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Topic: Once again, position of resistors in Project 2 (Read 1 time) previous topic - next topic

zhulien

Sep 19, 2014, 10:24 am Last Edit: Sep 19, 2014, 10:25 am by zhulien Reason: 1
I have read all the previous threads about this and still cant explain to myself why it doesn't really matter. And I cant continue with the projects ahead until I grasp that vital concept.

In LEDs(and diodes as a whole) the electricity flows in only one direction. In Project 1 we used the resistor to limit the voltage supplied to the LED. That makes sense. However, in Project 2 the resistors are placed after the LED. And again, electricity to the LED flows in one direction only. How is the LED then protected from receiving too much voltage?

Do the components(specifically LEDS) absorb energy or the the energy just flows through them? Can you please explain what is the actual behaviour of the electricity in such a case step by step. Thank you!

knut_ny

all diodes have a gives voltage drop (1.2V..3.5V depending of colour)
They do not limit the current!  This means that if you feed them 5V !! the voltage (5V - diode voltage drop) will
drive a current in your circuit.  The resistance is very low - som the current will be very big. (something get fried)
U can control the current to meet your needs.
example:
red led -> 1.5V drop
driving voltage is 5-1.5 = 3.5V
you may want 15mA through the LED
Then calulate R using Ohm law:  R=3.5 / 15m = 233   (Ohm)  
U select a resitor from your box - value 200 .. 250 ohm  (to big is never a problem)
The resistor placed befor or after led... doesnt matter at all..
Ny

nigelsoh

This is how I think of the Resistors and Electric Circuit.

An electrical circuit is a loop, like a circle.
So now imagine that there is this circular road (circuit) filled with cars (current).
I then decide to narrow the width of one part of the road. (resistance)
The amount of cars remains the same, but the cars will now move slower throughout the circular road, as lesser cars can pass through the narrowed part within the same amount of time as before.
As the circular road is a loop, all the other cars are thus affected, no matter which part of the circular road is narrowed.

Hope this helps.  ;) 

BennyBoyW

@nigelsoh
Brilliant answer - I finally understand this now with the circular road analogy - I wish I'd read this post 2 weeks ago

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