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University of Sorocaba Control and Automation Engineering

Mateus Djanikian Arrobas Martins

Circuits & Experiments for Beginners: Lighting up an LED

Sorocaba/SP 2017


Abstract This work has the purpose of teaching students how to correctly dimension a resistor, in order to protect the Light Emitting Diode (LED) used in the circuit, and also how to make a very simple electronic circuit, which lights up an LED. It is taught how to dimension the resistor through the Ohm´s Law, how to understand a resistor band color table, and also how to know if the desired resistor value exists according to the E24 resistor series. At the end of the work, the student will be able to successfully dimension the resistor, no matter the voltage source used to power the circuit. The student will be also able to figure out the value of a resistor just by its color code and determinate whether the value can be found at the E24 commercial resistors series. Keyword: Resistor. Dimension. LED.


Summary

1. Objective: ........................................................................................................ 4 2. Needed Material: ............................................................................................. 4 3. The Components: ........................................................................................... 4 4. Experimental Procedure: ............................................................................... 5 5. How does it work? .......................................................................................... 7 6. Math and the circuit: ....................................................................................... 8 i)

How was the resistor chosen? ................................................................... 8

ii) How was the resistor´s voltage drop determinate? .................................. 8 iii) Tolerance.................................................................................................... 10 iv) E24 Series .................................................................................................. 10 v) What´s the minimum value for the resistor? ........................................... 10 vi) What is the resistor´s dissipated power? ................................................ 11 vii) What is the LED´s dissipated power? ...................................................... 12 viii) What´s the power generated by the batteries? ....................................... 12 ix) Math and the circuit´s conclusion ............................................................... 12 7. Conclusion: ................................................................................................... 14 8. Additional Information I: Resistor Color Table .......................................... 15 9. Additional Information II: E24 Series .......................................................... 16


1. Objective: Build an electronic circuit that lights up a Light Emitting Diode (LED). 2.

Needed Material: 1 common LED 1 560â„Ś Resistor 1 Breadboard (any size will be fine). Powering the circuit: In order to power the circuit, you can use 4 cells (or batteries) of 1.5V each with a battery holder, or a 6V coin cell, or even a power supply regulated for 6V.

3.

The Components:

Figure 1 - Red LED used for the experiment

Figure 2 – Small Breadbord (170 pins)

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Figure 3 - 560Ω Resistor

4. Experimental Procedure: Below we can find the diagram from the circuit that will be made:

Figure 4 – Circuit´s Diagram

Keep in mind that it´s very important to pay attention to the LED´s polarity. If the LED is inverted the circuit won´t work. Please notice that the LED has two terminals that look like small metallic legs. How to know which one is the positive terminal and which one is the negative terminal? The negative terminal has a small chamfer or flat spot, as shown next page:

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Figure 5 - LED and it´s chamfer

The resistor has no polarization, so it can be inserted in the breadboard without any worries. As it´s possible to see below, the resistor has a golden band (to the right of the brown band). We´ll use it as a reference during the circuit making.

Figure 6 – Resistor golden band

Below we have an illustration of how the circuit should be assembled:

Figure 7 - Circuit Ready

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If the assembly was well successful, then the LED lit and the circuit worked perfectly. In case it didn’t work, it´s suggested that you turn it off, and try to figure out the problem without removing the components from the board. If the problem continues, then remove all the components and try again. 5. How does it work? The battery (or the circuit´s power source) supplies the electric current to the circuit. At the negative pole there is a bigger amount of electrons (negative electric charges) than at the positive pole. For balance reasons, the electrons at the negative pole are attracted to the positive pole so that there is a balance among the positive and negative charges. This attraction creates an electron flux which will flow through the circuit, passing through the electronic components (resistor and LED) and also through the wires which have been used, until the charges reach the positive pole. This flux is named electric current. However, the flux of electric current through the circuit has some consequences. When passing through the resistor, the electric current faces electric resistance or opposition, and in order to defeat this resistance, the current liberates energy as heat energy (Joule Effect). It would be the same thing as if an adult pushed a car, and liberated energy as sweat. The force that pushes the car is the voltage. The car´s weight would be the resistance and the sweat would be the heat produced by the current when passing through the resistor. The heat however is too low for us to feel with bare hands. When flowing through the wires, a small amount of heat is liberated, but in this case, it´s too small to be felt or measured, being in our case, irrelevant. When flowing through the LED, the electric energy is converted to light energy (energy is not created nor destroyed, it is transformed), producing the LED´s bright (the more intense the current is, the more intense will the bright be). It´s very important to remember that the LED´s color is determined by the impurities contained inside the LED, and not by the Epoxy case (sometimes, the Epoxy case is transparent). After passing through the LED, the electrons arrive at the positive pole of the battery or power source. There is however, a last part of theory to which we must pay attention. In order to have electric current, there must be a difference in the amount of electrons at the positive and negative poles from the power source (electric potential difference). If the amount of electrons is the same at both poles, the current ceases.To avoid this problem, the battery makes a chemical process named redox (the short for reduction-oxidation process). Through this chemical reaction, the battery transfers electrons from the positive pole to the negative, so that there is this potential difference, and thus, electric current. 7


However, to make this process, the battery loses energy (that´s the reason why batteries die; they don´t have any more energy to make the redox process), but that doesn´t happen when we use a power supply, because it´s connected right to the energy from the building.

