The usual method of operating an LED from a voltage which is higher than its forward voltage is well known. A limiting resistor is used to limit the LED current to its rated value. Learn how to make an economical LED driver circuit.
Calculating the value of the resistor is simple enough: supply voltage minus LED forward voltage divided by the maximum current rating of the LED. The formula is: Thus the voltage difference between supply voltage and forward voltage is dropped by the limiting resistor. However, the disadvantage is that the power dissipated by the limiting resistor is fairly high if the supply voltage is relatively high. Thus, for example, with a supply voltage of 24 V and a current of 25 mA the power dissipated is greater than 0.5 W. There is an alternative: the circuit shown here only requires 0.1 W. lt is effectively a switched current source. The current source is based on transistor T1 and the oscillator uses a 3140 operational amplifier.
When transistor T1 conducts, a current flows via coil L1, LED D1 and resistor R3 to earth. The current curve is shown in figure 2. As soon as T1 turns on and a current flows, the current rises together with the volt- age at R3 from zero volts. This voltage is now applied to the non- inverting input of the operational amplifier. A reference voltage of approximately 0.25 V is applied via voltage divider Rl/R2 to the inverting input. lf the rising voltage at the non-inverting input reaches the level of the reference voltage, the output of the operational amplifier switches to a high voltage potential. Transistor T1 turns off and the current through the LED flows via diode D2. As shown in figure 2, the current drops; the voltage at R3 therefore drops also. Once the current and voltage are sufficiently low, the operational amplifier switches over again and the transistor turns on.
This operation is repeated periodically. The switching point is adjusted with preset potentiometer P1. This governs the changeover voltage at pin 3 of the operational amplifier which, in turn, governs the maximum LED current. lt should not exceed 50 mA. The frequency of the oscillator (which is also the switching frequency for the transistor) is deter- mined by coil L1 and by the switching hysteresis adjusted with P 1. With the specified value of 4.7mH the switching frequency is about 15 kHz with a period of approximately 65 ps. Two other switching frequencies using different coil inductances can be found in the following table: Coil T F 2.2 mH 35 ,us 30 kHz 10 mH 150,is 6kHz P1 should be adjusted to obtain the lowest frequency at which the circuit still starts to oscillate.
Calculating the value of the resistor is simple enough: supply voltage minus LED forward voltage divided by the maximum current rating of the LED. The formula is: Thus the voltage difference between supply voltage and forward voltage is dropped by the limiting resistor. However, the disadvantage is that the power dissipated by the limiting resistor is fairly high if the supply voltage is relatively high. Thus, for example, with a supply voltage of 24 V and a current of 25 mA the power dissipated is greater than 0.5 W. There is an alternative: the circuit shown here only requires 0.1 W. lt is effectively a switched current source. The current source is based on transistor T1 and the oscillator uses a 3140 operational amplifier.
When transistor T1 conducts, a current flows via coil L1, LED D1 and resistor R3 to earth. The current curve is shown in figure 2. As soon as T1 turns on and a current flows, the current rises together with the volt- age at R3 from zero volts. This voltage is now applied to the non- inverting input of the operational amplifier. A reference voltage of approximately 0.25 V is applied via voltage divider Rl/R2 to the inverting input. lf the rising voltage at the non-inverting input reaches the level of the reference voltage, the output of the operational amplifier switches to a high voltage potential. Transistor T1 turns off and the current through the LED flows via diode D2. As shown in figure 2, the current drops; the voltage at R3 therefore drops also. Once the current and voltage are sufficiently low, the operational amplifier switches over again and the transistor turns on.
This operation is repeated periodically. The switching point is adjusted with preset potentiometer P1. This governs the changeover voltage at pin 3 of the operational amplifier which, in turn, governs the maximum LED current. lt should not exceed 50 mA. The frequency of the oscillator (which is also the switching frequency for the transistor) is deter- mined by coil L1 and by the switching hysteresis adjusted with P 1. With the specified value of 4.7mH the switching frequency is about 15 kHz with a period of approximately 65 ps. Two other switching frequencies using different coil inductances can be found in the following table: Coil T F 2.2 mH 35 ,us 30 kHz 10 mH 150,is 6kHz P1 should be adjusted to obtain the lowest frequency at which the circuit still starts to oscillate.
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