Crowbar circuits are known in the art and are typically employed for protecting an electrical device from damage by high currents resulting from fault conditions. For example, an inductive output tube (IOT) is frequently employed in UHF television transmitters. Such an IOT is connected to a high voltage (HV) power supply and may suffer adverse damage from a sudden high current resulting from internal tube arcing. Under such conditions, an unprotected IOT will draw excessive current from the HV power supply causing possible damage to the tube.
A crowbar circuit serves to detect a sudden rise in current drawn from the HV supply due to fault conditions. This will cause an electronic switching device, such as a deuterium thyratron, to be turned on and it serves to direct the fault current from the supply away from the IOT to prevent damage. When the crowbar switching device, thyratron, is turned on it informs an amplifier controller and the controller causes a circuit breaker to open disconnecting the HV power supply from its AC line voltage source.
The thyratron is connected directly across the HV supply and, hence, when it is turned on it provides essentially a short circuit across the IOT. An electrode, such as the anode, of the thyratron is connected to earth ground. In order to determine whether the crowbar circuit is operative, a fuse wire may be connected between the cathode of the thyratron and a point that may be shorted to ground quickly.
A crowbar circuit verification device known in the prior art is illustrated in FIG. 1 and it includes a fuse wire together with a vacuum switch, which, when closed, provides a short circuit around the thyratron to direct fault current to ground. The fuse wire together with the vacuum switch are disclosed in detail in FIG. 2 which will be described in greater detail hereinbelow. This vacuum shorting switch of the prior art has several shortcomings including the fact that it requires an external power source to operate the switch. Additionally, this form of a shorting switch cannot normally be installed inside a typical HV compartment because it requires transmitter interlocks to be defeated. The shorting switch does not provide positive provision for connecting the fuse wire. The switch is bulky and difficult to transport and is relatively expensive.