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I am looking to idiot proof some connectors I use, which are not intended to be hot plugged, but of course they are..

The typical problematic case is with a standard 4 pole connector I use where the pins are:

  1. 20V
  2. GND
  3. SDA (I2C)
  4. SCL (I2C)

I am using a 2mm pitch, 4 pole, JST connector (B4B-PH-K-S), where all pins are the same length so there is no certainty in order of connection, thus the hot plug issues.

I have no issues with these connectors when they are safely plugged/unplugged from the system when it is off. But they consistently have failed during unintended hot swapping.

I know many common hot plug interfaces (such as USB) have extended power pins to guarantee contact order of pins, in order to prevent damage. enter image description here

I guess my question is, are there any low cost, small form factor, similar to the JST connector I am using that has this sort of feature?

Other ideas on solutions are welcome as well!

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    \$\begingroup\$ sata connectors \$\endgroup\$
    – jsotola
    Commented Jun 6, 2018 at 18:59
  • \$\begingroup\$ ebay.com/itm/… \$\endgroup\$
    – jsotola
    Commented Jun 6, 2018 at 22:54
  • \$\begingroup\$ these? \$\endgroup\$
    – Tom Carpenter
    Commented Jun 10, 2018 at 11:31
  • \$\begingroup\$ Have you considered applying some conductive tape or other metallic housing around the connector? This would support negative reinforcement by providing a mild shock to anyone attempting to remove the connector while the unit is powered on. \$\endgroup\$
    – Zhro
    Commented Oct 13, 2018 at 15:42

2 Answers 2

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There are a few ways to hot swap:

1) Use a connector that attaches ground first and power last. The really good hot swap connectors have a shorter pin so you can attach a resistor to power first and let the rail "come up" to voltage with a current limiter before the other power pins are mated.

2) Use an electronic fuse or mosfet to bring up the rail

3) Maybe put an inductor in series on the power rail which would prevent inrush currents (this may have unintended consequences, so it may not be possible with the rest of your design.

4) Possibly use isolation or a buffer with protection for the digital lines.

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    \$\begingroup\$ The problem in the OPs situation is that he has a 20V power line, If the 20V power pin mates and one or both of the data lines mate before the ground line mates then bad things are likely to happen. \$\endgroup\$
    – Peter Green
    Commented Jun 6, 2018 at 19:35
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    \$\begingroup\$ Part of the problem is when you hot swap and the power connects before the ground it chooses an alternate path, and sometimes can go through a different line with an unexpected voltage. That's why you'd need to protect the digital lines. I ran into the same problem with SPI lines and a 15V rail, but part the problem was the digital lines were getting blown out. If you buffer and protect the digital lines on the other end (with diodes and a buffer, or a buffer that can handle a higher voltage, some go to 7V) then that can solve the problem. The route I took was digital protection on both ends \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 6, 2018 at 22:17
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Unfortunately I'm not aware of any PCB header style connectors with the feature you want (doesn't mean they don't exist).

The biggest issue I see with your design is what happens if the power and one of the IO lines connects, but the ground line doesn't. In that case, the powered consuming device will be powered via the logic lines, potentially exposing them to undesirable voltages and/or currents.

Other ideas on solutions are welcome as well

Two techniques I would consider to reduce the risk.

  1. Add some protection diodes, Schotky diodes from the IO lines to the logic power rail and gound and then a Zener diode across the logic power rail. IF your timings/voltage margins can tolerate it also consider in-line current limiting resistors.
  2. Add more ground pins, a ground pin at each end of the connector will dramatically increase the probability of at least one ground mating first.
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  • \$\begingroup\$ Hi, thanks for the suggest, I'm curious about solution 1. I am having troubles convincing myself that this would protect against damaging 3V3 logic ICs. Assuming the typical slave configuration has a 20V -> 3V3 regulator, powering a I2C slave connected directly to my connector, are you able to explain how a Schotky diode from SDA/SCL to 20V protects these inputs? Really interested in this type of solution. \$\endgroup\$
    – this_is_not_a_test
    Commented Jun 8, 2018 at 17:55
  • \$\begingroup\$ Sorry if I wasn't clear, the protection diodes go to/across the logic power rail, not the 20V power rail. \$\endgroup\$
    – Peter Green
    Commented Jun 8, 2018 at 18:42
  • \$\begingroup\$ I am looking at a part I am currently using for ESD protection on USB lines: USBLC6-2SC6 I would connect this to GND, 3V3, SDA, SCL. With 3V3 being regulated on board from the 20V input. Assuming that the zener breakdown of USBLC6-2SC6 is low enough to stay within the absolute maximum values of the ICs do you expect any issues? Appreciate the suggestions! \$\endgroup\$
    – this_is_not_a_test
    Commented Jun 11, 2018 at 16:05
  • \$\begingroup\$ As an update for anyone interested: I tried using USBLC6-2SC6 as protection on a 5V system, having an IC on both the master & slave near the connector. I hot plugged the connection a number of times and quickly saw failure in both of my two tests. Seems like this particular solution did not solve my problem. I will update If a solution is found and tested. \$\endgroup\$
    – this_is_not_a_test
    Commented Jun 14, 2018 at 14:43

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