AP 3302 Pt. 3
Section 2
CHAPTER 3
Square Waves applied to LR and LC circuits
We can now consider what happens in the circuit when the valve is switched on. The resistance of the conducting valve is effectively in parallel with the tuned circuit and causes these oscillations to die away fairly rapidly, giving the waveforms shown in figure 5.
In some circuits it is necessary to protect a transistor from high voltage rings as the circuit is switched off. This is often done by connecting a diode across the tuned circuit with its cathode connected to the HT+ line. As soon as the anode rises above HT+, the diode conducts and introduces losses in the tuned circuit. This prevents very high voltages being developed and causes the ringing to die away extremely quickly.
