Pulse Train Generator Based on Bidirectional Trigger Diode

1 Working principle of pulse sequence generator of voltage successive comparison method
The working principle waveform of pulse sequence generator is shown in Figure 1. A sawtooth wave is generated by a single capacitor, and then a pulse is generated by comparison method at different voltages of the sawtooth wave. Therefore, as long as the comparison voltage is stable, the number of output pulses remains unchanged. It can be seen from Figure 1 that when the capacitance becomes larger or smaller, only the pulse interval changes, while the number of pulses remains unchanged.

2 Pulse sequence generator with single capacitor timing The
pulse sequence generator block diagram is shown in Figure 2, which is mainly composed of a timing capacitor, a holding capacitor, two bidirectional trigger diodes, a constant current source, etc. The specific circuit is shown in Figure 4.

2.1 Bidirectional trigger diode working characteristics
The working characteristic curve of the bidirectional trigger diode is shown in Figure 3. After being triggered at VB0, the internal resistance decreases rapidly, and it automatically disconnects if the current is maintained <10 mA. The VB0 of the bidirectional trigger diode DB3 is 33 V, and the hysteresis voltage △V is 7 V. This characteristic can be used to form a pulse generator with automatic comparison point change.

2.2 Working principle of sawtooth wave generator
Figure 4 is the overall circuit diagram. Q2, R5, and C3 form a constant current charging type sawtooth wave generator. Therefore, C3 is a timing capacitor and requires high precision. C2, C4, and C5 are auxiliary capacitors without high precision requirements. C2 only requires low leakage and can use a small-capacity monolithic capacitor. The current of the constant current source charging C4 depends on formula (1)

R2, R3, R4 and R8 form a voltage divider network. The voltage divider ratio is changed by S1 and S2 to set different maximum values ​​of the sawtooth voltage, thereby changing the number of pulses within the range of 2 to 4. C4, Q4, R10 and Q5 form a maximum value controller of the sawtooth voltage. If the voltage at point D reaches 33 V, Q5 is triggered to turn on, the thyristor Q4 is turned on, and the voltages of C3 and C5 are quickly discharged to re-form the sawtooth wave.
[page]2.3 Working principle of the pulse generator with automatic comparison point change
C2, D1, Q3, R7 form a pulse generator with automatic comparison point change. At the beginning, the voltage of C2 is 0 V. Therefore, when the sawtooth voltage at point A reaches 33 V (comparison point 1), Q3 is triggered to turn on, and the thyristor Q1 that controls the strobe tube is turned on to realize a strobe. At this time, since C3 and C5 are much larger than C2 (experiments show that more than 10 times is sufficient), C2 is quickly charged with the hysteresis voltage of Q3, 7 V, and then Q3 is cut off. The sawtooth voltage at point A continues to rise. When the voltage reaches 33+7=40 V (comparison point 2), Q3 triggers C2 to quickly charge 7 V. At this time, the voltage of C2 is 7+7=14 V, and so on. Therefore, the voltages of comparison points 1, 2, 3, and 4 are 33 V, 40 V, 47 V, and 54 V, respectively. When the sawtooth voltage at point A reaches the set number of control voltages, Q4 turns on C3 through D2, and C2 quickly discharges through D1, restarting the next cycle.
Changing the voltage value of the voltage regulator D2 can change the starting point of the sawtooth wave, thereby changing the time T1 when the first pulse arrives.
2.3 Experimental results
Since the timing time is in the order of seconds and the trigger pulse time is in the order of microseconds, it is difficult to observe the trigger pulse, so the waveform at both ends of the strobe tube after being triggered is used to replace the waveform of the trigger pulse.
In Figure 5, the pulse number is set twice, the timing capacitor C3 in the upper figure is 4.7μF, and C3 in the lower figure is 4.7+2.2=6.9μF. The actual measured working waveform is when the change is about 50%.

In Figure 6, the pulse number is set to 4 times, and the actual measured operating waveform is when C3 in the upper figure is 4.7μF and C3 in the lower figure is 6.9μF.

It can be seen that when the capacity of capacitor C3 changes within the range of 50%, only the trigger pulse interval changes, while the set number of times remains unchanged. Moreover, due to the constant current charging method, the pulse interval time is almost the same when the trigger number is from 2 to 4. If the voltage divider ratio is changed, the number of pulses can be controlled within 1 to 8.

3 Conclusion
A pulse sequence is formed by using a capacitor and a bidirectional trigger diode and adopting the successive voltage comparison method. Since only one capacitor is used, various problems caused by the traditional pulse sequence generator using two capacitors for timing are overcome.
Experiments have proved that the single capacitor pulse sequence generator composed of a bidirectional trigger diode DB3 does not need to provide a low-voltage working power supply, and directly uses the 300 V high voltage for strobe, and the entire circuit works in a micro-power state.
Since the hysteresis voltage of DB3 is 7 V, the voltage difference of each comparison point is large, which is conducive to anti-interference.
When this circuit is used, even if the capacity of the timing capacitor C3 changes by 50%, the generator can accurately output the set pulse, but the time interval becomes larger, and C2, as a holding capacitor, has almost no effect on the timing time when its capacity changes by 100%. The general error of the capacitors used in production is ±lO%, so it is easy to meet the requirements of this circuit.