First look at the principle of the diode volt-ampere characteristic curve
Forward characteristics of the first quadrant : When the forward voltage rises from zero, before 0.4V, the forward current of the diode is small. But starting from 0.7V, the current increases rapidly. Reverse characteristics of the second quadrant: when the reverse voltage reaches -40V, the reverse current, that is, the reverse leakage current, is almost zero. It shows that after the forward voltage of the diode is greater than 0.7V, its equivalent resistance is very small, which is the forward characteristic of the diode; the reverse characteristic of the diode is that the reverse resistance is large.
In Figure 1, the diode is in forward connection, and its tube voltage drop is 0.7V. So the voltage across resistor R is: What about the current flowing through resistor R? Look at Figure 2 again: the anodes of the two diodes are connected to 12V, so both diodes are connected in the forward direction. Therefore, the anode of the D1 diode should be 6+0.7V=6.7V, and the anode of the D2 diode should be 2+0.7=2.7V. So what is the output voltage Usr of the circuit? Assuming Usc=6.7V, then diode D2 will be in forward connection. And because the voltage drop of diode D2 is 0.7V, the anode of diode D2 will be forcibly pulled to 2.7V. As a result, the diode D1 will be in a reverse biased state, that is, the voltage of the cathode of D1 is higher than the voltage of the anode. Note: After D2 is turned on, the positive pole of D1 becomes 2.7V, while the negative pole of D1 is 6V, so D1 is reverse biased and turned off. That is, the output voltage Usc is forcibly clamped at 2.7V. Whichever voltage is low, the output voltage of the circuit is the low voltage plus 0.7V.
This picture is a set of circuits used to control the triggering of thyristors . As shown in the figure, we can see the clamp circuit formed by the positive AND gate . The three input terminals are the voltage measurement and control terminal, PID control and trigger pulse circuit respectively . The normal output of the voltage circuit of the measurement and control terminal is pulsating DC, and the duty ratio of the high level is relatively large; the PID control output is also a high level, and the trigger pulse outputs a positive and negative alternating high- level pulse. It can be seen that under normal circumstances, the output of the AND gate is determined by the trigger pulse. After all, the zero level is also a part of the pulse. It can be seen that the application of the clamp circuit is still very extensive. Let's talk about Zener diodes . The voltage circuit of the measurement and control terminal in the above figure:
Assuming that the primary voltage of the transformer is 380Vac and the secondary voltage is 24Vac, after bridge rectification , the average voltage is 0.9X24=21.6V, which belongs to pulsating DC. However, it cannot be calculated in this way in actual calculation, and the maximum value must be used for calculation.
We know that the Zener diode works in the reverse breakdown region, see the third quadrant of the first picture. Its curve characteristics are: the current changes greatly, but the voltage change is small, which is its voltage regulation principle. However, it should be noted that the diode is in reverse connection at this time, that is, the Zener diode works under the reverse voltage. Assuming that the Zener diode stable voltage in the above figure is 12V, and the maximum stable current is 25 mA. We first open the resistor R2 to calculate the value of R1. Therefore, the value of R1 is 820 ohms, the power is 0.51W, and the nominal value is 1W. What is the waveform across the Zener diode at this time? This is the green part in the lower part of the waveform graph. Here, the Zener diode plays the role of clipping the half-wave DC waveform. Now, we plug in R2, so the current through the Zener diode becomes smaller. However, as long as the current flowing through the zener diode is still within its stable current range, the voltage stabilizing effect of the zener diode can be maintained. Assuming that the minimum stable current of the Zener diode is 5 mA, the current flowing through R2 and R3 is 25-5=20 mA. Therefore, the value of R2+R3 is: In fact, we see that the sum of R2+R3 is not less than 600 ohms, so the actual value of R2+R3 will be greater than the calculated value. The specific value has nothing to do with our answer and is ignored here. We can see that the collector of transistor T1 also has a Zener diode D2, which is also used for clipping, so that the highest value of the pulse amplitude output to the rear stage is equal to the stable voltage of the Zener diode.