2.
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 color LED you are using)
Type of LED

Resistance needed

Calculations

Red LED

R=105 ohm (assuming finite LED resistance)
R>166 ohm (assuming negligible LED resistance)

Assuming VLED = 1.85V,
On resistance = 61.7 ohm
61.7/(R+61.7)*5V=1.85
R=105 ohm
V=IR
5=30mA*R
R>166ohm

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.
It is a maximum. The lower the value of R, the more current flows through the LED. Akil: We actually meant the correct answer should be 'minimum' in the sense that we shouldn't use a lowerthanminimum resistor value because we'll exceed the absolute maximum current rating, but you're right too.
c. What is the resistance range of the potentiometer?
0  9.6kohm
3.
a. Does it matter what order the components of your circuit are arranged between power and ground? Why or why not?
Does not matter since the circuit is in series.
4.
a. Using this battery, what is the minimum resistance required for use with your LED?
Using 9V battery, we need a larger resistor to limit the current. For 30mA current,
30mA=9/(R+61.7) => R> 238.3 ohm
Or Assuming no LED resistance, R>9V/30mA = 300ohm
Appendix
Initial circuit
With POT With switch
Comments (1)
Akil Srinivasan said
at 6:32 pm on Jul 10, 2011
Nice of course!
You don't have permission to comment on this page.