Keeping Your Circuits Dry: How to Waterproof Electronic Circuits

For the home hobbyist, protecting their electrical devices usually means keeping the coffee cup or soda can away from anything that carries a large voltage. Good practice indicates that electrical devices should be housed in an enclosure to protect expensive components and reduce the risk of electric shocks from exposed circuitry. However, what do you do if the fantastic new device you’ve designed needs to work in a humid, damp, or dripping wet environment? Maybe it’s an automatic window opener in a humid glasshouse. Perhaps a weather sensor mounted out in the yard. It could even be a robot cleaner that wanders along the bottom of the pool if you’re lucky enough to live in a warmer country. Keeping water away from the electrics may appear daunting at first sight, but with the right enclosure and a few simple tips, it’s child’s play.

What’s the big deal?

Hopefully, this won’t come as a big surprise, but water and electrics don’t mix whether that’s water in its liquid form causing short circuits or as a vapour causing corrosion of metal parts. The result will always be permanent damage to your carefully designed device. 

The most common cause of damage comes from humidity rather than direct exposure to water. Humid air can enter what appears to be a sealed enclosure and cause water to form from condensation effects when there’s a temperature change.

Introduction to the Terminology

First, let’s start with a few basic terms. You may see things described as water-resistant, water repellent, or waterproof. It’s crucial to understand the difference as these are not technical terms but are often used interchangeably as vague marketing terms.

Water-resistant means that something is designed to make it hard for water to get inside under non-challenging conditions such as a light rain shower or a small splash of water. It does not mean that water can’t get inside. Of all the terms we discuss here, this is the lowest level of protection that something can have.

Water repellent means that something has been actively treated with a hydrophobic material or substance to repel any water that comes into contact with it. In practice, it offers similar protection to water-resistant protection, excellent in non-challenging conditions but not suitable to be left out in heavy rain or submerged in water.

Waterproof is the highest form of protection, providing a degree of protection that depends on exactly how waterproof it is. This can range from only protecting against light rain, akin to being water-resistant, through to capable of being submerged to depths measured in thousands of yards. The critical thing to note is that if something is described as waterproof, this device will never fail due to contact with water as long as that contact doesn’t exceed the waterproofing limits. Here we can call on the International Protection Marking Standard to precisely define the boundaries and solve the problem of what the term waterproof actually means. Items can be assigned an IP code that describes exactly how much protection it provides using this standard.

IP codes have two numbers; the first number refers to protection from solid objects (dust, sand, etc.), while the second number refers to protection from liquids. In this article, we are only interested in protection from liquids, so we’ll ignore the first number by setting it to ‘X.’ There are nine possible protection levels, numbered from 0 through to 9. While in principle, the higher the number, the better the protection, subtleties in the differences between resistance to directed jets of water, and resistance to immersion in still water mean that care needs to be taken in selecting the correct rating for your specific application.

As we can see, the IP rating makes it simpler to choose the best waterproof enclosure for your precious electrical device, bearing in mind that the better the level of protection then the more expensive the enclosure will probably be.IP Code

We should mention that there is no regulation to the use of IP ratings, and often products are self-tested by the manufacturer using methods that may not be completely thorough or conclusive. Like the term waterproof, IP ratings can be used as marketing terms and should be treated with caution. Always study an IP rated item’s datasheet carefully and employ the engineer’s greatest asset, your common sense!

How Do They Work?

Any enclosure with a reasonably high IP rating will comprise the main body, some lid or opening to access the insides, and a seal between the two. Your electrical device needs to be securely fitted inside the enclosure, and then all openings, including any connectors, are sealed. When done correctly, the outside environment cannot get inside. Sounds great, but hang on, what if the electrical device has a display that you need to see or buttons you need to press? Or what if there’s a sound like an alarm tone generated? How does the sound get out of a sealed box? And what about all the heat that the electronic components generate? Where does that go?

User Controls

It’s not unusual for devices to have buttons and switches for the user to operate. The simplest way to incorporate these into a waterproof enclosure is to use a flexible material such as thin plastic or rubber that allows the user to exert a force onto a button located directly under and close to the flexible material. An alternative option is to mount waterproof switches and buttons with the correct IP rating onto the enclosure and connect these to your circuit with wires or a ribbon. 

Suppose your device will be immersed in water. In that case, a great solution is a remote operation using a technology such as IR if your budget allows, and the signal will reliably get to your device. For example, Bluetooth is not an option as the signal won’t travel through water, which absorbs electromagnetic (EM) radiation at the frequencies that Bluetooth uses. IR, on the other hand, will travel short distances through water, which may be sufficient for your application if you are determined not to get your hand wet. A cheaper solution is to run a waterproof cable from the submerged device to somewhere a little dryer where you can mount your user controls. This will also potentially offer a solution to the problem of displaying information to the user. Displays can be provided on the user control rather than the user trying to peer at an LED mounted on a submerged enclosure through murky water.

