Circuit protection isn’t just about diodes and fuses. You can protect your circuitry from lightning with a special component, but how can you protect it from environmental stressors like moisture or vibration? Here are some tips to save your circuitry from events of the non-voltage variety.
Potting Materials
To “pot” a board or component is a technical term for literally pouring goo on top of it. This might not sound appealing right off the bat, but potting has definite advantages.
Waterproofing
PCBs are commonly potted entirely in a box so that the potting material completely surrounds the PCB and protects the build from all angles. When a water-resistance gel is poured over an entire board, the effect is much like wrapping it in plastic. It conforms to the surface of all parts and protects them from moisture even under full submersion.
Vibration Resistance
Potting your design also protects against vibration. Solder is good at many things, but not at forming a bond impervious to vibration. If a circuit board is subject to enough shock or vibration, the solder joints tend to start to crack and the parts will lose connection or even just fall off the board. Putting an adhesive around the components (or even just those particularly susceptible to vibration) acts as a shock-absorber around the component and prolongs the life of the solder joints.
Thermal Relief
Some potting materials are specially designed to help wick heat away from components and distribute it evenly through the material. They can be used to surround a metal heatsink or by themselves in a potting box.
Potting materials that are thermally conductive aren’t just for pulling heat out of a hot chip – they can also be used to distribute heat from a hot spot in the environment. If a potted PCB in a box is resting on a car engine, the potting material will help evenly distribute that heat through the box rather than concentrating the heat on the components closest to the bottom of the box. But if your circuit needs to live in a very hot environment, metal heat sinks may not be enough to thermally manage your board. Active cooling, like a fan, can significantly lower the temperature of a PCB just by keeping the air moving. Active cooling can be as simple as a small axial fan, or as complicated as a custom fluid or gas cooling system.
Strain Relief
Anyone who’s tried soldering down a wire knows how hard it can be to keep in place – now imagine putting it in an environment where it’s constantly tugged on for one reason or another. Strain relief is a broad term for putting something on the wires that can absorb the strain of being pulled upon. Typically, this is a gasket that hugs the wires where they enter an enclosure containing the rest of the system. The rubber scrunches down around the wires and holds them in place, so any strain on them does not go straight to the board connections and rip them out.
Of course, there’s always the poor-man’s strain relief option – tie a knot in the wire that cannot fit through the hole of the enclosure on either side of the enclosure wall. If you don’t have an enclosure to brace against, look for a board mount connector with built in strain relief.
If a disconnected wire could have dangerous consequences to the circuit (runaway voltage on a switcher, for example), a third option is implementing a strain sensor that cuts power to the entire system if strain on a wire exceeds a certain threshold at which it is likely to get pulled off. Also, some wire to board connectors are specially designed to withstand vibration, like Phoenix Contact’s Spring Cage connectors. The spring-cage connection always exerts the same force on the conductor regardless of external stresses, guaranteeing a constant connection.
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So when you’re thinking of circuit protection solutions for your design, remember to think outside of the box by thinking inside of the box. These physical circuit protection solutions might be the difference between design success or design disaster.