I am writing regarding an article that I read, "Protecting Inputs in Digital Electronics" written by Solutions Cubed, Inc.in Digi-Key's technical library, who posted this article on their website. Figure 6 of that article has the following caption "Current limit protection for an input." This figure is under the section headline "Current limiting" of that article. It was stated that "a good value for the current limiting series resistor is between 100 Ohms and 10 kOhms." What is the exact equation that was used to calculate this range for the current limiting series resistor? Could you explain with the math too? Thank you very much.
There is no exact equation, because it is not a science, but based on rules of thumb and practical guidlines.
The point of the resistor is to limit the peak current when (usually) a static electricity/ESD event happens. There are rules-of-thumb for the capacity of a human body, and usual values to which they charge walking across a carpet, etc.
Chip Manufacturers will also sometimes list energy figures for ESD tolerance of their chips.
In general, the higher the value of the resistor, the more protection it offers, but too high a value will cause malfunction due to slowing or attenuation of the input signal, so you have to compromise, and it is probably best done by experiment rather than calculation. Increase the value until the input stops working, then reduce by half...something like that.
Adding diodes (reverse biased in normal operation) to VCC and ground on the outboard end of the resistor (the end that connects to outside world) means that the resistor will only see more than a diode drop of voltage for the Ton of the diode.
Consider the publication date of anything you read on this topic. 20 years ago, CMOS was rather notorious for latching-up if anything spiked the inputs. Manufacturers have made great strides in reducing this problem, though it can still crop up occasionally. If cycling power fixes a problem, latch-up is one likely cause.
Which is another point about protection: It may be impossible to protect the circuit from malfunction in all cases, but still possible to protect it from damage. A non-critical application where the user can cycle power when required is an easier problem than trying to make sure nothing malfunctions when a 1/2" spark jumps from the user's finger.
Thank you Kevbo. I appreciate your response.
Hi Kevbo - great answer!
It's interesting that you mention latch-up. It is still quite common in some devices. (AVR Dragon anyone?) It's a trap for beginners too.
I don't know how many times I've heard experienced engineers tell me they don't need to take static precautions because they say it's just not an issue. I bet these are the same ones who tell you that "sometimes chips just go bad".
The rule of thumb I use is this (it may not suit everyone). For any pins which go to headers that can be "touched" I always add TVS (transient voltage suppressors) and I sometimes add in-line resistors.