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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.

I used the RED LED:

Max forward current = 30 mA = 0.03 A = I_{max}

Typical forward voltage = 1.85 V = V_{f }(this is what we will use in our calculations)

Power supply voltage = 5 V = V_{power}

V_{power} - V_{f} = I_{max}R

5 - 1.85 = (0.03)R

R = 105 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 resistance from (a), 105 Ohms, is a minimum resistance.

We know from Ohm's law (V = IR) that current and resistance are inversely related. This means that, holding voltage constant, as resistance increases, current through the circuit decreases. We know that the maximum forward current for the LED we're using is 30 mA and we used this I_{max} to calculate a value for R. If voltage is held constant and resistance decreases to less than 105 Ohms, the current through the circuit will increase beyond the max current of 30 mA, making the LED fail. On the other hand, if voltage is held constant and resistance increases to greater than 105 Ohms, then the current will be less than its maximum value, so the LED will still be safe. This means that the value we calculated for resistance using an I_{max} of 30 mA is the R_{min} necessary for the LED to succeed.

✔

c. What is the resistance range of the potentiometer?

Using the multimeter, I measured the resistance range of the potentiometer to be between 0 Ohms and 10 kiloOhms (10,000 Ohms).

✔

3. Basic LED Circuit with Switch

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

No, the order of components in the circuit between power and ground does not matter.

✔ Great

From Kirchoff's laws, we know that if the LED is located before (with respect to conventional current flow) the resistor and the switch, the current coming into and out of the LED will be the same as the current coming into and out of the resistor which will be the same as the current coming into and out of the switch. This will also be true if the switch is located after the LED and before the resistor, and in fact, in all other possible arrangements of the components of the circuit.

As long as the circuit is closed (which happens when the button switch is pressed -- the switch makes it so that the circuit is normally open), and there is potential difference (voltage) between power and ground, we know that the current going into and out of each of the components will be the same and that the circuit should function as we expect regardless of the order of the parts.

4. Battery-Powered LED with Switch on Breadboard

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

V_{power} - V_{f} = I_{max}R

V_{power }= 9 V

V_{f} = 1.85 V (see 2(a))

I_{max} = 0.03 A (see 2(a))

V_{power} - V_{f} = I_{max}R

9 - 1.85 = (0.03)R

R = 238.3 Ohms

I used a 270 Ohm resistor in my circuit.

✔ Excellent

5. Make a Video of Your LED Being Switched On and Off.

## Comments (1)

## zahraa@... said

at 4:42 pm on Jul 6, 2015

Great

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