economic LED

29 economic LED 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. 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: R = UB — ULED ILED 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 24V 5 mA 7-47 elektor july/august 1983 2 ILED UR3 periodically. The switching point is adjusted with preset potentiometer Pl. This governs the changeover voltage at pin 3 of the operational amplifier which, in turn, governs the maximum LED current. It should not exceed 50 mA. The frequency ...