Protecting logic inputs and outputs that go to external connectors

[Northy]

Hi,

I have two boxes that I'd like to connect together so that when one triggers the other one triggers too. I've come up with the following circuit which I've prototyped and it seems to work well.

The logic signals will end up going to a connector though, and I'm nervous about damage to the devices through over voltage connection and ESD etc.

What sort of protection should I be looking at using to protect the devices?

Thanks,

G

 

[Harald Kapp]

1) Add a pull-up or pull-down resistor to any input that goes to a connector. In case nothing is connected you still have a valid logic state on that input. 100kOhm, for example, are sufficient and will typically not overload the driving source when connected to that input.

2) Any further protection depends on the type and magnitude of overvoltage you expect. Here is an application note explaining the effects and how to use TVS (transient voltage suppressor) diodes to protect signal lines. For the inputs, an additional series resistor for limiting the current can be used. Take care: The series resistor and the pull-up/pull-down make a voltage divider, therefore the series resistor needs to have a considerably lower value than the pull-up/-down. Other wise the signal level will degrade.
If your input signals are slow, an additional capacitor parallel to the pul-up/down can help eliminate disturbances. A small one (100pF-1nF) should suffice. Note that with slow input signals and especially when using the capacitor you should use logic circuits with Schmitt-Trigger inputs to regenerate the signal to a defined logic state.

The outputs are less problematic due to their low output impedance. A TVS diode is good for protecting against overvoltages. You may add a small series ressitor, too, to limit output current. As with the series resistor on the input take care to keep the signal in the valid voltage range for high or low when the output is loaded and some voltage drops across the resistor.

 

[Arouse1973]

Good advice Harald, May I add to this. I generally use the newer version of static suppressors called MLVs. These are much faster than most TVS on the market and the static guard manufactured by AVX has lower capacitance than most TVS diodes.

A series resistor and back to back zener diodes is sometimes all you need. Also most CMOS inputs have static protection on them for this purpose so a series resistor will supress an ESD event.

If you are designing your own PCB then a simple spark gap made from a PCB trace can be a good first point of contact and will extend the life of the added TVS or MLV by preventing repeated large discharges of current through the device.

Adam

 

[Northy]

Thanks for the replies

Does it matter that this is both an input and an output? I'm really sharing the "OR'ed" point between boxes?

Thanks,

G

 

[Harald Kapp]

Protect the output AND the input if they can be separated (as I came to understand the original request). If done correctly, this will not interfere with normal operation.

 

[Northy]

The point that will connect to the outside world is the line that goes to the external connector on the drawing on the first post.

What's a spark gap?

Thanks,

G

 

[Harald Kapp]

This is a spark gap.

You probably don't need one unless you expect really high electrostatic voltages from people handling the circuit.

 

[Arouse1973]

This is a spark gap.

You probably don't need one unless you expect really high electrostatic voltages from people handling the circuit.

I thought he was concerned about ESD? A spark gap is cheap and easy to implement, you just bring it closer until you have a the correct spacing for the amount of expected ESD. A few millimetres is normally all that's needed.

Thanks
Adam

 

[(*steve*)]

What sort of protection should I be looking at using to protect the devices?

If you rely on the inbuilt protection, simply adding series resistors to the inputs and outputs is likely to be sufficient.

However, if you want to be more secure, have a resistor, then diodes to the supply rails, then another resistor. (the diodes are normally reverse biased).

The resistance should be such that you get an acceptable voltage drop in normal operation. For the outputs, this might be a few tens to a few hundreds of ohms. For the inputs it could be tens to hundreds of k.

If you think you'll see kV on the input or the output, you may been to do a lot more -- especially if it is a high energy source. That's where spark gaps and the like come in. There are also other devices (varistors, zener diodes, and similar devices) that can be used.