Hysteresis comparator circuit diagram, schematic diagram

Hysteresis comparator circuit diagram, schematic diagram

Hysteresis comparator The hysteresis comparator can also be understood as a single-limit comparator with positive feedback. For the single-limit comparator introduced above, if the input signal Uin has a slight interference near the threshold value, the output voltage will produce corresponding jitter (fluctuation). Introducing positive feedback in the circuit can overcome this disadvantage. Figure 1a shows a hysteresis comparator. The familiar "Schmitt" circuit is a comparator with hysteresis. Figure 1b shows the transmission characteristics of the hysteresis comparator. It is not difficult to see from Figure 1 that once the output state is converted, as long as the interference near the jump voltage value does not exceed the value of ΔU, the output voltage value will be stable. But the resolution is reduced. Because for the hysteresis comparator, it cannot distinguish between two input voltage values ​​with a difference less than ΔU. The hysteresis comparator with positive feedback can speed up the response speed of the comparator, which is one of its advantages. In addition, since the positive feedback added to the hysteresis comparator is very strong, much stronger than the parasitic coupling in the circuit, the hysteresis comparator can also avoid self-oscillation caused by parasitic coupling of the circuit. If a jump point needs to be fixed at a certain reference voltage value, a nonlinear element, such as a crystal diode, can be connected to the positive feedback circuit. The unidirectional conductivity of the diode can be used to achieve the above requirements. Figure 2 is its schematic diagram. Figure 3 is the grid overvoltage detection circuit part of a certain induction cooker circuit. When the grid voltage is normal, U4 of 1/4LM339 is <2.8V, U5=2.8V, the output is open, the overvoltage protection circuit does not work, and the emitter follower BG1 as positive feedback is turned on. When the grid voltage is greater than 242V, U4>2.8V, the comparator flips, the output is 0V, BG1 is cut off, and the voltage of U5 is completely determined by the voltage divider value of R1 and R2, which is 2.7V, making U4 greater than U5, which makes the state after flipping extremely stable, avoiding the instability caused by the small fluctuation of the grid voltage near the overvoltage point. Due to the creation of a certain hysteresis, after the overvoltage protection, the grid voltage has to drop to 242-5=237V, U4