Operating a simple LED with an IC...

[zlash]

Hello!

I'm trying to teach myself some basic electronics and this is giving me several brain damage because of the little knowledge I have.

In most of the circuits involving an IC driving a LED, a resistor is usually used between the IC's output and the LED: The explanation given is that the resistor is needed to protect both the IC and the LED from becoming a nice puff of magic smoke. I can believe that this is true, but I really want to understand it.

After some research, I understand why a LED should not be connected directly to a power source: When the LED is operating it has a very small resistance so the current gets very high and it burns the LED.

The problem comes when an IC is added to the circuit, and I'll talk precisely about a PIC because its what concerns me.
Most PIC schematics I've seen don't use a resistor to power it making it seem that the PIC don't suffer the same fate as the LED when directly connected to a power source. This lead me to deduce that the PIC is offering some resistance. Immediately I went to this PIC's data sheet and found an "Unit Resistor Value" of 2k.
So, If the PIC is working as a 2k resistor, why do I need to use resistors at its outputs?

Thanks in advance, and sorry for my poor English.

 

[IanP]

First of all, most LEDs will withstand currents of up to max 30mA ..
However, they will happily work with currents as low as 1mA, specially more recently manufactured LEDs ..

On the other hand PIC pins can source (or sink) currents of up to 25mA ..
So if you allow an LED to “work hard” at 25mA (instead of doing the same work at, say 2mA + 1k resistor) you can connect an LED directly to the PIC’s leg ..
Otherwise use a resistor that “shares” to output voltage with an LED and in the same time lowers the current to something reasonable ..

So, If the PIC is working as a 2k resistor, why do I need to use resistors at its outputs?

The answer is – you don’t need to drive LEDs with almost (or above) the maximum allowable current ..

Goto:
http://www.kpsec.freeuk.com/components/led.htm
see how to divide the driving voltage between an LED and a resistor ..

Rgds,
IanP