Zero-crossing point detection circuit

A zero-crossing point detection circuit includes a hot line input, a neutral line input, a first zero-crossing point output, and a first optical coupler. The first optical coupler includes a first light-emitting diode (LED) and a first optical transistor. The hot line input and neutral line input are respectively connected to two terminals of the first LED. An emitter of the first optical transistor is grounded. A collector of the first optical transistor is connected to a direct current (DC) power source. The collector of the first optical transistor is also connected to the first zero-crossing point output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to detection circuits, and particularly to a zero-crossing point detection circuit.

2. Description of Related Art

Zero-crossing point detection circuits are well known for detecting the instances at which a signal voltage changes from negative to positive and from positive to negative, such as in uninterrupted power supply (UPS) system, or the like.

Generally speaking, such a zero-crossing point detection circuit includes a level comparator circuit consisting of a differential operational amplifier of high gain, some resistors, and some capacitors. However, zero-crossing point detection circuits are directly connected to a power switch circuit and thus are susceptible to static jamming, coupling jamming and the like. Furthermore, the comparator circuit is a simulation circuit which is very complicated to select proper resistors and capacitors.

What is desired, therefore, is to provide a zero-crossing point detection circuit resistant to noise jamming.

SUMMARY OF THE INVENTION

An exemplary zero-crossing point detection circuit includes a hot line input, a neutral line input, a first zero-crossing point output, and a first optical coupler. The first optical coupler includes a first light-emitting diode (LED) and a first optical transistor. The hot line input and neutral line input are respectively connected to two terminals of the first LED. An emitter of the first optical transistor is grounded. A collector of the first optical transistor is connected to a direct current (DC) power source. The collector of the first optical transistor is also connected to the first zero-crossing point output.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, a zero-crossing point detection circuit10in accordance with a first embodiment of the present invention is provided for detecting zero-crossing points of an alternating current (AC) power source20such as a 110V AC power source. The zero-crossing point detection circuit includes three resistors R1, R2, and R3, a first optical coupler12, a second optical coupler14, a direct current (DC) power source Vcc, a first zero-crossing point output X, and a second zero-crossing point output Y. The first optical coupler12includes a first light-emitting diode (LED) D1and a first optical transistor Q1. The second optical coupler14includes a second LED D2and a second optical transistor Q2.

A first end of the resistor R3is a hot line input adapted for connecting to a hot line L of the AC power source20. A second end of the resistor R3is connected to an anode of the first LED D1. A cathode of the first LED D1is a neutral line input adapted for connecting to a neutral line N of the AC power source20. An emitter of the optical transistor Q1is grounded. A collector of the optical transistor Q1is connected to the DC power source Vcc via the resistor R1, and connected to the first zero-crossing point output X.

A cathode of the second LED D2is connected to the anode of the first LED D1, and an anode of the second LED D2is connected to the cathode of the first LED D1. An emitter of the optical transistor Q2is grounded. A collector of the optical transistor Q2is connected to the DC power source Vcc via the resistor R2, and connected to the second zero-crossing point output Y. In this embodiment, the first zero-crossing point output X is a positive zero-crossing point output, and the second zero-crossing point output Y is a negative zero-crossing point output. Resistances of the resistors R1and R2are both approximately 10KΩ, and a resistance of the resistor R3is approximately 22KΩ. Voltage of the DC power source Vcc is 5V.

Referring also toFIG. 2, output waveforms40and50of the zero-crossing point outputs X and Y corresponding to an output waveform30of the AC power source20are shown. The output waveform30of the AC power source20is a sine wave and includes a plurality of positive zero-crossing points a1, a2, a3. . . and a plurality of negative zero-crossing points b1, b2, b3. . . . At the beginning, the zero-crossing point outputs X and Y are both at a high voltage level. When output of the AC power source20passes the first positive zero-crossing point a1, the first optical coupler12turns on, so that the positive zero-crossing point output X is changed to a low voltage level until output of the AC power source20passes the first negative zero-crossing point b1, at which time the first optical coupler12turns off, so that the positive zero-crossing point output X is changed to the high voltage level until output of the AC power source20passes the second positive zero-crossing point a2, and then it goes on like before.

When output of the AC power source20passes the first negative zero-crossing point b1, the second optical coupler14turns on, so that the negative zero-crossing point output Y is changed to a low voltage level until output of the AC power source20passes the second positive zero-crossing point a2, at which time the second optical coupler14turns off, so that the negative zero-crossing point output Y is changed to the high voltage level until output of the AC power source20passes the second negative zero-crossing point b2, and then it goes on like before.

When a backup power source (not shown) needs to take over from the AC power source20, the zero-crossing point outputs X and Y of the zero-crossing point detection circuit10are connected to a power source switch circuit (not shown). The power source switch circuit cuts off the AC power source20from the electrical device and connects the backup power source to the electrical device via zero-crossing point signals from the zero-crossing point output X or zero-crossing point output Y.

If the power source switch circuit cuts off the AC power source20and connects the backup power source to the electrical device via zero-crossing point signals from the zero-crossing point output X only, a switching time will be operated in at least a cycle of the AC power source20. However, if the power source switch circuit cuts off the AC power source20via zero-crossing point signals from the zero-crossing point output X and connects the backup power source to the electrical device via zero-crossing point signals from the zero-crossing point output Y, the switch time can be operated in a half cycle of the AC power source20which reduces switching time. Further, because the zero-crossing point detection circuit10uses two optical couplers to detect the zero-crossing points, it will suffer less noise jamming when two power sources switch, and the zero-crossing point detection circuit10is very simple, which costs less.