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Everybody has had the time where water gets all over their electronics, and the whole thing gets ruined, like cell phones.

At that low of a voltage (3-5 volts), I don't understand why it is such a short-term harm (long term makes sense - corrosion, etc.)

If an LED was in parallel with water, maybe a bit more current would be drawn, but it doesn't seem to be enough at all to short out the system, and the LED would still shine.

So what is it that permanently damages some electronics and what is the cause?

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  • \$\begingroup\$ Imagine there is a device, say some output pin, that can only source 5mA. When submerged while powered-on, this pin will become shorted to ground, and due to Ohm's law, will try to source much more current than that, often damaging sensitive internal components. This is just one mode of failure. \$\endgroup\$ – Jonathon Reinhart Sep 5 '13 at 3:26
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    \$\begingroup\$ @JonathonReinhart - You seem to be making the assumption that water is extremely conductive. This is generally incorrect. \$\endgroup\$ – Connor Wolf Sep 5 '13 at 3:35
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    \$\begingroup\$ @ConnorWolf Not necessarily. I said "shorted to ground", not "dead shorted to ground". \$\endgroup\$ – Jonathon Reinhart Sep 5 '13 at 3:45
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    \$\begingroup\$ @ConnorWolf for practical purposes, it's extremely conductive. Sure it's no superconductor. But it's not air. Let's not be pedantic about this, this isn't the physics se. \$\endgroup\$ – Passerby Sep 5 '13 at 8:55
  • \$\begingroup\$ There's some parts of circuits which are sensitive to minute changes too - look at DDR RAM tracks, crystal oscillators, etc. - wherever tiny currents, high frequencies, or low voltages exist, a small change can screw the pooch. \$\endgroup\$ – John U Sep 5 '13 at 14:21
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Pure water is actually not bad for electronics. Pure water does not conduct electricity. I've seen whole PCBs submerged in pure water and they run just fine. The problem is that pure water does not stay pure for long. It will quickly dissolve/absorb various contaminants from the environment, and those contaminants will cause the now not pure water to conduct electricity.

These contaminants come from the environment-- including the air. So dust, dirt, and even CO2 will cause the water to conduct. Tap water has lots of minerals and salts in it that will also conduct.

But normal water (not pure water) won't destroy most electronics when the circuit is off. I frequently rinse off PCB's in the sink, or even a normal dishwasher, to clean it. I just have to make sure that the water completely dries and doesn't leave a residue before turning it on.

But the reason why normal circuits, submerged in normal water, doesn't work is because normal water is conductive. It isn't a perfect conductor, but it is enough of a conductor. If you get enough electricity flowing in/through places that it wasn't intended to then that's bad. If you're lucky it will just make the circuit temporarily misbehave. If you're not lucky then you'll have permanent damage.

Simple circuits, like an LED+Resistor+Battery will likely work just fine when submerged. The LED might not stay lit, and the battery might be completely discharged. But dry it off and replace the battery and it should work fine. But some circuits are more sensitive. Think of a MOSFET that is switching hundreds of amps/volts. It takes only a little bit of electricity to turn the MOSFET on, and the water is just conductive enough to cause it to turn on. But now you have huge amounts of power turned on when it shouldn't be-- so it is no surprise that something can be damaged.

Or think about the resistive voltage divider on the feedback of a DC/DC converter. That is what sets the output voltage. Add some water and the output voltage could be forced too high. It wouldn't take much for water to mess up that divider. Now, instead of outputting 3.3v it is spitting out 9v. Of course, any chip that is being powered from 9v instead of 3.3v is probably dead.

So, non-pure water is bad. It kills things.

