Output Protection

Hi!

I was just wondering, how does one protect an output pin? There is lot of info about protecting inputs, such as the ruggeduino project.

If i’d like to protect output from anything, negative voltage, overvoltage, overcurrent, you name it, is it similar to the ruggeduino solution?

or do i have to do the same thing they did to the 5v output pin?

thanks.

lotof: Hi!

I was just wondering, how does one protect an output pin? There is lot of info about protecting inputs, such as the ruggeduino project.

If i'd like to protect output from anything, negative voltage, overvoltage, overcurrent, you name it, is it similar to the ruggeduino solution?

or do i have to do the same thing they did to the 5v output pin?

thanks.

OK, tackle these separately:

Simple over current (with no fancy voltages around), just use 150 ohm resistor in series to limit current to below 40mA.

Voltage out of range you want a divider circuit with schottky diodes from the pin to the two supply rails (normally reverse biased so don't interfere with the circuit). A series resistor (say a 150 ohm one!) will then take the extra voltage as the diodes conduct the current to the rails (at -0.3V and +5.3V).

Gross over voltage needs more resistance to limit the current, and you have to be sure the extra current to the +5V rail won't pull it above 5.0V (extra dummy load or a voltage clamp might solve that, extra decoupling can help absorb spikes). The schottky diodes are mimicking the standard CMOS input protection circuit but with better diodes.

The ruggeduino circuit is a little different, using a single zener to protect from over and under voltage (I personally don't like this, a 5.1V zener doesn't clamp very hard - schottky diodes turn on hard (and fast) in a fraction of a volt). They use a 220 ohm resistor that doubles as a fuse (its resistance increases on gross overload due to overheating).

Apart from over current and over-voltage what other thing did you have in mind? High power RF-interference?

Protect against what? If you're connecting an inductive load a diode will protect against the reverse voltage spike when it's turned off. Other than that, the only protection an output needs (that I know of) is against drawing too much current. That's what resistors are for. But there's no magic bullet that can be applied to all of your output pins. How you protect them depends entirely on what's connected, and anything you connect will effect performance. A 1N4001 diode, for example, will lower the output voltage by about 0.7V. If you need 5v, that's not going to work. The same goes for resistors. 220 ohms will protect against drawing too much current through an LED, but might not be the value you need for controlling a transistor. Each circuit should be evaluated individually when determining what sort of protection is needed.

Thanks so much!

I was going to connect a shift register driven 7-segmend display via, for example, a db9 or ethernet cable. Then, I thought, you never know what people will poke to your connectors.

So, if i had the reverse-biased diodes and a resistor (how about a pct fuse?) would that affect the shift register communication? If i had a diode between the output and ground rail, then i wouldn't need a diode across the output?

It will depend on what you are outputing and how much control you have with the devices being driven.

Short of that, I would stick in a 330ohm (or thereabout) resistor and call it a day.