Question on how resistors work in series

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Question on how resistors work in series

Post by Dom » Wed Aug 31, 2016 7:49 pm

Hi, regarding the simple project with 1 led and 1 resistor, why isnt the resistor on the positive side of the led connection? I thought current flows from positive to negative. If this is correct then to my mind the current is going directly to the led at 5v then getting reduced afterwards before going to gnd. This doesn't make sense to me as I thought it has to be reduced before it hits the led to stop it from frying. It obviously works but my understanding of why it works on the negative rather than positive is clearly wrong. Can someone explain please?
Thanks, Dominic

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Joined: Tue Sep 24, 2013 11:58 am

Re: Question on how resistors work in series

Post by GraemeSPa » Sat Aug 18, 2018 10:24 am

Been a year and nobody answered your question......You might already have worked this out for yourself, but here's my take on it.

In a simple resistor/LED circuit, it really doesnt matter which way round they are connected. The resistor is there to reduce the voltage drop across the LED and control the current through it. Without the resistor, the LED would not last long as they are only meant for the forward volt drop - between 2.5 and 3.5 Volts , depending upon the LED type and colour.

Build a circuit with the positive supply to the resistor, the other end of the resistor to the LED anode and take the LED cathode back to the supply negative. Measure the volt drop across the resistor and across hte LED. Now switch off and rebuild the circuit with the positive supply connecting directly to the LED anode, the LED cathode connecting to the resistor and the free end of the resistor connecing back to negative. Measure the volt drops again - see? same-same. You calculate the resistor value using the forward volt drop and forward current of the LED that you get from the LED specifications . Resistor R = (Supply voltage - Forward voltage) divided by forward current. So for a 5 volt supply, to get a red LED with 2.1volt and 20mA of current, this works out as 145 ohms - nearest standard value is 150 ohms, so go with that value.

There are lots of tutorials on line covering basic electronics. Keep going!!

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