This paper designs a single-phase parallel anti lightning surge circuit based on varistor and ceramic gas discharge tube, and applies it to the switching power supply of instrument. The whole circuit includes lightning protection circuit and switching power supply circuit, wherein the lightning protection circuit is composed of three varistors and a ceramic gas discharge tube, and the common mode and differential mode are fully protected. The power supply circuit of the lightning protection instrument is composed of the classic switching power supply circuit. The varistor is used in parallel to extend the service life. After the short-circuit failure of the varistor, it is separated from the switching power supply circuit and will not cause fire.

    In order to achieve the above objectives, the design scheme adopted is to apply the single-phase parallel lightning surge resistance circuit of varistor and ceramic gas discharge tube to the power supply of the instrument. It is mainly divided into lightning protection circuit part and switching power supply circuit part. The circuit is simple. It adopts compound symmetrical circuit, common mode and differential mode full protection, and can be connected without L and n terminals. The varistor rv1 is located at the front end of the patch rectifier module and connected in parallel with the power supply L and N respectively, mainly to clamp the voltage between the L and N lines. The varistors rv0 and RV2 are connected in series with the ceramic gas discharge tube FD1 and grounded. The series connection of rv0 and FD1 is mainly to discharge the lightning surge current induced on the L line. The series connection of RV2 and FD1 is mainly to discharge the energy on the 24V reference potential connected by the signal port. After the short-circuit failure of rv0 and RV2, FD1 can separate them from the power supply circuit, No fire will occur. A wire wound resistor is connected in series on the front-end line of rv1. When the rv1 short circuit fails, the wire wound resistor can act as a fuse to disconnect the short circuit circuit circuit. The varistor is a voltage clamp type protection device, and the selection of its clamp voltage point, i.e. the parameter of the varistor, is relatively important (the one with higher pressure-sensitive voltage and larger flux is safer, more durable and has a low failure rate); The overall size and packaging form are selected according to the requirements of current capacity. In this circuit, 561k-10d varistors and ceramic gas discharge tubes are connected in series to prolong the service life and ensure safety. The flow capacity of the ceramic gas discharge tube is selected according to the required flow capacity. The circuit adopts 3rm470l-7.5-l and the flow rate is 5000A. The wire wound resistor R1 serves to limit the current and divide the voltage; The chip mounted rectifier module is the front-end rectifier of the switching power supply circuit, C1 is the high-voltage filter capacitor, Y1 is the decoupling capacitor, the resistor R2, the capacitor C2 and VD2 constitute the absorption clamp circuit of the MOS tube of the switching power supply chip, and the protection chip. The switching power supply chip adopts the tny27 series of PI company, Tr1 is the high-frequency transformer, VD3 and C3 constitute the secondary filter of the high-frequency transformer, U2, VD4, R3, R4 and R5 constitute the feedback circuit of the switching power supply circuit, The secondary output voltage of the transformer can be stabilized at the design value, and the lightning protection and anti surge circuit has achieved good results in practical use.