Designing for extreme temperatures: -50°C to +50°C ambient

[RyanT]

I've never had to design anything to run in less than 0°C ambient or more than about 50% RH, which certainly made things simple.

Project brief: I want to add some custom smarts to my garage, car, and porch. Where I live, this means -50°C to +50°C ambient, and humidity from 0-100% RH (usually well below 50%), with temperature swings of up to 30°C overnight. Nothing directly exposed to the elements. Nothing mechanical, aside from a few buttons. All circuits below 14VDC, and low enough power that passive cooling should be more than adequate. Most of these would be either mains rectified or battery powered, while at least one would run off of car wiring. Expected lifespan would be a few years at <1% duty cycle. In other words, not setting the world on fire (pun simultaneously intended and not). Cost considerations: these are one-off fun projects, so I'm not trying to shave pennies off of the BOM by any means.

I realize the "correct" answer to my implied question is "duh, read the datasheet!", and of course I will when it comes down to it, but more specifically, I'm looking for higher level, rule-of-thumb type observations that apply most of the time. Acceptable battery chemistry, component types to avoid (I'm looking at you, standard temp. range LCDs!), PCB layout considerations, use (or not) of potting compound, enclosure considerations, that sort of thing.

I also realize that, in some respects, designing for cold temps is very different from designing for hot, so if you have expertise in one but not the other, please share what you know.

[MosherIV]

Hi

You are pretty much looking at the kind of design used for military electronics with your -50°C spec.

I can pass on some techniques.

Choose hermetically sealed aluminium encloses.
Choose components rated to high temps (50°C is actually not that bad for mil spec)
Mount components/boards so that they use the enclosure as a heat sink.

Most people do not know that some mil spec equipment use built in heaters to cope with low temps.
Use heaters (power resistors) where components are not rated for below 0°C

Use conformal coating to seal boards against moisture. You will get condensation inside your enclosure!
You will get condensation inside your enclosure even though it is sealed (yes, I repeated myself becuase it is that important).

Use mil spec circular connectors. They are guarented to be sealed against moisture. Use any other type and you will loose the seal the enclosure gives you. Be warned, they are EXPENSIVE. So use sparingly.

Hope that helps.

[voltsandjolts]

^ what Mosher said...and don't use any batteries or displays (other than LED) and you'll be fine!

[Rerouter]

If your using a microcontroller, be aware RAM can take many minutes to clear at significant negative temperatures when power is disconnected / interrupted, so be sure to clear and initialize any relevant registers on every start up.

equally capacitors can get weird, most datasheets dont go that low,

[Ian.M]

Specifically any wet or gel electrolyte capacitor (e.g. Aluminium or wet Tantalum electrolytics, and supercaps) may be be damaged by -50°C, and ion mobility in solid polymer electrolytes is likely to be so much lower at -50°C that  the ESR will dramatically increase.

[RyanT]

Hi

You are pretty much looking at the kind of design used for military electronics with your -50°C spec.

O Canada, eh!

I can pass on some techniques.

This is exactly the sort of information I needed. It's all helpful. Thanks very much!

The power resistor to warm the enclosure is a neat idea. Even well-insulated, that could increase my average power consumption by an order of magnitude, though, but then again voltsandjolts also said batteries were a no-go, so I guess I'll have to mains power all of these. Not a big deal, just a little more work.

[max_torque]

The answer, often missed these days is "keep it simple"

If you only need to show  few states, just use an LED and not an LCD

If you don't need loads of buttons, don't fit loads of buttons

If you don't need a 32b, floating point 250Mhz processor, don't use one!

-50 to 50 is actually not too bad.  I'd also suggest that it's actually really pretty difficult to actualy soak a running electronic device to a genuine -50 degC due to the heat that device intrinsically makes, and the thermal impedance to the environment (ie, put it in a plastic box and not a metal one)


Humidity wise, use a "gore vent" to provide pressure equalisation but not allow water vapour in/out, use a large sachet of desicant in the enclosure to absorb any mositure that does get in (obvs, once temp drops below about 3 to 5degC humidity drops to pretty much zero)


The biggest issue will be sealing for pass throughs.  This means decent sealed connectors and thinking about what controls need to actually pass through permanently. There are a wide range of commercial connectors designed for arduious off-highway applications like excavators and dump trucks etc, from companies like deutsch (google DT and DTM series) which use plastic housings to keep costs down.  Full MIL spec metal connectors are EXPENSIVE and an overkill for this sort of application. One option for controls is to use a "remote" control pad, which could be standardised, kept indoors, and just be connected to each device when needed.  You can get "capped" connectors that use a sealing cap to fully seal them when not used, and hence you could have a std 4 wire interface (power + coms) and use one "controller" for all your devices?

Most off-the-shelf capacitor ratings stop at around -45 degC, but in reality, as mentioned, i doubt you'll actually see any lower, especially in a sheltered or semi-sheltered environment.  Certainly avoid big electrolytics for critical items.

One important item i fit to all my control units now, is an internal temp sensor (some micros now include this internally).  That enables me to log the temps the device has actually seen in the field, and provides me with important design data!

[RyanT]

The answer, often missed these days is "keep it simple"

If you only need to show  few states, just use an LED and not an LCD

If you don't need loads of buttons, don't fit loads of buttons

Only one device will need a display, and that'd be the car-mounted one, which would only operate while the car is running, in which case the car's interior would be considerably warmer. As long as I can find a reasonably priced display that isn't actually damaged by extreme temps, I don't care if it needs to warm up a bit before it will update.

All of the other devices will only communicate wirelessly, so they can be more or less completely sealed units if needed. Aside from a power/status LED, I don't really need anything external, though at least in the prototyping stages I might fit a couple of buttons for ease of debugging. It's very early in the design process. I'm designing a single PCB layout for all of them, and will just populate what I need, as they are all substantially similar, just different configurations of the same firmware.

If you don't need a 32b, floating point 250Mhz processor, don't use one!

Whoa, hey, now that's just crazy talk! I was thinking my 8-bit PIC running at minimum speed was already way overkill.

I'd also suggest that it's actually really pretty difficult to actualy soak a running electronic device to a genuine -50 degC due to the heat that device intrinsically makes, and the thermal impedance to the environment (ie, put it in a plastic box and not a metal one)

Based on my experience as a mammal living in this climate, I have a lifetime of practical experience to back up your above statement. The duty cycle of my devices will be below 1% (how much below, I'm not sure yet). But I'd imagine even the "low power" modes and mains power supply (if captured somewhat thoughtfully) would buy me several degrees at least.

One important item i fit to all my control units now, is an internal temp sensor (some micros now include this internally).  That enables me to log the temps the device has actually seen in the field, and provides me with important design data!

Indeed, temperature recording is an application requirement already, so I'd get that diagnostic information for free.

Thanks to you (and everyone I've not directly replied to) for all of the great information.

[Vtile]

Hi

You are pretty much looking at the kind of design used for military electronics with your -50°C spec.

O Canada, eh!

Huh, Nordic.