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    \$\begingroup\$ Additionally, try sticking two probes from a multimeter into a glass of tap water and measure the resistance. Distance between the probes will affect the measurement slightly, but you may be surprised at what order of magnitude the result comes out to be \$\endgroup\$ – Shamtam Sep 5 '13 at 5:45
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    \$\begingroup\$ Corrosion is another issue. Water tends not to be exactly pH 7, but rather somewhere in the range 6.5 to 8 depending on your mineral content. The various salts in the water start to oxidise solder joints and other component surfaces, leading to increased wear and damage. We used to see this a lot in mobile phone repair - the fix involved using a vibration bath and solvents to clear off the residue, then manually touching up any corroded joints. \$\endgroup\$ – Polynomial Sep 5 '13 at 9:43
  • \$\begingroup\$ @AlvinWong That depends on which chips you are referring to. Most 1.8 Volt ICs will be destroyed even with 5 Volts, let alone 9. \$\endgroup\$ – Anindo Ghosh Sep 5 '13 at 10:11
  • \$\begingroup\$ @AlvinWong It depends on what you mean by CMOS. Technically, almost every modern chip is CMOS, the the Intel i7 will die from 2v to the wrong pins. Although the 4000 series are certainly more robust in this regard. \$\endgroup\$ – user3624 Sep 5 '13 at 12:21
  • \$\begingroup\$ Bear in mind even a couple of AA batteries can deliver damaging and dangerous currents if shorted across the wrong pins. Reverse biasing, scr latching on a 5v device, with a 5v supply, may cause permanent damage to the part. \$\endgroup\$ – Danny Staple Sep 5 '13 at 13:11
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Although I have never heard of this happening, it is also possible that powering-up a water soaked device containing a cheap SRBP board could be a fire risk.

As a reckless teenager I used to take delight in wiring 12V dc across a pair of adjacent tracks on a piece of cheap stripboard, then placing a drop of tap water across the tracks. At first, all you get is a load of hydrogen & oxygen but eventually the heated water partly evaporates and partly soaks into the cheap SRBP base material. Eventually, the board gets so hot it starts to carbonize, then sparks appear between the tracks and ultimately the board catches fire!

I don't know what the minimum voltage would be for this to occur (I haven't tried recently!) but 12V at a few hundred milliamps will do it with 0.1" pitch Veroboard.

I have a disused in-car mobile phone charger which would be an ideal candidate for an experiment later ...

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    \$\begingroup\$ Reckless? This is how I got into electronics. That and gluing two halves of a coin to the pavement wired to a high voltage generator. \$\endgroup\$ – Rocketmagnet Sep 5 '13 at 12:06
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    \$\begingroup\$ @Rocketmagnet - Looks like the killjoys have put a solder resist layer on my old phone charger. Shame, I was looking forward to reliving my youth - and setting myself on fire :) \$\endgroup\$ – MikeJ-UK Sep 5 '13 at 13:34
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Low-voltage microelectronics often have low tolerances to higher currents and voltages. It's the nature of improved micronization and energy efficiency. By adding water, you are adding electrical paths for various parts that were not meant to be there. Things get shorted out, protection parts get skipped, parts can receive higher voltages than they can tolerate.

A given device might be powered by a 3.7 V battery or 5 V USB connection, but there could be step-up regulators for certain subsections of its electronics. You might have a device with a step-up to 18 V. Add water to create an unwanted electrical path, and that 18 V subsection just shorted out to a 5 V-only section, killing every chip on there.

An IC could only support sinking or sourcing 10 mA. Add water and a short to ground or Vcc would cause a lot more than 10 mA to be pulled, frying that IC's pin, if not the entire chip. Poof, there goes the LCD on your phone.

The main reason this happens is because it is not individual, unpowered parts, but an entire board of possibly thousand parts, all with various maximum voltage and current thresholds that are carefully laid out in a manner where electrical paths are carefully controlled.

To compare, your car can sit in the rain without water getting in (if it's well designed and maintained, naturally). Drive it into a river (or the river comes to you), and water will destroy the inside of the cab and engine. That's what you are doing when you introduce water to electronics.

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  • \$\begingroup\$ I suspect that one could probably immerse the vast majority of stuff on a typical board in a slightly-conductive liquid (e.g. adding paths with ~1Meg resistance between every connection and a common "bus") without damaging anything--perhaps even without affecting operation. Many boards, however, include power-supply circuits that (to minimize quiescent current draw) use high-resistance feedback paths. Adding a parallel 1M resistor to ground could cause a supply circuit which should output 3 volts to instead output 6. \$\endgroup\$ – supercat Sep 5 '13 at 21:25

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