![]() ![]() In this project you will make two circuits: an LED circuit and a light-to-voltage converter circuit. Schematic diagram showing how to use Ohm's Law to calculate the correct value for the current-limiting resistor (Hewes, 2006). The diagram explains how the starting voltage of the battery loses some value of volts to the LED bulb as it passes through, and to figure out the voltage going across the resistor you must subtract the voltage lost to the LED from the starting battery voltage.įigure 3. You can then use Ohm's Law to calculate the resistance, R, needed to produce a desired current, I: The voltage drop across the resistor will equal the supply voltage minus the voltage drop across the LED (or, V S − V L). Using Ohm's law, the current, I, through the resistor will be V/ R = 7 V/1kΩ = 7 mA.įigure 3 shows you how to use Ohm's Law to calculate what size resistor you need to limit the current through the LED to the desired value. Seven volts fall across the resistor, and 2 V fall across the LED. There is 9 V available at the top of the waterfall. Think of the circuit as a waterfall loop. That leaves 7 V to drop across the resistor. ![]() Of the 9 V available, 2 V is required to turn on the LED. Voltage drop is defined as a loss in voltage across components in an electrical circuit. This means that 2 V is required for the LED to turn on and conduct or create a path for current. The voltage drop across an LED is about 2 V (except for blue or white LEDs, where the voltage drop is about 4 V). Schematic diagram of an LED in series with a 1kΩ resistor and a 9 volt battery. In this case, the battery is providing 9 V to the series combination of the resistor and the LED.įigure 2. The cathode of the LED is connected to the negative terminal of the battery. The resistor is connected in series with the anode of the LED. In this circuit, the positive terminal of the battery is connected to the resistor. To keep the LED current at or below the recommended operating current level, LEDs are typically connected in series with a current-limiting resistor, as shown in Figure 2. Catastrophic failure can be defined as when the LED no longer emits light. Far above this value, the LED will fail catastrophically. Above this value, the lifetime of the LED will be decreased significantly. Typically, the recommended current for an LED is 20 milliamperes (mA) or less. LEDs are available in many different colors.Īs the current through the LED increases, the brightness also increases. The color of the light depends on the material from which the semiconductor is made. When current flows through the diode in the forward direction, some of the electrical energy is converted into light of a specific color (i.e., wavelength). In the schematic symbol for an LED (bottom), the anode is on the left and the cathode is on the right (Hewes, 2006). The longer lead is the anode (+) and the shorter lead is the cathode (-). ![]()
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