| RF Exciter Monitor
This circuit monitors RF voltage from a sample and either opens or closes a circuit if the voltage drops below a preset level. This is useful as a fail-safe system if you have a transmitter that could be damaged if the RF drive fails while transmitting.
Circuit Description:
An RF sample from the source to be monitored is fed to the input, reduced to a low level by R1, and rectified resulting in a DC sample voltage at "TP". The 2N2222 switching transistor is normally biased off by the 10k resistor between the base and emitter, so as long as there is enough sample voltage (about 5 volts or more), the transistor conducts. A 12V relay would be energized as long as the RF sample remained high enough. The LED in parallel with the coil indicates that the RF level is sufficient.
The relay de-energizes when the RF level drops below the level set by the 10k potentiometer. Choosing which relay contacts to use would depend on whether this circuit would interrupt the keying circuit (N.O.) or trigger an overload shutdown circuit (N.C.). Having the relay energized under normal operating conditions also provides a safety measure if the circuit loses power.
R1 is selected based on the RF source. This circuit can be used to monitor VFOs with as little as 200mW using low values for R1. The higher the RF level to be monitored, the higher the value of R1. The goal is to measure about 5 volts at "TP" with the input control set midway.
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