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I am running 3.3V over a USB cable from circuit A to supply voltage to another circuit B. I am worried that someone may plug 5V into this line and fry my board(s)

There are two scenarios.

  1. 5V supplied will go to output of my 3.3VLDO on circuit A and all of my components some of which cannot handle 5V

  2. 5V supplied to my 3.3V circuit B which may handle 5V but I'd rather clamp it to 3.3V max

My initial solutions are:

  1. Is to either use a crowbar (may be overkill) or just put a schottky in series and run 3.1V over the lines to power circuit B (which should be okay, maybe put a boost on the other side to regulate back up to 3.3V)

enter image description here

  1. Clamp the Voltage to 3.3V using a 3.3V Zener in Parallel

enter image description here

Anyone have any thoughts on how this might be done better, or is this a reasonable/cheap way to get the desired protection?

EDIT: at this stage I took the advice about using a standard jack in a non standard way. I think ill use a TRRS jack (yes I know the irony of using another standard header) This minimizes the risk to almost zero. I can use the switching capability of some TRRS jacks to cut off the pins until fully connected if I need: but I dont think I need to with my circuit. Thanks

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    \$\begingroup\$ Don't use a standard connector for a non-standard application. Find another connector for your 3.3 volt power connection. \$\endgroup\$ Jun 12, 2019 at 3:25
  • \$\begingroup\$ I wholly agree with you on not using a standard connector for a non-standard application, its definiely a big "con" in the design choice: but in this case I feel regulating/protecting the 5v to 3.3v shouldn't be too much of a problem and worth the non-standard application. The lines only run ~1m and need 4 lines: seems perfect for a USB cable. I've also seen micro-usb used for many non standard applications like ECG nodes etc, and means if the cable is lost: any other cable can be used. \$\endgroup\$
    – CalDow
    Jun 12, 2019 at 3:36
  • \$\begingroup\$ @CalDow Why not try using cat5 cable instead? It's also very common, but less likely to get accidentally plugged into something else. \$\endgroup\$
    – Hearth
    Jun 12, 2019 at 4:19
  • \$\begingroup\$ @Hearth That's what I currently have on the prototype actually haha, the rj45 connectors and cable is just a bit bulky for my application: hence the change to micro USB. 100% would use a different cable / connector if there was one that was low profile and common enough. (4 wire is all thats needed) \$\endgroup\$
    – CalDow
    Jun 12, 2019 at 4:24
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    \$\begingroup\$ Design your system to distribute 5 V and regulate it down to 3.3 V at each peripheral. \$\endgroup\$
    – The Photon
    Jun 12, 2019 at 5:19

3 Answers 3

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Sometimes a stupid but convenient design just needs stupid protection.

enter image description here

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  • \$\begingroup\$ I like it. Not sure I like the lipo explosion/risk if things went wrong. But I definitely will have this on there anyway. \$\endgroup\$
    – CalDow
    Jun 12, 2019 at 4:25
  • \$\begingroup\$ You can always add a PTC \$\endgroup\$ Jun 12, 2019 at 4:43
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An ideal diode controller on the output and hotswap controller on the input would probably accomplish this.

There are many options, but in general each category will lead to the correct non-blocking diode orientation for each side. Most of these devices are higher min voltage though, so that will be something to watch out for.

There are also large surge-stopper type devices, but the cost and size can get out of hand really fast. ADI (previously LT) parts are mostly in that category.

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  • \$\begingroup\$ bit much for this application, but an ideal diode would work AOK on the output. Its probably what I would do if I were still rolling with this idea \$\endgroup\$
    – CalDow
    Jun 14, 2019 at 2:45
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Use a voltage regulator. 3.3 Volt regulators are easy to find out.

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    \$\begingroup\$ How will that work when it's plugged correctly into a 3.3 V supply? \$\endgroup\$
    – Transistor
    Jun 12, 2019 at 9:50
  • \$\begingroup\$ A voltage regulator can reduce the input voltage to a certain voltage. If you want to lower the supply voltahe then you should use one. \$\endgroup\$ Jun 13, 2019 at 13:33
  • \$\begingroup\$ S/he doesn't want to lower the voltage. S/he justs wants protect if the wrong voltage is fed in. Your regulator idea will drop the 3.3 V to less than 3.3 V so the circuit will not work when plugged into the correct power supply. If you read the question again you will see that the OP knows about voltage regulators, LDO regulators and crowbar circuits so I don't think you have answered the question at all. You can use the edit link below your answer to improve it. \$\endgroup\$
    – Transistor
    Jun 13, 2019 at 23:44
  • \$\begingroup\$ Depending on the minimum acceptable supply voltage this might actually work, if an LDO with very low dropout voltage was used. \$\endgroup\$ Jun 14, 2019 at 3:14

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