Frequency sensors measure the frequency of an electrical signal. Frequency sensors are used in aircraft and industrial applications where equipment is sensitive to changes in frequency or may be subject to damage if power line frequency fluctuates. Frequency sensors detect line frequency. Trip-point frequency sensors may have fixed or adjustable trip points. Trip-point frequency sensors continuously monitor the line frequency and when a specific frequency trip point is reached, the frequency sensor will switch relays to connect or disconnect the device from a power source or generator to avoid damage or sub-standard operations.
Another type of frequency sensor is the band pass type. Pass band frequency sensors can be either fixed or adjustable, but they are designed so that the circuit continuously monitors the input frequency and whether the input frequency is within the band pass – that is above the frequency that defines the bottom of the pass band yet below the frequency that defines the top of the pass band. If the input frequency drops below the bottom of the pass band or rises above the top of the pass band, the frequency sensor detects it, and switches the output relay so that the power source or generator is disconnect from the device to which power is being supplied. The pass band frequency sensor will stay that way until the input frequency once again falls within the upper and lower limits of the pass band.