6. Math and the circuit: It´s important to pay attention to the fact that there is some math behind this circuit, and it´s very important to know it, to better understand how the circuit works. i)

How was the resistor chosen? When choosing the resistor, it´s very important to know how much current will flow through the circuit, because a current that is too low won´t light up the LED, and a current that is too intense will damage the component, even burn it. The component´s datasheet gives us vital information for who are willing to project the circuit (the LED´s nominal voltage, the maximum current, and the forward current). For the used model, the nominal voltage is 1.7V and the maximum current is 20mA. This means that a current above 20mA will damage the component. We can, using good sense, determinate the intensity of the current which will produce the best result. It´s also very important to work using a safety margin. In our case, a 15mA current has been chosen. Knowing that the current is 15mA, and the LED´s working voltage is 1.7V, we can determinate the value of the resistor, by using Ohm´s Law. We need to know the voltage drop in the resistor in order to determinate which commercial value is the best option for the circuit.

ii) How was the resistor´s voltage drop determinate? We know the sum of the voltage drops from the components is equal to the battery´s voltage drop. Since the power source has 6V: Vsource = VLED + Vresistor Replacing with numeric values, we have: 6V = 1.7V + Vresistor

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Subtracting 1.7 from both side of the equation: 6 – 1.7 = 1.7 – 1.7 + Vresistor 4.3 = 0 + Vresistor Vresistor = 4.3V Now we know the resistor´s voltage drop. Appyling Ohm´s Law: R= R = Resistor´s value (Ω) Vres = Resistor´s voltage drop (V) I = Electric Current that passes through the resistor (A) Now, changing into numbers: R= R = 286.67Ω We can also find the resistance by using the current´s value in mA, and we´ll have the same value for the resistance: R=(

)

R = 286.67Ω It´s not possible to find a commercial resistor value of 286Ω, so we´ll be using the closest commercial value. To determinate the best value for the resistor is just a matter of following these rules: 1. Determinante the resistor´s value (286Ω). 2. Search for the E24 resistor series, and see what resistor value is the closest to the value found through the calculations. By the E24 series we would have the following values: 240Ω 270Ω 300Ω 330Ω

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The value found through math was 286Ω; therefore, the most recommended values are 300Ω or 330Ω. Though the best theoretical value is 330Ω, a 560Ω resistor was chosen for safety reasons, to ensure the best operation for the circuit. The change from 330Ω to 560Ω will only reduce slightly the LED´s bright. In case of curiosity a 330Ω resistor can be used with no problems. iii) Tolerance The E24 has a tolerance of ±5%. This means that the value of the resistor can vary 5% of the theoretical value for more or for less. Example: Let´s say we have a 1000Ω resistor (1kΩ) with a ±5% tolarance. We know 5% of 1000Ω is equal to 50Ω. In this case, our resistor´s value would vary 50Ω for more or for less. Though it´s a 1000Ω resistor, its value (when measured) would be between 950Ω and 1050Ω. The resistor´s value found through math is equal to 286Ω. If a 300Ω resistor were used, its resistance would be between 285Ω and 315Ω. If a 330Ω resistor were used, its resistance would be between 313.5Ω and 346.5Ω, which is safer for our case, once we want to work with a safety margin. Since we´re using a 560Ω resistor, its resistance will be between 532Ω and 588Ω. iv) E24 Series The E24 resistor series is the most popular and easy to be found series, when talking about electronics. Most of the resistors we buy in our daily lives are from the E24 series and that´s why it was quoted during this work. At the end, it´s taught how to read the E24 series table. v) What´s the minimum value for the resistor? In order to find the minimum value for the resistor we should consider the current as being the maximum current that the LED can stand. In this case Imax = 20mA.

