Lab 1

Part C

2a. 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.

For a red LED, the voltage drop is 1.85V. This means that 5V-1.85V=

3.15V will drop across the resistor. Using a 5V supply, a resistance

of

R=V/I=3.15V/.03A=105 ohms

is needed to limit the current to 30 mA.

For a green LED, the voltage drop is 2.2V. The voltage drop across the

resistor will be 5V-2.2V=2.8V. To limit the current to 25mA, the

resistance should be at least

R=V/I=2.8V/.025A=112 ohms

For a yellow LED, the voltage drop is 2V. The voltage drop across the

resistor will be 5V-2V=3V. To limit the current to 25mA, the

resistance should be at least

R=V/I=3V/.025A=120 ohms

✔

2b. 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 resistance is a minimum resistance. By increasing the resistance,

we can further decrease the current. This means that it is safe to

increase the resistance, while decreasing the resistance will increase

the current and may lead to an LED failure.

✔

c.What is the resistance range of the potentiometer?

.3 ohms - 9.76 kohms.

✔

3a. Does it matter what order the components of your circuit are

arranged between power and ground? Why or why not?

The order that the components are arranged between power and ground is

not important. Because all of the elements are in series, the current

through all of the elements is the same.

✔

4a. Using this battery, what is the minimum resistance required for

use with your LED?

For the red LED, the minimum resistance will be

R=V/I=(9V-1.85V)/.030A=238.34 ohms.

For the yellow LED

R=V/I=(9V-2V)/.025A=280 ohms.

For the green LED

R=V/I=(9V-2.2V)/.025A=272 ohms.

✔

## Comments (1)

## Benjamin Tee said

at 12:02 am on Jul 11, 2012

Nice work Christian! Do upload a video of your working LED or devices next time! Keep it up!

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