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I have a circuit that should be protected from high voltage. Is there any component other than a fuse that I don't have to change every time? Something like a switch that activates only when high voltage is supplied, diverts the flow and grounds it, and then closes.

Sorry, I can't scan the schematic right now but I can give you information. I have a triac connected to a submersible motor via a MCU IC mounted on PCBA. The problem is sometimes there is a short circuit due to water. I want to know how to protect the circuit in this case as I already have fried it twice. Is an optocoupler or solid state relay advisable for isolation?

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    \$\begingroup\$ We're going to need a LOT more information. What voltage? What duration? A schematic would go a very long way to showing us what you want. There is a huge difference between a static discharge and a direct lightning strike, and different solutions. \$\endgroup\$ – user3624 Dec 19 '12 at 5:21
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    \$\begingroup\$ Still not very clear. Are you trying to protect from high currents (caused by shorts within your submerged equipment) or from high voltages (maybe caused by a short circuit between a higher-voltage input power wire and internal nodes)? \$\endgroup\$ – The Photon Dec 19 '12 at 5:34
  • \$\begingroup\$ short between ground and live wire \$\endgroup\$ – Deep Dec 19 '12 at 5:36
  • \$\begingroup\$ In case you're protecting from high currents, do you know about polyfuses (resettable fuses)? In case of protecting from high voltages, do you know about zener diodes and transorbs? \$\endgroup\$ – The Photon Dec 19 '12 at 5:36
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    \$\begingroup\$ I'd try to fix the leaking water first. \$\endgroup\$ – jippie Dec 19 '12 at 7:12
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So in general the way I do this is that I have a circuit which looks something like this:

schematic of a water shut-off circuit

So essentially, the idea is that there is an input power mosfet which controls the flow of power to your circuit. You push a button, which causes the gate voltage to rise, and power flows to your circuit. "VDD" in this diagram is the battery positive rail. Ground is assumed to be available and shared all over this circuit.

The "small gap" is a physical one. If water connects ground and the wire coming off of that op-amp, then the op-amp will output low, and you'll suddenly lose power.

Now this circuit needs a lot of things, it's only meant to be the beginnings of a circuit that you could use. But it should push you in the right general direction. The robotics team I was on built an underwater vehicle and I can tell you for a fact that the idea behind this circuit is tried and true... the schematic I have for that part of the craft though was a bit more complex because it was dealing with a lot of other parts, so you get the simplified but incomplete version for now!

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In a comment you say you're worried about shorts between live wires and ground, causing high currents. But in your question you say you maybe want

a switch that activates only when high voltage is supplied, diverts the flow and grounds it, and then closes?

This is unlikely to work because when there's a short from a power net to ground, it tends to cause the voltage on the power net to fall (due to resistance of the power wires), not rise. In any case, the behavior you ask for is essentially a short from power to ground.

It's not clear to me how shorting power to ground would protect you from shorts between power and ground. However if I've misunderstood your requirements and you really do want this behavior, what you're looking for is called a crowbar circuit.

other options

One thing that might help you is a polyfuse or resettable fuse. These work like fuses, in that they increase their resistance dramatically when current rises too high, typically shutting down the load. Their advantage over a traditional fuse is that they revert to normal operation after cooling down, so they don't have to be physically replaced every time they operate.

However like fuses, they may take tenths of a second or seconds to operate, which could be too long to avoid damage for some sensitive equipment. Fuses are not really meant to protect your equipment, just to stop it catching fire and damaging things around it.

Another option would be to modify your power supply to include overcurrent protection. Linear Technology, for one, offers several regulator ICs with an overcurrent protection feature.

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  • \$\begingroup\$ Thanks, I am a bit confused at the moment, will check it out again by applying a polyfuse in series \$\endgroup\$ – Deep Dec 19 '12 at 5:50
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Unfortunately I would say "you're doing it wrong" - if there are common shorts to ground that blow your kit up, you need to design the drive circuit to sense & shut-down, not just to not explode. This could include feedback, passive current limiting (NTC / inrush limiter), active current limiting/load-sensing, fail-safe principles, and some sort of lockout (is the short when someone is accessing the connections, can the operation cause some feedback that kills the circuit while the user is fiddling?).

This is ignoring the safety issues around whatever the heck you're doing with power & water, as I'm guessing this project is not going into commercial production.

Some important distinctions: Fuses protect from over-current. Tranzorbs/MOV's throw themselves across the rails when a large voltage comes by, but may or may not survive repeated hits.

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