DC voltage stabilizing circuit and zero-crossing detection circuit in the indoor unit control circuit of Hisense KFR-25GW-06BP inverter air conditioner

Shown is the DC voltage stabilizing circuit and zero-crossing detection circuit in the indoor unit control circuit of Hisense
KFR-25GW/06BP ​​inverter air conditioner . The DC voltage regulator circuit is a circuit that generates 5v DC voltage to power the microprocessor. The circuit is
composed of a step-down transformer Tl, a bridge rectifier circuit and a three-terminal voltage regulator. After the AC 220 V input voltage is stepped down by the transformer Tl, the voltage-resistant AC signal is output, and then
converted into a pulse DC voltage through the bridge rectifier circuit (VD1-VD4). This voltage is then rectified by VD5, and filtered by C4 and C40 to form +12 V DC
regulated power supply. After the +12 V voltage is stabilized by the three-terminal voltage regulator U4 (7805), it is filtered by c5 and c27 to form a DC 5v power supply, which supplies power to the microprocessor and related circuits in
the control circuit .
    The zero-crossing detection circuit is composed of VT1 and peripheral components.
    The 100 Hz pulse signal output by the bridge rectifier is added to the base of the VT1 transistor, as shown in the figure. When the input signal is O, the transistor
VT1 is turned off, and its collector outputs a high level. When the VT1 base input voltage exceeds 0.7 V. VT1 is turned on and outputs a low level, so
a pulse signal will be formed around the zero point of the input signal, as shown in the figure. This pulse signal is synchronized with AC 50 Hz.