work again. Sorry to say I did not find any parts to link you up with, hopefully someone else will give that detail

Images of thermal fuses

 

There are a variety of types and sizes of bimetallic switches which auto-reset when the temperature falls.
http://www.ebay.co.uk/sch/i.html?_f...nkw=bimetallic+temperture+switch&_sacat=12576

 

Thanks for the reply's.

I did have a look at thermal cut-off's but they don't make economic sense for something I want to make multiple quantities of.

PPTC's can be used as an thermal fuse. The load is constant, but the trip rating of the fuse decreases as the temperature increases, I see no reason why this shouldn't work given I can find the correct value for my particular scenario.

I didn't want to trail and error it but that might just be the way forward, there's actually not a lot of choice now anyway, the most common left in my particular range is 750ma and 900ma. I can tweak it by decreasing the load if need be to hopefully hit a sweet trip point.

 

Thanks for the reply's.
I did have a look at thermal cut-off's but they don't make economic sense for something I want to make multiple quantities of.

How many do you need and what's your target cost per unit?

 

Thanks for the reply's.

I did have a look at thermal cut-off's but they don't make economic sense for something I want to make multiple quantities of.

PPTC's can be used as an thermal fuse. The load is constant, but the trip rating of the fuse decreases as the temperature increases, I see no reason why this shouldn't work given I can find the correct value for my particular scenario.

I didn't want to trail and error it but that might just be the way forward, there's actually not a lot of choice now anyway, the most common left in my particular range is 750ma and 900ma. I can tweak it by decreasing the load if need be to hopefully hit a sweet trip point.

You can get exactly what you want/need by using a negative temperature coefficient thermistor and a comparator with hysteresis set to trip and disconnect the load at a particular temperature and to reconnect the load at a lower temperature.

Or, if you want, the load can be permanently disconnected until manually reset once the enclosure temperature drops below a given temperature.

Interested?

 

I don't have a target cost, but just by looking at the prices of thermal cut-offs I know that's not going to be the way to go. Quantity wise we're talking on-going small hobbyist amounts.

So an NTC and comparator? Interesting. I don't have a lot of board space to spare, board size is 18x18mm and is mostly taken up by the fan controller.

Can you just explain why a resettable fuse wouldn't work? Everyone seem to be suggesting other methods and I'd rather not overcomplicate it.

 

I don't have a target cost, but just by looking at the prices of thermal cut-offs I know that's not going to be the way to go. Quantity wise we're talking on-going small hobbyist amounts.

So an NTC and comparator? Interesting. I don't have a lot of board space to spare, board size is 18x18mm and is mostly taken up by the fan controller.

Can you just explain why a resettable fuse wouldn't work? Everyone seem to be suggesting other methods and I'd rather not overcomplicate it.

Goto:
http://www.littelfuse.com/products/resettable-ptcs.aspx
and read: "PTC characteristics and terms."

If a PTC still looks good to you after that, then the following Littelfuse graphic, which I've edited by plotting the PTC's derating at about 65C, shows that the ambient temp is eating up about 62% of the PTC's capacity, which means that if you want to use a PTC and you want it to open at, say, 750 mA, it must be rated to trip at 750mA / 62% = 1.2 amperes at 25C.