Resistor placed AFTER L.E.D?
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Posts: 46,084
Hi, newbie here. Looks like this should be a great forum. For my first
beginner's question, I'd like to know why a resistor is placed AFTER a LED
instead of before.
Maybe it's my audio engineering background, but it makes more sense to me to
put the resistor, (a rough equivalent to a limiter or compressor in audio
terms, in my mind anyway) BEFORE the LED instead of after. Isn't there a
problem with the +5 voltage being too much for the LED? How could something
placed after the current reaches the diode serve its purpose in protecting
it?
Thanks so much for any help. I look forward to getting my first responses.
Nick
beginner's question, I'd like to know why a resistor is placed AFTER a LED
instead of before.
Maybe it's my audio engineering background, but it makes more sense to me to
put the resistor, (a rough equivalent to a limiter or compressor in audio
terms, in my mind anyway) BEFORE the LED instead of after. Isn't there a
problem with the +5 voltage being too much for the LED? How could something
placed after the current reaches the diode serve its purpose in protecting
it?
Thanks so much for any help. I look forward to getting my first responses.
Nick
Comments
The function of a resistor in series with the LED is to limit the current.
As you trace the circuit, the LED and resistor can occur in any sequence in a
given path.
The rationale is like this:
Say the LED has an operating voltage of 2.3 volts and an operating current of
10 milliamps ( I think these are typical, but you should check). Your supply
voltage is 5 volts. You need to reduce the voltage to 2.3 volts with a
current of 10 milliamps.
The voltage drop across the resistor needs to be 2.7 volts at 10 milliamps.
R = E/I = (2.7v)/(0.01A) = 270 ohms. Pick a standard-value resistor that's
near 270 in value (270 or 330 ohms, for instance) and all will be well.
Just to confuse the issue a little, some LED packages actually have a current
limiting resistor built in, but they're designed for a particular voltage
(e.g., 5, 6, or 12). I think that such LEDs will be identified, but don't
know how.
Tom O'Brien
>beginner's question, I'd like to know why a resistor is placed AFTER a LED
>instead of before.
>
>Maybe it's my audio engineering background, but it makes more sense to me
>to
>put the resistor, (a rough equivalent to a limiter or compressor in audio
>terms, in my mind anyway) BEFORE the LED instead of after. Isn't there a
>problem with the +5 voltage being too much for the LED? How could
>something
>placed after the current reaches the diode serve its purpose in protecting
>it?
Before or after, it doesn't matter. The purpose of the resistor is to limit
current. The current will be the same regardless of where the resistor is
at, as long as the applied voltage is across the resistor and the opposite
end of the LED. An LED is a light emitting diode. Diodes serve to conduct
current in only one direction.
Rob
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