Simple input protection question?

[AndrewM]

Hi,

I need some way to get a microcontroller to read a signal from a dry-contact relay. Easy enough, but the interface is banana jacks, and this thing will be handled and used by people with no knowledge of electronics or much respect for the unit. I'm not expecting anyone to plug a billion volts into it, but I'd like it to be able to survive 12, and the usual static shock from handling.

The unit is powered by a single lipo cell.

Cost isn't super important here, as long as it's not stupid expensive. The rest of the unit probably costs around $150 in parts and a few hours of labor for me to assemble, so I'd rather not cheap out on input protection and have it bite me later.


So, what's a reasonable way to do input protection on this?


My current guess:
- power the powered output jack via a PNP transistor connected to the battery, not a direct connection to a microcontroller pin
- TVS diode between the two banana jacks


Things I'm not sure about:
- PTC fuse resistors on the banana jack inputs? If I do this, does it go before or after the TVS diode? If there's a constant voltage, I want the fuse to blow up before the TVS diode does, but will the TVS even work to suppress static shocks if it's after the fuses?
- Is there any point in using an optoisolator here? I have a bunch lying around, but considering it's not actually an isolated power source, is there any advantage over just using a transistor?
- Is there any point in adding a fuse here? I figure by the time even a small fuse blows, there's been enough damage to the rest of the unit that the fuse won't matter.


Some ideas:

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Thanks!

[sanwal209]

I dont think you need PTC and transistor here. Just replace transistor with diode so when user plug higher voltage the diode will block it. Add series resistor to opt coupler led. If user apply 12V(Doesnt matter the polarity) system will be protected.

TVS should be place as close as possible to Banana Jacks.

PS: Use Bi-directional TVS

[AndrewM]

Transistor is so that the jack isn't live when the device is not turned on.

[Renate]

Step one: Years from now, you or somebody else will look at this panel and wonder if it's isolated or the high is +5 or how the current flows.
Rule one: One banana jack/binding post should be ground and labelled as such.
That still leaves ambiguous what the other jack is, but a good guess (and a voltmeter check) will show that it's a positive current source.

Opto is not bad, but non-opto could work out too, since you're not really isolating all that much.
Use a constant current source on top and then it won't matter if someone wants to hook up -12V.

[TimNJ]

TVS are most effective with some value of series input impedance (preferably resistance). Doesn’t have to be a ton. Usually on the order of 10-100ohms is okay. Even 1 ohm is better than nothing. A TVS, basically a chunky zener, becomes a “short circuit” above some threshold value. When the TVS conducts, the series resistance limits the TVS current so that it doesn’t self destruct. Check your TVS’s peak energy handling capability. If the source of overvoltage is expected to be from a low impedance source (like an actual power supply) then the additional series resistance is most useful. If you are mainly concerned with ESD, then the resistor may not be necessary, as ESD is modeled as a higher impedance source.

Can your dry circuit terminals handle a few 10s of ohms in series? I’m not familiar with what these interfaces look like.

If you use a fuse, whether one-time, or PTC, put it before the TVS. TVS tend to fail short if they are forced to handle too much energy.