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Therefore: R= R = 215Ω The commercial value that better fits in this case is the 220Ω resistor. Smaller values than 220Ω might damage the LED, even burning it. vi) What is the resistor´s dissipated power? First of all, we need to know that power is the rate at which work is done. The higher the power, more work is done at a period of time. There are a few different ways that can be used to find out the dissipated power: 1. Through the resistor´s voltage drop Power = Voltage x Electric Current Power = 4.3 x 15mA Power = 0.645W or 64.5mW 2. Through the resistance and the electric current Power = Resistance x (Electric Current)2 Power = 286.67 x (0.015)2 Power = 64.5mW 3. Through the voltage and resistance Power = Power =

(

(

)

)

Power = 0.6451 W or 64.5mW

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vii) What is the LED´s dissipated power? In order to know the dissipated power, we can use the LED´s voltage drop. Power = Voltage x Electric Current Power = 1.7 x 15mA Power = 25.5mW viii) What´s the power generated by the batteries? It´s exactly the same idea as the LED: Power = Voltage x Electric Current Power = 6 x 15mA Power = 90mW We can also clearly see that: Powersource = Powerresistor + PowerLED 90mW = 64.5mW + 25.5mW 90mW = 90mW ix) Math and the circuit´s conclusion: After these calculations, it´s possible to ask the following questions: 1. What if the power supply has it´s voltage increased? Answer: If that happens, then we need to calculate a new resistor for the circuit, by using the following formula: Resistor´s Value =

2. What if the power supply voltage is exactly the same as the LED´s working voltage? Answer: In this case, a resistor won´t be necessary. However the LED will be working at its limits and might be damaged.

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3. What if the power supply voltage is smaller than the working voltage of the LED? Answer: The LED won´t need a resistor to protect it. However, it won´t work at full capacity and the bright may not be as good as expected. Note: It´s very common at electronic components stores, when buying an LED, for the salesperson to connect it directly to a 1.5V coin cell to test the component. This makes sure that the LED you´re buying is working perfectly, without damaging the component. 4. What if the resistor is replaced by another resistor, with a higher resistance? What if it´s replaced by another resistor, with a lower resistance?Note: Consider Vsource = 6V. Answer: If it´s replaced by a higher value, then the electric current will be less intense. This results in a lower LED´s power, and thus, the brightness will be reduced. If the resistor is replaced by a lower value, inside the safety margin, then the electric current will have higher intensity. This results in a higher LED´s power, and thus, the brightness will be increased. 5. What would happen if we changed the LED´s color (a green one, for example)? Answer: The circuit would work perfectly, as long as the resistor has been correctly chosen.

Note: If you want to change one of the components, please make sure that the circuit is not powered! If the circuit is powered you might damage one of the components. Turn it off first, change the components and turn it back on. If you want to increase the power supply´s voltage, first make sure that the power source is not connected to the circuit, to avoid any damage to the components due to excessive voltage.

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7. Conclusion: During this first experiment, it was possible to have the first contact with electronic circuits and the some basic calculations. The Ohm´s Law has been explored along with the power definition and math. It was also taught how to dimension the resistor in order not to damage the LED and how to calculate the dissipated power for each one of the components from the circuit. For the next experiment, it will be explored how to add a switch to the circuit, in order to control the LED.

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8. Additional Information I: Resistor Color Table Below it is possible to see a resistor color table. By using it, it´s possible to find out the value of a resistor without using any equipment such as multimeters.

Source: http://physicsabout.com/resistor-color-code/ This table is very simple to be used. The resistor is made of 4 bands: 1st figure, 2nd figure, 3rd figure, multiplier and tolerance, as it´s possible to see in the table. a. Let´s assume that our resistor has the following colors: brown, black, red e gold. According to the table, we have: brown: 1 (1st figure) black: 0 (2nd figure) red: x100 (multiplier) gold: ±5% (tolerance) Therefore: 1000Ω or 1kΩ b. Let´s now assume that our resistor has the following colors: yellow, violet, brown and gold: yellow: 4 (1st figure) violet: 7 (2nd figure) brown: x10 (multiplier) gold: ±5% (tolerance) Therefore: 470Ω

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c. For last: red, red, blue and gold. We´ll never have a resistor that starts with the black band. That´s a rule! red: 2 (1st figure ) red: 2 (2nd figure) blue: x1M or 106 or x1000000 (multiplier) gold: ±5% (tolerance) Therefore: 22MΩ 9. Additional Information II: E24 Series

Like it was said before, the E24 resistor series is the most common commercially. It has a ±5% tolerance and it´s easily found everywhere. The available values for the E24 series are: 1.0

1.1

1.2

1.3

1.5

1.6

1.8

2.0

2.2

2.4

2.7

3.0

3.3

3.6

3.9

4.3

4.7

5.1

5.6

6.2

6.8

7.5

8.2

9.1

Source: http://eletronicassim.blogspot.com.br/p/a-importancia-das-unidades-medidae.html Having these numbers at hand, we can multiply them from x1 to x100000000. For exempla, a resistor with a 1.2 initial value can be found at the following values: 1.2Ω 12Ω 120Ω 1200Ω (1k2Ω) 12000Ω (12kΩ) 120000Ω (120kΩ) 1200000Ω (1M2Ω) 12000000Ω (12MΩ) 120000000Ω (120MΩ)

This rule works for any value from the table shown above, with no exceptions.

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