How to suppress interference during the switching of measurement range in an electrical force measurement switch (PMS)?

When switching measurement ranges in a PMS (such as switching current transformer taps or voltage divider resistors via relays), the mechanical action of the relay and the bounce of its contacts can generate transient interference. Simultaneously, the change in the signal path caused by the switching may introduce charge injection, affecting the stability of the metering chip. Suppression requires a simultaneous approach involving relay selection, drive circuitry, and software. Sealed relays with magnetic blowout or permanent magnet arc suppression are preferred, or solid-state relays (SSRs) should be used. A freewheeling diode or TVS diode (such as SMBJ24CA) should be connected in reverse parallel across the relay coil. An RC snubber circuit (such as 0.1μF+10Ω) should be connected in parallel across the contacts.

Twisted-pair or shielded cables should be used for the signal lines driving the relays. On the PCB layout, the relay should be kept away from the metering chip, and the switching signal line should be routed perpendicularly to the sampling signal line. At the software level, after the range switching command is issued, metering sampling should be delayed by 10-20ms to avoid the interference period. For high-precision metering, synchronous switching technology can be used, i.e., range switching should be performed when the AC voltage crosses zero, at which point the current and interference are minimal. These measures can suppress the fluctuation of the measurement reading caused by range switching from 10% of the full scale to less than 0.5%, and quickly restore stability.