AC undervoltage detection circuit of liquid crystal display television

Abstract

The utility model provides an AC undervoltage detection circuit for a liquid crystal display television. The AC undervoltage detection circuit is characterized by comprising an AC sampling circuit, an operational amplifier U2, a reference voltage Vref and an optical coupling circuit, the positive input end of the operational amplifier U2 is electrically connected with the AC sampling circuit, the negative input end of the operational amplifier U2 is electrically connected with reference voltage Vref, and the output end of the operational amplifier U2 is electrically connected with the optical coupling circuit; wherein the reference voltage Vref is a known fixed voltage value. According to the circuit, the special AC detection circuit is additionally arranged, when AC input voltage is too low, a detection signal is transmitted to the secondary side mainboard, the mainboard sends out an instruction and shuts down operation of a power supply, abnormal heating and damage of devices cannot be caused, and meanwhile, due to the adoption of the special detection circuit, the detection precision is greatly improved.

Description

Technical Field

[0001] The utility model belongs to the technical field of high-temperature airflow recovery equipment, in particular to an AC undervoltage detection circuit for a liquid crystal television. Background Art

[0002] For safety reasons, current LCD TV power supplies on the market generally include output overvoltage and overcurrent protection, as well as open-circuit and short-circuit protection for the light strip. However, these power supplies typically lack dedicated detection circuits for abnormally low AC input voltages due to power grid fluctuations, relying instead on current sampling in the switching circuit for overcurrent protection. Because the sampling resistors are typically small and subject to significant error, this protection lacks precision and results in a wide range of deviations. In severe cases, this can lead to overstressed components, resulting in heating and damage. Summary of the Invention

[0003] Technical solution: In order to solve the above technical problems, the utility model provides an LCD TV AC undervoltage detection circuit, specifically:

[0004] It includes an AC sampling circuit, an operational amplifier U2, a reference voltage Vref, and an optical coupling circuit; the positive input terminal of the operational amplifier U2 is electrically connected to the AC sampling circuit, the negative input terminal is electrically connected to the reference voltage Vref, and the output terminal is electrically connected to the optical coupling circuit; wherein the reference voltage Vref is a known fixed voltage value.

[0005] As an improvement, the AC sampling circuit includes a resistor R1, a resistor R2, a resistor R3, a resistor R4, and a capacitor C1; wherein the resistors R1 and R2 are connected in parallel and then in series with the resistors R3 and R4; the input ends of the resistors R1 and R2 are both electrically connected to the AC voltage output end, and the output end of the resistor R4 is electrically connected to the positive input end of the operational amplifier U2; the capacitor C1 is used for filtering in the AC sampling circuit.

[0006] As an improvement, it also includes a diode D1, a resistor R5, a resistor R6, and a capacitor C2; the positive pole of the diode D1 is electrically connected to the output end of the operational amplifier U2, and the negative pole is electrically connected to the resistor R5 and the resistor R6 in sequence; the output end of the resistor R6 is electrically connected to the optical coupling circuit; the capacitor C2 is used for filtering the output circuit of the diode D1.

[0007] As an improvement, the optical coupling circuit includes an NPN transistor Q1, an optical coupler U1, and a resistor R7; the emitter of the NPN transistor Q1 is grounded, the collector is electrically connected to the input end of the optical coupler U1, and the base is electrically connected to the output end of the resistor R6; the output end of the optical coupler U1 is electrically connected to the resistor RT, and the resistor RT is electrically connected to a power supply VCC.

[0008] Beneficial effect: The LCD TV AC undervoltage detection circuit proposed by the utility model adds a special AC detection circuit. When the AC input voltage is too low, the detection signal is transmitted to the secondary side mainboard, and the mainboard issues an instruction to shut down the power supply. It will not cause abnormal heating and damage to the device. At the same time, due to the use of a special detection circuit, the detection accuracy is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 This is a schematic diagram of the principle of the circuit of the utility model. DETAILED DESCRIPTION

[0010] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0011] See Figure 1 As shown in the figure, the detection circuit of this utility model adds AC input L and N sampling circuits and a detection IC. When the AC voltage is detected to be abnormally low, the detection IC outputs a low level. Furthermore, an optocoupler and related circuits are added to transmit the control IC output level to the secondary detection motherboard, which issues a command to shut down the power supply.

