Author Topic: LED Resistor, before or after?  (Read 3498 times)

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Offline Mint.

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LED Resistor, before or after?
« on: January 17, 2012, 03:25:36 PM »
I am slightly confused. On which side is the resistor placed on and what effect does it have on the circuit? ::)
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Offline vk6hdx

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Re: LED Resistor, before or after?
« Reply #1 on: January 17, 2012, 03:30:12 PM »
For a simple circuit with a single led and resistor it does not matter if the resistor is on the anode or cathode side of the LED.

Offline metalphreak

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Re: LED Resistor, before or after?
« Reply #2 on: January 18, 2012, 12:27:09 AM »
The resistor is there to limit the current.

Each type/colour of LED has a "forward voltage drop". The rest of your supply voltage is dropped across the series resistor. The voltage across the resistor combined with the resistor value gives you the current. Without it, the LED would burn out because it would pass far too much current.

Since the LED and resistor are in series, it doesn't matter which side it is on.

A red led would be say 2.0V forward voltage. If you use a 5V supply, this means 3V across your resistor. If its a 1k ohm resistor, 3 / 1000 = 3mA current

Blue/White LEDs are usually ~3V so you need different resistor values for the same brightness/current.

Offline RJSC

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Re: LED Resistor, before or after?
« Reply #3 on: January 18, 2012, 12:35:21 AM »
I put the resistor between the LED and the element that switches it on (IC, transistor, switch) and connect the led directly to a power rail, just because it makes sense to me visually and if facilitates fault finding on the circuit, but I can't find any electrical reason to do it.


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