IN4148 - Datasheet Archive

 

Appnote 6 by David M. Barton Introduction Figure 3. Series capacitor value versus average LED current for 120 VRMS 60 Hz

 

Operating LEDs on AC Power Appnote 6 by David M. Barton Introduction Figure 3. Series capacitor value versus average LED current for 120 VRMS 60 Hz Frequently it is desirable to operate LEDs on AC power rather than DC. Typically, the power source is 120 VRMS 60 Hz. The most obvious method is to rectify this power with a series diode and use a resistor to limit LED current as shown in Figure 1. 2.0 1.0 R Rectifier C (µF) Figure 1. Power Resistor Method LED AC .5 .2 .1 2 This method, though sound, results in very high power dissipation in the resistor since the LED operates on only 1.6 volts. Figure 2. Capacitor method RS AC 50 The current in the LED, of course, flows almost exactly in quadrature with the line voltage. For this reason, power dissipation is low, being limited to the expected LED and rectifier power loss, the loss in series resistor and to losses in the capacitor. The latter term will be extremely low if high quality capacitors are used. Although power consumption of a circuit may not be of much significance in terms of the cost of the power, it certainly can be important to reduce heat generation within an enclosure. The Method C ( Diod e IN41 48 10 20 30 ILED (average)­mA LED If more than one LED is to be operated from the same source, simply put the LEDs in series in the same circuit, as shown in Figure 4. For small numbers of LEDs the current will be, for practical purposes, the same as for one. Figure 2 shows a better method-using a capacitor to control LED current and a shunt silicon diode provides rectification. Since, for current in either direction, voltage drop across the LED or rectifier is a negligible part of the supply voltage, current in the capacitor is almost exactly equal to the AC supply voltage divided by the reactance of the capacitor. Average capacitor current is then: Figure 4. LEDs in series C ( Diode IN4148 IN4148 RS 1. IC (AV) = .9 x VRMS/XC AC and average half-cycle LED or rectifier current is: LEDS 2. ILED (AV) =1/2 ID (AV) = .45 VRMS/XC or, for 120 VRMS, 60 Hz operation, Conclusion 3. ILED (AV) = 20 mA x C µF I LED ( AV ) or C µ F = -20 mA Figure 3 shows the value of the series capacitor needed for a range of average LED currents assuming 60 Hz, 120 volt power. A resistor is necessary in series with the capacitor to limit turnon transient currents. A value of 100 ohms will be adequate in most cases. Cost of the series capacitor (mylar) will be similar to the cost of a series power resistor. The shunt diode, a IN4148 IN4148 or similar, will cost about two cents; much less than a series rectifier which must have a several hundred volt PIV rating. So the capacitor method is both lower in cost and lower in heat generation and power consumption than the resistor method. 14­20