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Sharma Naman Lab 1

Page history last edited by xinyi xie 9 years, 9 months ago

2. Controlling the Brightness of LEDs

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.

Ans (a)

When using the Super Bright Green LED,

We have the LED in series with a resistance of known value. Further the voltage drop across the LED is known (using the Kingbright datasheet provided for the same).

We decide to take the maximum value of voltage drop from the data sheet(i.e. 2.5V) because the problem statement needs to find the MINIMUM resistance for the working conditions. Typically the value for voltage drop however is 2.2V, (from the datasheet), and if we calculate the problem using this value the answers would vary slightly.

So we have:- a 5V dc source => V_s

voltage drop across the LED= 2.5 V (from datasheet), we'll call it Vled

max current that can flow through the circuit= 25 mA, lets call it I_max

Let the resistance that should be used be "R".

Then using ohm's law we get,

I_led= V_s/(R+(V_led/I_led))

Inserting the values,

25mA = 5V/(R+(2.5/25mA)

solving, we get=>

R= 100 ohm

Hence, a resistor of at least a !00 ohms must be used with the GREEN LED.

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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.

Ans(b)

The value of resistance "R" found above is the MINIMUM resistance. Which is to say, that our circuit must have a resistor of at least a 100 ohm or more in order to prevent the LED from failing. Any more the circuit would work just fine even though the brightness of the LED may be affected. Any less, and we run the chance of the LED failing, or worse EXPLODING, unlike what happened in the lecture.

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c. What is the resistance range of the potentiometer?

Ans(c)

The range of the potentiometer, was found to vary from a minimum of 0.2 ohm to a max of 9.856K ohm.

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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?

Ans(a)

No, the order of components arranged in a circuit does not matter at all. The net current in the circuit will be found using OHM'S LAW. That is, the total current flowing through the circuit will be equal to the ratio of the source voltage and net equivalent resistance of the various components in the loop, regardless of the order in which they are arranged.

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