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I'm designing a small module that takes power from USB and provides it to a breadboard for prototyping. My target audience will just be hobbyists so I want to include some protection. I'm more concerned about protecting the USB supply from mistakes made whilst prototyping rather than protecting the board from faulty supplies.

I'll be including a PTC resettable fuse to protect the USB supply from shorts and over overloads. One thing I am concerned about is when a higher voltage (say 12v) is accidentally connected to the 5v rail, I don't want this to be backfed to the computer or other device that is providing the 5v over USB.

Normal use of a zener diode would be to be place it after the PTC fuse, however I am thinking for my circuit it might be better to place it before the fuse. This way if a user was to accidentally connect 12v to the 5v line, it would first flow through the PTC and then the zener, hopefully tripping the PTC.

Is my thinking here correct?

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    \$\begingroup\$ Designed for use by hobbyists? Isolate and be done with it. \$\endgroup\$ – Matt Young Mar 18 '16 at 20:05
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    \$\begingroup\$ If you're concerned about protecting the USB host, then just don't make it possible to connect to the USB 5V directly. USB voltage comes in, goes through a PTC, and that net is your system 5V. \$\endgroup\$ – Brendan Simpson Mar 18 '16 at 20:42
  • \$\begingroup\$ That is what I am doing, but if 12V was accidentally connected to the 5V rail, it would still flow through the PTC back to the USB 5V. The fuse may trip shortly after, but that may be too late. \$\endgroup\$ – noggin182 Mar 18 '16 at 20:50
  • \$\begingroup\$ Your thinking isn't too bad but this is not a great solution. A zener diode needs some resistance in series with it to limit the current through it, otherwise the zener will fail due to over-current. The PTC fuse will take several seconds to go high resistance, so it will not protect the zener. The best solution to protect against back-feeding voltage is to block it with a schottky diode, but this will drop 0.3V so your module will only output 4.7V. \$\endgroup\$ – Steve G Mar 19 '16 at 10:50
  • \$\begingroup\$ @SteveG An ideal diode controller (like the ones made by Linear Tech/Analog Devices) would also work but would typically have < 50mV forward voltage drop. \$\endgroup\$ – user4574 Jun 6 '19 at 18:49
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Your thinking works, but seems unnatural. Why is the 5v likely to get connected to 12v?

Personally I would put ptc fuses on the output of any user accessible 5v terminals, as well as a standard fuse and voltage clamp on power input the normal way round.

Incedentally, a zener diode is not the right component for this. check out tvs diodes for overvoltage psu protection

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  • \$\begingroup\$ Thanks. All my module will be doing is providing power to the 5V rail of a breadboard and the target audience are the kind of people who tinker with arduinos. It's quite common for them to be using a 5V microcontroller to turn on 12V motors (as an example) where wiring a transistor incorrectly or inserting a wire into the adjacent socket by mistake is all too easy to do. My understanding was that TVS diodes were used mostly for signal/data lines rather than power, I'll read up a little more on them. But for this case a TVS that has its annode connected between the USB 5V and PTC is fine? \$\endgroup\$ – noggin182 Mar 19 '16 at 0:02
  • \$\begingroup\$ Tvs diodes can be used for a variety of protection schemes. The ones used for signal or data lines are typically esd protection diodes, which cannot handle large currents and will die if you connect a power rail to them. Look at the 5kpa series for the other end of the spectrum. Does your module do anything other than breakout the 5v for breadboard use? \$\endgroup\$ – Loganf Mar 19 '16 at 1:09
  • \$\begingroup\$ Thanks! I'm reading a little more into them now. No, this module is solely to breakout the 5V from USB to a breadboard with some protection. I just realised (with your help) that for this a crowbar device would most likely be more suited, if there is an overvoltage condition it would be best to blow the polyfuse to isolate the harmful voltage from the USB. \$\endgroup\$ – noggin182 Mar 19 '16 at 1:43

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