Placing resistors before or after the LED load...

Other Hardware General Electronics
1

Greetings:

My question involves wiring LED lights on a breadboard and I hope to make this easy to understand.

Every example I've seen places one 200-ohm resistor between the Arduino card's I/O pin and the anode of the LED, with the LED's cathode wired to ground on the card. Would it matter if the resistor was placed between the LED's cathode and ground, instead?

One reason I ask is because an RGB LED has three anodes and one cathode, and it makes sense to me (long-term production wise) to use one resistor instead of three.

Thank you for your time.

Side note: as my profile indicates, I've got newbie written all over me. I know just enough about electronics and C/C++ programming to get myself in trouble. Again, thank you for your time and input.

2

It does not matter where in the circuit the resistor goes as long as the LED is in series with the resistor and so limits the current through the LED

If you put a single resistor in the common connection of an RGB LED then different currents will flow depending on how many of the LEDs are lit which at best will alter the brightness of the LEDs. It will work but is considered bad practice, so use 3 resistors

3

ItsCaseyDambit:
One reason I ask is because an RGB LED has three anodes and one cathode, and it makes sense to me (long-term production wise) to use one resistor instead of three.

Absolutely correct - but only if you only ever want to show one colour at any one time. That is what makes genuine sense - one colour, one resistor. :sunglasses:

UKHeliBob:
If you put a single resistor in the common connection of an RGB LED then different currents will flow depending on how many of the LEDs are lit which at best will alter the brightness of the LEDs. It will work but is considered bad practice, so use 3 resistors

Actually, since the LEDs generally have different threshold voltages, not only will the different LEDs have different brightness when enabled together, it is likely that the brightness of one or more will be - invisible. :astonished:

The red LED has the lowest threshold, so having it on will generally prevent the green or blue showing if you have one common resistor.

4

Thank you kindly! This was the answer I was hoping for.

We'll consider this issue resolved.