I just published a new video where I answer the question:
“Does the resistor go before or after the LED?”
You can find the video here:
https://youtu.be/zWC_AiycHbo
If you don’t want to watch the video, I’ve attached the script below so you can read it instead:
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[VIDEO SCRIPT]:
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.
You see…
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.
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Keep On Soldering!
Oyvind @ build-electronic-circuits.com
Jameel says
what is the procedure to convert 15v dc to 5v dc
LouF says
That is the best write-up I’ve ever come across, but I still have unanswered basics. “Same current everywhere” with “the LED has a forward voltage”: Why hasn’t it typically been called “voltage drop”? It is so much more clear and standard talk. I find “forward voltage” very, very confusing. Concurrent with my question and the example, if it has a voltage drop of 2V, why isn’t that effecting the circuit’s total resistance? 0.007A was only figured on the resistor, but each gives a voltage drop. Is this the reason “forward voltage” is used because there is no resistance yet a v. drop? (Also it’s forward voltage would be much more intuitive if that were the voltage TO BE SUPPLIED). What do I care that the downstream voltage is 2V after normal conditions (of the LED)? It’s the supply to it that is important. Besides all that, it’s said they’re “current operated” (while voltage and current are related, and voltage is what creates the current anyway). This just baffles me. Please, please clear this up for me.