I just published a new video where I answer the question:
“Does the resistor go before or after the LED?”
If you don’t want to watch the video, I’ve attached the script below so you can read it instead:
This is an LED.
If the LED gets too much current flowing through it, it will burn out and die.
So it always needs a resistor to protect it.
But, in which order?
Here’s the thing…
It doesn’t matter!
The resistor can go before – or after – the LED, and it will still protect it.
the current that flows out of a battery is always equal to the current that flows back into the battery.
So in a circuit like this – with only one path for the current to flow – the current is the same everywhere in the circuit.
The current through the resistor is the same as the current through the LED.
Now, you might wonder – what controls the current then?
An LED has something called a “forward voltage”.
That’s the voltage drop the LED will have under normal conditions.
A typical forward voltage is 2V.
In a circuit with a 9V battery, an LED, and a resistor, you will have 2V across your LED.
The rest of the voltage – 7V – will be across the resistor.
Ohm’s law tells you that current equals voltage divided by resistance.
So if you have a 1000 Ohms resistor, you would get 7 divided by 1000 equals 0.007 – which is 7 mA.
Since the current is the same everywhere in the circuit, you’ll also get 7 mA through the LED – no matter if the resistor is before or after the LED.
What matters is the size of the resistor.
Keep On Soldering!
Oyvind @ build-electronic-circuits.com