Load sensor application principle circuit analysis

Load sensors (LSE for short) are sensitive devices specially used to monitor the connection and disconnection of the load at the output end of power supply equipment. Domestic manufacturers have currently developed three series of load sensor products, namely, the I series general-purpose type (the manufacturer's design concept is to be used as a no-load energy-saving sensor in the inverter); the II series is specially used for voltage regulator equipment; the III series is a DC load sensor (the manufacturer's design concept is mainly used as a sensor for energy saving and protection in recorders). This article only introduces the application principle, basic electrical parameters and relevant precautions of the I series load sensor LSE, together with some newly developed application circuits, to the majority of electronic enthusiasts.

Ⅰ Series Application Principle

I series load sensors are universal devices that can be used in any substation. Since they require a set of DC power supply to work, they are most convenient to use on inverters with DC power supply. They can directly use the DC power supply. The basic function is to use the load sensor to directly control the power on and off of the oscillation circuit in the inverter, thereby realizing automatic startup and shutdown to achieve the purpose of energy saving.

The application principle wiring diagram of the I series load sensor is shown in Figure 1 (a) and (b). A load RL is connected in series between the ① and ② pins of the load sensor LSE (no voltage is required), a DC voltage (DC6-15V) is added to the ③ pin of the LSE, the ⑤ pin of the LSE is grounded, and the switch S is closed. At this time, there is current output at the ④ pin of the LSE. In addition, a load RL can be connected in series between the ① and ② pins of the LSE, and connected to an AC or DC circuit less than 2500V, and the switch S is closed. At this time, there is current output at the ④ pin of the LSE, the transistor VT is cut off, and the relay J is in a released state. Disconnect the switch S, and at this time the ④ pin of the LSE outputs a low level, the transistor VT is turned on, and the relay J is excited and attracted.
The I series load sensor has a total of 5 pins: ① and ② are the loop on-off detection terminals; they can be connected in series in a 0-2500V AC or DC circuit. The ③ pin is the positive end of the power supply; a 6-15V DC power supply is used. The ④ pin is the output terminal; the manufacturer recommends a maximum parameter of 50mA. (According to actual measurement, as the voltage of pin ③ increases, the output current of pin ④ can reach up to 80mA.) Pin ⑤ is the ground terminal. The action sensitivity of the sensor is 50K (that is, it can act when the internal resistance of the load RL is less than 50K). The static loss current is less than 30uA; the rated load current is determined by the device model, as shown in the table.

When using this device, please pay attention to the following matters:
① The parameters marked on the device are all rated values. It is not advisable to use it beyond the rated values, otherwise the safety of the device cannot be guaranteed.
② The output terminal ④ of the load sensor LSE cannot be short-circuited to the ground.
③ If the output terminal ④ directly drives the load, when the load current is greater than 50mA, the current can be amplified by a transistor to drive the load to work, but the base of the transistor used must be connected in series with a current limiting resistor, otherwise the transistor may be damaged.
④ When welding the pins of LSE, its temperature should be controlled below 260℃, and the welding operation time should not exceed 6s.
⑤ When used in the presence of strong electromagnetic pulse interference, in order to enhance the reliability of the circuit, a 0.22uF capacitor or an adjustable resistor can be connected between the ② and ⑤ pins of LSE.