How to protect a connector small signals and power signal

[Halban]

Hello dear community,

I have a question, for an Eurorack project (electronic musical instruments), I'm using a special board-to-board connector (for connecting an expander).

As users can plug it, I want to protect the small signals (connected to microcontroller) and power signals (the +12V or the -12V can damage the microcontroller if something is wrong, for example if the ribbon cable to insert in the header is faulty).

The schematic of the "expander" connector (to connect the other board with the microcontroller and the power supplies) :


Do you know what is the most effective way to protect the pins, especially for the microcontroller pins? For example, are special protecting diodes references for that purpose?

Thanks a lot!
Alban

[jnk0le]

Separate HV from uC with a ground. (e.g. pin 1,2 +-12V, 3,4 GND)
Power supplies often have overcurrent protections, and it is easier to repair if not. (or just the connector electrolysis happened) 

And the most important: connector must fit only in one orientation only, including the row/column shift. Otherwise users will always manage to connect it wrong, no matter how expensive that equipment is.

BTW, some unnamed companies intentionally put like 80V+ rails right next to signals from expensive/irreplacable components, in order to fry them in case of liquid presence. (and milk the customers of course).

[Halban]

Thanks for your feedback!

1) Unfortunately the connector port is already in production and can't be remapped (it corresponds to the image attached).

2) The connector has 1 orientation, you can't put it in the reverse way. But we never know And the ribbon cable can be faulty and can mix the signals.

[tom66]

If the signals are not high bandwidth / high current then adding 1-2k ohm series resistance will limit the current by a lot.  The microcontroller won't be happy about being biased with 6/12mA through the ESD diodes (I've seen it cause issues with ADC and oscillator blocks), but it probably won't destroy it.

High bandwidth in this case means over about 100kHz.

[Halban]

Thanks, I will definitely add the resistors, it's low frequency signals.

Maybe adding these ESD protections can be great to protect the GPIOs ? https://www.ti.com/lit/ab/slvafq4/slvafq4.pdf?ts=1734554848876

[tom66]

ESD diodes are not rated for continuous biasing, you will destroy the diode without a way to limit the current, so you will need the resistors too.  But most ESD diodes also suffer from poor tolerancing, they are designed for impulses where the magnitude of the spike is 10-20x the normal operating voltage, not to precisely break down at a specific voltage. 

If you want to reduce the biasing through the microcontroller, use something like BAT54S dual Schottky diode, with cathode tied to the MCU supply rail and the anode to ground, then the centre tap of the diode connected to your signal.  The BAT54S will bias a bit before the MCU does, and it will take 12mA bias without much problem.    You will need one diode for each signal.  The diode should go after the resistor, on the same side as the MCU pin. 

[Halban]

Thanks a lot!

I've made a schematic example to be sure I got it right:



(only 1 GPIO pin is protected on the connector for the example)

I also added an other resistor on the Expander board to add protection on that side, in order to limit current if a pin of the connector is directly connected to a digital IC, maybe it's a good idea?

[tom66]

Looks good to me.  You need protection on any digital pins where transposition is possible.

It looks like it is not possible to connect either +12V/-12V to +3.3V,  but if it is, you should consider some kind of protection there.  A TVS diode is a common approach. 

[Halban]

Got it!

Updated my design with a TVS 3V3 diode

[jnk0le]

a two more ideas:

- sacrificial 74xxx etc. chips to buffer from MCU. (locating faults is not always obvious, but irreplacable MCU can survive meeting with 230V  )
- use comm buses, of which transcivers are designed to survive shorts to highest voltage on that connector (e.g. CANbus, sometimes rs485)

[Halban]

Thanks, good idea!

[jnk0le]

- sacrificial 74xxx etc. chips to buffer from MCU. (locating faults is not always obvious, but irreplacable MCU can survive meeting with 230V  )

The ultimate of that is full galvanic insulation, but that would be an overkill for simple board to board connections.