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Lab1 -LED

Page history last edited by Vivien Tsao 7 years ago

 1.a. What resistance do you need to limit current to 30 mA  (if using red LED) or 25 mA (if using yellow or green)? Be sure to state which colour LED you are using. This resistance refers to the total resistance in series with the LED. 

Hint: Make sure that you account for the forward voltage drop (Vf) of the LED that you're using.


 With a 5v power supply and a red led whose forward bias voltage is 1.85v,in order to limit the current to 30mA the resistance we need to use is-

V=I*R(Ohm's Law)



 =105 ohms


For a yellow or green led whose forward voltage is 2.2v the resistance used is:



 =112 ohms  



b. Is the resistance from question a) a maximum or minimum resistance? That is, in which direction if you change the resistance (higher or lower) would the LED likely fail.


The  value of the resistance in the above question is the minimum value required to make the led work properly. If we decrease the 

value of resistance then it would exceed the value of the maximum forward current (i.e. 30mA for red led) and the led would fail.



c. What is the resistance range of the potentiometer?


The resistance range of the potentiometer is from 0-10k ohms. It can be varied by using the knob or the wiper.



2.a. Does it matter what order the components of your circuit are arranged between power and ground? Why or why not?


It does not matter how the components are arranged between power and ground as all the components are arranged in series connection.

In series connection the same electrical current flows through all the components.



3.a. Using this battery, what is the minimum resistance required for use with your LED?


When we are using a 9v battery and a red led, whose forward voltage is 1.85v and maximum forward current is 30mA, the minimum value of resistance required is:

V=I*R (Ohm's Law)


 =(9-1.85)/ 0.03

=238 ohms (We use 240 ohms) 



Link to the video: http://www.youtube.com/watch?v=bUwRiqzgvlk&feature=youtu.be




Comments (1)

Vivien Tsao said

at 6:02 pm on Jul 11, 2013

Good job!

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