current limiting resistor for output pins?

Hi! I have one of those dumb questions, because I'm a perpetual noob. :slight_smile:

Do I need current limiting resistors for the Arduino (or micro controllers in general) output pins when using analogWrite() or digitalWrite()? I'm sending these outputs into this SN74ABT125N buffer chip.

Here is the spec sheet if needed :

If yes, what size? I'd like to know the reason behind the yes or no too.

Thanks so much! Happy Holidays!

Hi,
No, An Arduino digital output to a 74-series chips input needs no series resistor. This is a common connection..

Thanks, Terry!

The AVR microcontroller output pins are high current logic outputs designed for driving logic
chips directly, or small loads like 20mA LEDs. For an LED a series resistor is needed to
limit the current because the forward voltage is less than 5V, but other logic chips take
(almost) no current when their inputs aren't changing - they are called "high impedance",
ie high resistance.

Here "high current output" means below 40mA (most logic outputs are more like 5mA limit).

So high current for logic is actually quite low current (lower than relays and motors etc).

If in doubt use 150 ohm series resistor on an output pin to protect it (for instance if the
signal might get shorted to ground).

Good to know. Thanks, Mark!