User Displays

The issues with displays very much follow the same pattern as controls. A transparent panel in the enclosure allows the user to view any lights or displays within the enclosure if it’s practical and possible to view them. Alternatively, mount waterproof lights of the correct IP rating into the enclosure body itself and connect these to your circuit with wires or a ribbon to make them more visible.

Sound Generation
We can see that mounting a buzzer or speaker within a standard waterproof enclosure isn’t going to be very effective. At best, the sound will be muffled, and at worst, it will be too quiet to hear. Waterproof buzzers and speakers are readily available and can be connected to your device, either as a separate item connected by a lead or fixed to the device’s enclosure as a single package. 

We don’t need to point out that these solutions aren’t appropriate if the device will be submerged in water unless you’re planning on sticking your head below the surface to try and hear any sounds. But, for a device that will be located outside where it will be exposed to rainfall, these IP rated devices will perform satisfactorily.

Heat Dissipation

All electrical devices generate heat to some extent; the greater the efficiency, the lower the heat generated, but nothing is 100% efficient – the second law of thermodynamics tells us that. If you have a low power device with pretty good efficiency, then the heat generated may well not be an issue. The temperature inside the sealed enclosure will rise but will probably stabilize at a value that’s well below your components’ maximum temperature ratings and the PCB. But what do you do if your electronics generate excess heat that you need to get rid of to stop things overheating?

We usually rely on convection to dump heat; air circulating over the device carries heat away to the colder environment. Sometimes we’ll add a fan or a heatsink to speed the convection process up. However, we can’t drill holes in the enclosure to allow the air to flow, which would rather defeat the purpose of having a sealed enclosure in the first place!

The next option is to rely on radiated heat, great for electric room heaters running at many hundreds of degrees but not very effective at the temperatures our electrical components will be running at. 

That leaves conduction as the viable option, transferring heat through the enclosure walls and into the environment. The only issue here is ensuring that the enclosure is made with a material that is a good conductor of heat, such as aluminum or steel. 

The careful consideration of how and where components are mounted can help get any heat energy away from the components and into the enclosure’s body. Heat conducting materials can be used to bridge the components directly to the casing or via the PCB. Once the heat energy reaches the outside of the enclosure, the conduction effect can be boosted. This can be done by maximizing the surface area (metal fins) or using a mechanical device such as a fan or impeller to increase the air or water flow (depending on whether the enclosure is submerged or not) to carry away heat.

So, the Problems are Solved Then?

Unfortunately, placing your electrical circuit inside an IP rated enclosure doesn’t automatically solve all the problems. Bear in mind that the IP ratings correspond with set test conditions that may not completely match the real-world environment you’re operating in. For example, an IPX6 enclosure that provides protection when tested using a high-pressure water jet may well still allow air to enter and leave the enclosure. This means the inside of the enclosure may be susceptible to moist air entering if it’s in a high humidity environment. This humid air can then condense out as water inside the enclosure. Over time this moisture may well build up until it causes a short circuit. When choosing the enclosure, check the specification in thorough detail and consider whether it is airtight. If there is a risk of condensation, consider using a water-absorbing material like silica gel inside the enclosure but remember that it doesn’t work forever. There’s a limit to how much moisture it can absorb, and once it reaches that limit, it stops working. Another common solution in the commercial world is to fill the enclosure with a dry inert gas like nitrogen to keep humid air out. This is only effective if the inert gas can’t leak out over time. In the petrochemical industry, they use slow-release gas canisters inside enclosures to provide a steady replacement for the inert air. However, this is an expensive solution that takes up a lot of space, so unless you’re concerned with keeping potentially explosive gases out of your enclosure, it’s not practical.

If you have wires entering the sealed enclosure from the outside, you could well find that you have water traveling along the wires thanks to capillary action and entering the enclosure by that method. Any connectors fitted to the enclosure must be the correct IP rating; they must be the same rating as the enclosure as a minimum. Don’t forget that a larger number doesn’t automatically mean that it’s better when it comes to IP ratings. Which ratings are better than which other ratings depend on if you are looking to protect against immersion, sprays, jets, or humidity? Also, make sure that they are fitted correctly, that all sealing devices such as O-rings are present, in good condition, and fitted properly.

Another critical factor is that water isn’t the only environmental factor you need to consider. For example, if your enclosure is going to be located outside in direct sunlight, it will need to be made from a material that is resistant to UV light. Plastic products may offer great low-cost solutions for waterproof enclosures, but it doesn’t take long for sunlight to make the material brittle, compromising its waterproofing abilities. On the other hand, a metal enclosure located in direct sunlight may well absorb IR and heat up during the day and then cool down at night. This daily cycle of expansion and contraction from the temperature change may compromise any waterproof seal over time.

Finally, there’s the assembly of the enclosure itself. If the parts are sealed using screws or bolts to join them together, both under or over-tightening may well distort the enclosure or any sealing parts.

Conclusion

The key lessons that should come from this article are check, check, and check again. Check that you precisely know what protection your device requires from all environmental factors. Check that the enclosure provides the protection that you need. Then check that the enclosure has been correctly assembled, so it offers the required level of protection.

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