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Serial Connection And Parallel Connection Figure 1 shows the most basic and commonly used circuits for driving light-emitting diodes. In Figure 1(A), a constant voltage source (VCC ) is connected through a current limiting resistor (R) to an LED so that it is supplied with forward current (IF). The IF current flowing through the LED is expressed as IF = (V CC - VF)/R, providing a radiant flux proportional to the IF. The forward voltage (V F) of the LED is dependent on the value of IF, but it is approximated by a constant voltage when setting R. Figures 1(B) and. | SHARP Opto Application Note Optoelectronics INFRARED LIGHT-EMITTING DIODE APPLICATION CIRCUITS Serial Connection And Parallel Connection Figure 1 shows the most basic and commonly used circuits for driving light-emitting diodes. In Figure 1 A a constant voltage source VCc is connected through a current limiting resistor R to an LED so that it is supplied with forward current IF . The IF current flowing through the LED is expressed as IF Vcc - Vf R providing a radiant flux proportional to the IF. The forward voltage VF of the LED is dependent on the value of If but it is approximated by a constant voltage when setting R. Figures 1 B and 1 C show the circuits for driving LEDs in serial connection and parallel connection respectively. In arrangement B the current flowing through the LED is expressed as IF Vcc - VF x N R while in arrangement c the current flowing through each LED is expressed as IF Vcc - Vf R and the total supply current is N x IF where N is the number of LEDs. VCC N VCC C OP1-1 A B Figure 1. Driving Circuit of LightEmitting Diode LED The VF of an LED has a temperature dependency of approximately -1.9 mV c. The operating point for the load R varies in response to the ambient temperature as shown in Figure 2. Constant Current Drive To stabilize the radiant flux of the LED the forward current IF must be stabilized by using a constant current source. Figure 3 shows a circuit for constantly driving several LEDs using a transistor. The transistor Tr1 is biased by a constant voltage supplied by a zener diode ZD so that the voltage across the emitter follower loaded by resistor Re is constant thereby making the collector current Ic IF constant. The Ic is given as Ic IE VZ Vbe Re. If too many LEDs are connected the transistor enters the saturation region and does not operate as a constant current circuit. The number of LEDs N which can be connected in series is calculated by the following equations. Vcc - N x VF - VE VcE sat Ve Vz - Vbe These equations give N
