# Options for a simple over-temperature cut-off?

I am designing a heater circuit that will be used in a product to be produced in small to medium quantities (hundreds to thousands of units, not tens of thousands). This heater will be PWM controlled to maintain a set temperature in the heated medium, however as an additional safety measure I would like to include a separate over temperature cut-out to prevent fires or high surface temperatures in case of a fault in the control loop (failed sensor, switching FET shorts out, load is removed from the heater element etc. etc.).

The cut-off should return to an operating state once the over-temperature condition is resolved (ie. normal, trip-once thermal fuses are not appropriate). Ideally the solution would be as simple as possible and contain few parts, the device would be used in remote locations so reliability is a concern.

The heater would be driven from 12V, maximum power would be 40W (ie 3.3A). The cut-off should operate at 50 to 60 degC.

The options I am considering and their relative merits are below:

### a. Normally-Closed Bi-metallic Switches, placed in series with the heater element (eg. Klixon 7AM):

• For:
+ Simple, single component.
+ Seems to be the most common solution for this application.
+ Significantly more expensive than PTC (>$3.00/pc). • Against: - I have not been able to find any surface mountable options. - Has moving parts which raises reliability concerns in my mind although they do seem to have quite acceptable lifetime ratings. ### b. Positive Temperature Coefficient (PTC) Resistors, placed in series with the heater element: • For: + Simple, single component. + No moving parts. + Surface mountable. + Cheap (less than$0.40/pc).
• Against:
- Generally designed for use in over-current protection and data is given to suit
eg. available data does not give resistance vs temperature curves.
- Temperature trip point would be dependant on current through the resistor.
- Few options available for use over 3A.

### c. Temperature Switch IC (eg. Analog ADT650x Series) driving a FET:

• For:
+ No moving parts.
+ Surface mountable.
• Against:
- Multiple parts (reliability).
- Additional design work (ok, it's negligible but I'm lazy).
- Requires additional power supply (5v).
- Total BOM cost is similar to bi-metallic switch.

### My questions are:

1. Any feedback on the merits of these options?
2. Can anyone suggest alternative options for implementing this cut-off?
3. Is anyone aware of any PTC resistors designed for this usage or higher current applications?
• Apologies for the slightly funky formatting, I couldn't get the markdown list functionality to properly nest my for/against items. – Matt B Nov 20 '13 at 9:51
• Bi-metallic strips are usually very reliable aren't they? The one in my home thermostat seems to have lasted a long time. What MTBF are you seeking? – RedGrittyBrick Nov 20 '13 at 9:56
• You are probably right (I don't have any experience with them), the bi-metallic strips I was looking at were rated to 10,000 cycles. We're looking for a five year lifetime, so at very worst case with a couple of cycles per day the part should be good enough. The comment was based more on a general suspicion of anything mechanical/moving and may not actually be a valid concern. – Matt B Nov 20 '13 at 10:21
• For a fail-safe my gut feeling is the choose the most simple device, which would be I think a bimetal thermostat. When I worked for a space company these things were used all over the place in satellites, so I think the reliability (at least of the parts they used) can be quite good. But, being space, they were used in groups of 4 (series+parallel) to achieve 1-fail operational. – Wouter van Ooijen Nov 20 '13 at 10:26
• Bimetal stats crop up in a hell of a lot of mass-produced devices - electric cookers, microwave ovens, kettles, Webasto/Eberspacher heaters, etc. - all safety critical & must pass safety tests / avoid lawsuits, I'd go with the herd there. – John U Nov 20 '13 at 11:40