EE 47 Lab 1: Orientation and LED
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. 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.
I used the yellow LED. From the spec sheet, forward voltage is 2.0V to 2.4V max.
V(resistor) = V(battery) – V(LED) --> 5V-2V = 3V
V=IR --> 3V=0.025Amps x R --> R = 3/0.025= 120 Ohms.
So, we need 120 Ohms of resistance to limit current to 25mA for the yellow LED.
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2 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 minimum resistance. The LED would likely fail if you lower the resistance, which increases the current thru the circuit.
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2 c. What is the resistance range of the potentiometer?
The resistance range of the potentiometer is 0 ohms to 10,000 ohms, with a likely tolerance of 10%.
✔ Good answer with the tolerance.
3 a. Does it matter what order the components of your circuit are arranged between power and ground? Why or why not?
No, it does matters what order the components are arranged between the power and the ground. The resistor can come either before or after the LED, since the resistor controls the current for the entire circuit, not just what comes after it. The switch can likewise can go anywhere since it simply breaks or connects the circuit; order doesn’t matter.
✔ Another way of seeing it is that the circuit is in series. According to Kirchoff's current law, current entering the circuit is the same as current leaving, so it does not matter.
4 a. Using this battery, what is the minimum resistance required for use with your LED?
The battery is 9V. From the spec sheet, forward voltage of the yellow LED is 2.0V to 2.4V max.
V(resistor) = V(battery) – V(LED) --> 9V-2V = 7V
V=IR --> 7V = 0.025Amps x R --> R = 7/0.025 = 280 Ohms
So, 28 Ohms is the minimum resistance required for use with the yellow LED.
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YouTube link for demonstration of LED being switched on and off: http://youtu.be/lL3nuXPd1e0
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