Half-wave rectification circuit with a low-pass filter for LED light strings
View Patent ↗Disclosed is a low cost LED string circuit design that uses an inexpensive half-wave rectification and low-pass filter circuit that is designed to produce minimal flicker in the LED string that is connected to the circuit. The components of the half-wave rectification and low-pass filter circuit are selected in accordance with design principles that prevent glittering and flickering of the LED string. The circuit components of the half-wave rectification and low-pass filter circuit can be embedded in an outlet plug or as a separate independent unit between an AC power plug and the LED string.
1. An LED string circuit comprising:
a plug that is adapted to fit in a standard household electrical socket;
a half-wave rectification and low-pass filter circuit disposed in said plug comprising:
a resistor having a resistance (R) that is connected to a first lead of an alternating current power source having a frequency (f o );
a diode connected in series with said resistor;
a capacitor connected between an output node of said diode and a second lead of said alternating current power source;
an LED string, having a resistance (R LED ) that is connected to said output node of said diode and said second lead of said alternating current power source, said LED string having an effective resistance (R LED );
said capacitor having a capacitance (C) selected in accordance with:
C
=
R
+
R
LED
2
π
RR
LED
f
c
,
and
f
c
=
2
η
f
o
π
,
and
where f c <<f o and η is the change in voltage that is applied to the LED string divided by the average voltage that is applied to the LED string.
2. The LED string circuit of claim 1 wherein f c <<f o to comprises f c having a value that is at least approximately two orders of magnitude less than f o .
3. The LED string circuit of claim 2 wherein said printed circuit board is encapsulated in an over-molded plastic housing for said plug.
4. A method of generating a substantially constant voltage for an LED string from an alternating current power source comprising:
connecting a resistor having a resistance (R) to a first lead of said alternating current power source;
connecting a diode in series with said resistor;
connecting a capacitor having a capacitance (C) between an output node of said diode and a second lead of said alternating current power source;
connecting said LED string between said output node of said diode and said second lead of said power source, said LED string having an effective resistance (R LED );
selecting the value of the capacitance of said capacitor in accordance with:
C
=
R
+
R
LED
2
π
RR
LED
f
c
and
f
c
=
2
η
f
o
π
where f c is the cut-off frequency of the circuit and f o is the frequency of said alternating current power source and η is the change in voltage that is applied to the LED string divided by the average voltage that is applied to the LED string;
selecting f c as follows:
f c <<f o .
5. The method of claim 4 wherein said process of selecting f c <<f o comprises selecting f c to be at least approximately two orders of magnitude less than f o .
6. The method of claim 5 wherein said process of connecting said resistor, connecting said diode and connecting said capacitor further comprises:
connecting said resistor, said diode and said capacitor to a printed circuit board;
encapsulating said printed circuit board in an over-molded plastic housing for a plug.