# How to design circuits to work under -55 Celsius degree?

Possible related post:Warming PCB in a low temperature environment

I am trying to design a circuit which should work properly under -55 degree. There are two options in front of me: to choose specific components and carefully design the circuits to fulfill the environment requirements, or to heat the circuit up.

After searching the internet for a while, I decide to choose the second one, due to the complexity of our circuit. It would be easier just to heat the system up.

Here comes some of my initial thoughts, which are contributed by the mentioned post.

1. The circuit shall be divided into two parts: heating part and the functional part. The heating part is to create a suitable environment for the functional part. The functional part can only work properly, say, above +10 degree.
2. The functional part shall not work when the temperature is below +10 degree. It shall wait until the heater heat up the environment.
3. An analog temperature sensor like this one will be used to measure the environment. It will control a 1*2 switch: one end system power, the other end heat up circuit OR the functional circuit.

My questions:

1. Could I get some comments? Or any better designs?
2. How should I calculate the number of heat-up resistors?
3. How to calculate the heat-up power needed?
4. Some sort of heat-isolation material shall be used. How to find them?
• From what I know about low-temperature pitfalls: Although most electronic components are not rated to work at -55 C probably most components will just work. Except electrolytic caps I guess. Maybe some "military specification" components are guaranteed to work at -55 C ? Batteries in general do not like the cold ! Maybe condensation can be an issue if part of the circuit is still cold but other parts heat up ?? I think many smaller heat-up resistors is a better idea than a few large ones. How much power is needed depends on the time it will need to heat up and how well the system is isolated. Jun 22, 2015 at 12:16
• Take note that beside a minimum operation temperature components also have a minimum storage temperature. So 'letting it cool down and then heat it p to above operational temperature' might result in a permanently dead system. Jun 22, 2015 at 14:57
• As @WoutervanOoijen also hinted to, if you heat up your circuit from a very low temperature to +10°C too quickly you risk thermal stress that can permanently damage your parts, for example tiny bond wires inside semiconductor devices, since the materials they are made of have different thermal expansion coefficients. BTW, is -55°C the worst case scenario, or is it an average prediction? I.e. could an exceptional event (e.g. a snow storm) bring the circuit to much lower temperatures? Jun 22, 2015 at 17:15

1. The sensor part you suggest is only rated to work to -55°C, not below. It probably will work somewhat below that but if you really need it .. that part is not guaranteed. There are PTC heater elements that can be used that don't require active circuitry.

2. Number and rating of resistors (or other heater) depends on the physical arrangement. It's not easy to calculate. You could to a heat flow FEA or build a prototype and test it (you don't necessarily have to go that low for testing the heat flow).

3. Covered by 2.

4. You mean insulation? Engineering textbooks have thermal resistance figures. Materials commonly used that have good insulation usually have some trapped air in them. You may have other requirements (fire rating, toxicity, cost) that will limit choices. Urethane foam is one common one. There are much more expensive insulations that perform even better, but you don't necessarily want a really good insulation. If you have a super-insulation your circuit may overheat during normal operation in a warm environment.

So I think your design process would be to determine the mechanical design and insulation you want to keep the internal temperature to an acceptable level with a hot environment (and covering the other factors), then estimate the power required to increase the internal temperature from the lowest ambient at least to operating temperature. You can also decide where to turn the heater off at- if the internal power is less than the heater power you may end up with the heater turning on and off (or if you control it smoothly the power will decrease as the ambient temperature increases).

You also need to check that the storage temperature of all the parts in both circuits includes the lowest possible ambient and that every part in the temperature control circuit will operate at the lowest possible ambient (or if not operate, at least not shut off).

Some consideration may have to be given to frost and condensation, but that's outside of the scope of this question.