Protecting digital inputs opto vs. R-C-D

[Jester]

I'm curious to hear opinions on input circuit protection.

This topic could really go off on a tangent if I don't define a few points up front:

  1) Preventing a failure of the digital input (from ESD etc.) is the main focus of this post.
  2) Inadvertent false triggering is also important
  3) Optical isolation is not required.
  4) The input wiring will be short and within a grounded metal enclosure, (push button type inputs)
  5) ESD is expected at the push buttons, and there is no expectation that it won't jump over whatever isolation is provided by the push buttons.

For pretty much every industrial type application I have designed in the past the opto isolator method has proven reliable even with long input wires and noisy environment.

The question is how reliable is the non-isolated (top circuit) over the long term and is it more or less immune to false triggers for example you just walked across a carpet and just touch the button with a healthy discharge to either the button itself or the enclosure but not actually push the button. I have certainly observed crappy equipment act up from a simple ESD discharge.

[moffy]

If it is mainly ESD protection that is necessary, opto isolators are not needed, for  the opto to truly isolate you need isolated power supplies for both sides. But that doesn't prevent ESD damaging the input LED junction, however the rest of the circuit should be safe. Heard it a while ago that chips when mounted on a PCB are much more resistant to ESD damage because the internal protection diodes have a path through the power rails to dissipate the charge. Can't remember where I heard it but it makes sense. I personally would go with the R-C-D solution.

[wraper]

Heard it a while ago that chips when mounted on a PCB are much more resistant to ESD damage because the internal protection diodes have a path through the power rails to dissipate the charge. Can't remember where I heard it but it makes sense. I personally would go with the R-C-D solution.

It's more protected on PCB only if particular component is protected by other components. If you say expose IC pins directly on connector, it's way more susceptible to ESD rather than bare IC as you added where current to flow. Bare IC would be less susceptible to be hit with discharge to begin with, and even if it gets hit, less current will flow through it as capacitance is what provides energy of the discharge.

[Zero999]

Is the enclosure metal? Is it earthed, or connected to either the 0V or + V of your power supply, either directly, or via a capacitor?

If the enclosure is metal, then don't worry about ESD on the push-button. Any arc will simply hit the enclosure and be absorbed. It's even less likely to be a problem if it's earthed and/or coupled to the PSU, since it will be held at the same voltage.

How about a BJT in common emitter configuration, with a diode in reverse parallel with the base? The base-emitter junction of a small transistor is more resilient than an MCU input and is cheaper to replace, if it does get damaged. It will invert the signal, so the software will need modifying.

[Terry Bites]

I'd put a TVS across the line.
The long wire will pick up RFI so a CM choke will help.
I assume the switch ground and receiver ground are not adjacent to one another.

Consider current loop signalling as an alternative.
It mitigates ground differences and the loop's low impedance makes it less susceptable to induced currents in the line.

[Jester]

I'd put a TVS across the line.
The long wire will pick up RFI so a CM choke will help.
I assume the switch ground and receiver ground are not adjacent to one another.

Consider current loop signalling as an alternative.
It mitigates ground differences and the loop's low impedance makes it less susceptable to induced currents in the line. (Attachment Link)

This topic could really go off on a tangent if I don't define a few points up front:

  1) Preventing a failure of the digital input (from ESD etc.) is the main focus of this post.
  2) Inadvertent false triggering is also important
  3) Optical isolation is not required.
  4) The input wiring will be short and within a grounded metal enclosure, (push button type inputs)
  5) ESD is expected at the push buttons, and there is no expectation that it won't jump over whatever isolation is provided by the push buttons.

[Zero999]

Is the enclosure metal?

To reinforce my point about a metallic enclosure.

There's a control system for a machine where I work. It had been malfunctioning for some unknown reason. It's in a big, earthed medal cabinet and the 0V of the power supply is earthed. Someone suspected ESD was to blame. I tested it by zapping the pushbuttons on the front with a piezoelectric igniter.  The sparks just jumped across the plastic surface of the buttons to the earthed enclosure. It wasn't ESD which was causing the malfunction but a software problem, due to an intermittent network connection.

  4) The input wiring will be short and within a metal enclosure, (push button type inputs)

All the less to worry about then.

[Jester]

Is the enclosure metal?

To reinforce my point about a metallic enclosure.

There's a control system for a machine where I work. It had been malfunctioning for some unknown reason. It's in a big, earthed medal cabinet and the 0V of the power supply is earthed. Someone suspected ESD was to blame. I tested it by zapping the pushbuttons on the front with a piezoelectric igniter.  The sparks just jumped across the plastic surface of the buttons to the earthed enclosure. It wasn't ESD which was causing the malfunction but a software problem, due to an intermittent network connection.

  4) The input wiring will be short and within a metal enclosure, (push button type inputs)

All the less to worry about then.

We developed a piece of portable industrial equipment (metal enclosure) that had a bunch of opto isolated inputs, I can't recall a single failure of those inputs. Same piece of equipment had a plastic body metal shaft rotary encoder the Honeywell type, it did not happen often, but we would get the odd failure of that encoder most likely from ESD. I think the ESD prefered the down the 4 wire interface path to the jump to metal chassis path.

This encoder.... https://www.digikey.ca/en/products/detail/honeywell-sensing-and-productivity-solutions/600128CBL/53504?s=N4IgjCBcoLQdIDGUBmBDANgZwKYBoQB7KAbRABYAOABhAF0BfBggJlJAEUBXNAEwCc0AFy78cAAhwA7RIV45%2B9BkA

ESD failures can be a bit tricky.

[Zero999]

Is the enclosure metal?

To reinforce my point about a metallic enclosure.

There's a control system for a machine where I work. It had been malfunctioning for some unknown reason. It's in a big, earthed medal cabinet and the 0V of the power supply is earthed. Someone suspected ESD was to blame. I tested it by zapping the pushbuttons on the front with a piezoelectric igniter.  The sparks just jumped across the plastic surface of the buttons to the earthed enclosure. It wasn't ESD which was causing the malfunction but a software problem, due to an intermittent network connection.

  4) The input wiring will be short and within a metal enclosure, (push button type inputs)

All the less to worry about then.

We developed a piece of portable industrial equipment (metal enclosure) that had a bunch of opto isolated inputs, I can't recall a single failure of those inputs. Same piece of equipment had a plastic body metal shaft rotary encoder the Honeywell type, it did not happen often, but we would get the odd failure of that encoder most likely from ESD. I think the ESD prefered the down the 4 wire interface path to the jump to metal chassis path.

This encoder.... https://www.digikey.ca/en/products/detail/honeywell-sensing-and-productivity-solutions/600128CBL/53504?s=N4IgjCBcoLQdIDGUBmBDANgZwKYBoQB7KAbRABYAOABhAF0BfBggJlJAEUBXNAEwCc0AFy78cAAhwA7RIV45%2B9BkA

ESD failures can be a bit tricky.

Encoders are notorious for being flaky and prone to failure, but are not very prone to damage from ESD, so it's unlikely it was the cause.