risistor Before/After LED

slinxy · 20 Oct 2012 at 16:25

Hi

I've always placed an resistor before the component to stop them being damaged. But I've read today that it doesn't matter on a LED, but can't understand why.

Can some nice person explain it to me in plain English.

Thanks

EDIT: sorry it posted before I finished typing.. I blame my touchpad..

Pestilence · 20 Oct 2012 at 16:26

Thanks for the info. Truly.

EDIT: Damn edited OP ruins my post. Down with you and your advice seeking!

RomanNose · 20 Oct 2012 at 16:26

It doesn't make a difference..

Tealc · 20 Oct 2012 at 17:03

The resistor will affect the current flow of the entire circuit wherever it is in the circuit.

It's only when you have other components in parallel with the LED and resistor that can affect the current flowing through any given part of a circuit.

MikeHunt79 · 20 Oct 2012 at 17:19

It doesn't matter if it goes before or after on other components also, as long as its in series it will work

bitslice · 20 Oct 2012 at 17:29

I've always put it before, on the vague possibility that a random short to ground will not kill the LED. Conceptually it's easier to visualise too.

Dr.EM · 20 Oct 2012 at 18:40

It can go on either the +V or ground side of the LED, if that's what you're asking? In either position it limits the current flow through the circuit; in this case just an LED and resistor.

Jotun · 20 Oct 2012 at 18:55

The LED will have a voltage drop, typically around 2V. If you had a 5V supply with your resistor and LED connected, you will always have your 2V dropped across the LED. The remaining 3V will be dropped across the resistor, regardless of the value. This allows you to change the resistor value to change the current through the LED.

I have done a very quick sketch:

If you put your LED after your resistor, then working from 0V upwards, it will have a 2V drop across it meaning that the mid point is at 2V. Your resistor will then be connected between 5V and 2V, giving it a 3V voltage drop.

If you put your LED before the resistor, then working from the 5V rail downwards, you will have 2V dropped across the LED and the mid point will be at 3V. The resistor is connected between 3V and 0V, and so has 3V across it again, giving the same current as the previous configuration.

Excessive current will cause the LED to heat up and become damaged. Either configuration means that a given resistor will result in the same current.

slinxy · 20 Oct 2012 at 19:33

Thank you all..

neodude · 20 Oct 2012 at 19:39

Electricity doesn't really have a direction of flow. It's just as valid to say that positive charge travels from the positive end of the supply to the negative as it is to say the usual negative charge travelling from the negative end of the supply to the positive. Hence it doesn't make a difference in what order series components are wired.

RomanNose · 20 Oct 2012 at 20:15

Well, electrons go from negative to positive.
However in electronics, we use the convention that current is from positive to negative. It's incorrect but it's just a convention.
When we are talking about Electricity, 99% of the time we are talking about the flow of electrons and not protons. Electrons are free to move in conductors, not protons.