[0012] in, Figure 1 The circuit in this paper consists of two main parts: an optocoupler circuit and an operational amplifier circuit.

[0013] 1. Optocoupler Circuit: The optocoupler (U1, model TLP521-1) is used for electrical isolation. It converts the input signal into an optical signal and then back into an electrical signal to isolate the input and output circuits. An NPN transistor (Q1, model 2N2219A) works in conjunction with the optocoupler. When the optocoupler turns on, it turns on Q1, allowing current to flow.

[0014] 2. Operational Amplifier Circuit: The operational amplifier (U2, model 3554AM) is configured as a comparator. It compares the voltages at its positive (+) and negative (-) inputs. The positive input is connected to an AC voltage through resistors R1, R2, R3, and R4. These resistors sample the AC voltage and convert it to a voltage level suitable for the comparator input. The negative input is connected to a fixed reference voltage (Vref, 12V), which is provided by an external circuit.

[0015] The circuit also includes protection and filtering circuits: Diode D1 (1N3600) protects the op amp input, preventing reverse voltage from damaging the circuit. Capacitors C1 and C2 provide filtering, reducing the effects of power supply noise and voltage fluctuations on the circuit.

[0016] The operating principle of this utility model is as follows: AC voltage enters the positive terminal of the comparator through sampling resistors R1-R4 and is compared with a reference voltage at the negative terminal. Under normal circumstances, when the voltage at the positive terminal of the comparator is greater than the reference voltage, the comparator output is high, turning on NPN transistor Q1 and the optocoupler. When the comparator voltage falls below the reference voltage, the output is low, turning off Q1 and the optocoupler. The optocoupler switches on and off, transmitting the relevant signal to the secondary side motherboard. The motherboard issues a command based on the detection signal to prevent the power supply from operating in AC undervoltage mode.

[0017] The above-described embodiments merely represent several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the utility model patent. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements fall within the scope of protection of the present invention. Therefore, the scope of protection of the present utility model patent shall be determined by the appended claims.

Claims

1. An LCD TV AC undervoltage detection circuit, characterized by: It includes an AC sampling circuit, an operational amplifier U2, a reference voltage Vref, and an optical coupling circuit; the positive input terminal of the operational amplifier U2 is electrically connected to the AC sampling circuit, the negative input terminal is electrically connected to the reference voltage Vref, and the output terminal is electrically connected to the optical coupling circuit; The reference voltage Vref is a known fixed voltage value.

2. The LCD TV AC undervoltage detection circuit according to claim 1, characterized in that: The AC sampling circuit includes resistors R1, R2, R3, R4, and a capacitor C1. Resistors R1 and R2 are connected in parallel and then in series with resistors R3 and R4. The input ends of resistors R1 and R2 are both electrically connected to the AC voltage output end, and the output end of resistor R4 is electrically connected to the positive input end of an operational amplifier U2. Capacitor C1 is used for filtering in the AC sampling circuit.

3. The LCD TV AC undervoltage detection circuit according to claim 1, wherein: It also includes a diode D1, a resistor R5, a resistor R6, and a capacitor C2; the positive electrode of the diode D1 is electrically connected to the output end of the operational amplifier U2, and the negative electrode is electrically connected to the resistor R5 and the resistor R6 in sequence; the output end of the resistor R6 is electrically connected to the optical coupling circuit; The capacitor C2 is used for filtering the output circuit of the diode D1.

4. The LCD TV AC undervoltage detection circuit according to claim 3, characterized in that: The optical coupling circuit includes an NPN transistor Q1, an optical coupler U1, and a resistor R7; the emitter of the NPN transistor Q1 is grounded, the collector is electrically connected to the input end of the optical coupler U1, and the base is electrically connected to the output end of the resistor R6; the output end of the optical coupler U1 is electrically connected to the resistor RT, and the resistor RT is electrically connected to a power supply VCC.

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