Triode tube breakdown

Crystal oscillator

Mobile phone crystal 3.2*2.5mm 3225 26M (26.000MHZ) 7.5PF 10PPM 20PPM 30PPM

Single chip microcomputer STM32L151CCU6

When the base of a transistor is open, and the voltage between the collector and emitter increases to a certain level, a breakdown phenomenon occurs as shown in Figure 1. The current (IC) rises rapidly, and once it reaches point A, the voltage drop across the transistor suddenly decreases, while the current spikes. The region from A to B is known as secondary breakdown, with a transition time on the order of milliseconds. During this phase, the transistor exhibits negative resistance characteristics, making it suitable for use in a relaxation oscillator.

Refer to Figure 2. A 220V AC input is first stepped down using resistor Rl, then rectified by diode D, and filtered by capacitor Cl to produce a stable DC power supply. This power source charges capacitor C2 through resistor R2. As the voltage across C2 increases, it eventually reaches the breakdown voltage of transistor V. At this point, the CE junction of V breaks down, significantly reducing its resistance. C2 then discharges quickly through the speaker and V. Once C2 is fully discharged, V turns off, and the power supply begins recharging C2 via R2. Due to the short conduction time of V, its power consumption remains within acceptable limits, preventing damage. When C2 is charged again, V turns on, initiating an oscillation that drives the speaker to emit sound.

The circuit in Figure 3 is similar to Figure 2, but the transistor is connected differently. In this case, the transistor used is 3DG12. According to the configuration in Figure 2, the breakdown voltage is 30V, while in Figure 3, it is only 7V. Therefore, when the resistor and capacitor values are the same, the oscillation frequency in Figure 3 is higher than that in Figure 2. When the power plug is removed, the charge stored in C1 continues to provide DC power to the oscillator. As the charge in C1 decreases, the oscillator's frequency gradually drops. This makes the circuit ideal for use as a hypnosis device. It is recommended to plug in the power before use and then unplug it for operation, ensuring safety. By selecting appropriate values for R2 and C2, the speaker can produce a sound resembling rain that starts dense and gradually fades, creating a calming effect. The capacity of C1 determines the working time of the hypnosis device, and increasing the time can be achieved by connecting multiple capacitors in parallel. For example, with C1 at 22μF, the device can operate for about eight minutes.

Component selection includes: R1 = 2.2KΩ, R2 = 1MΩ, D = 1N4007, C1 = 22μF/400V, C2 = 0.33μF/160V, V = 3DG12, and a speaker with 8Ω impedance. The withstand voltage of C2 needs to be greater than the breakdown voltage of V. The transistor V can also be replaced with other small-power transistors such as 9013, 9014, 9018, or 3DG series, though their breakdown voltages may vary slightly.

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