First look at the forward characteristics of the first quadrant. When the voltage rises from zero, the current of the diode is very small before 0.4V. However, starting from 0.7V, the current increases rapidly.
Looking at the reverse characteristics of the second quadrant. When the voltage reaches -40V, the reverse current (the reverse leakage current) is almost zero. This shows that after the diode gets a forward voltage greater than 0.7V, its equivalent resistance is very small. This is the forward characteristic of the diode, and the reverse characteristic is that the reverse resistance is very large.
In Figure 2.
In Figure 2(a), the diode is in forward connection, and its tube voltage drop is 0.7V. Therefore, the voltage on the resistor R is:
Then the current flowing through R is:
In Figure 2(b), we can see that the anodes of the diodes are all connected to 12V, so both diodes belong to the forward connection. Therefore, the anode of the D1 diode should be 6+0.7V=6.7V, and the anode of the D2 diode should be 2+0.7V=2.7V. What is the output voltage Usr of the circuit?
Assuming Usc=6.7V, then the diode D2 will be in forward connection. The voltage drop of diode D2 is 0.7V, so the anode of D2 will be forced to 2.7V. In this way, D1 will be in a reverse biased state, that is, the negative voltage of D1 is higher than the positive voltage.
Note: After D2 is turned on, the anode of D1 will become 2.7V, and the cathode of D1 will be 6V, so D1 is reverse-biased and cuts off. In other words, the output voltage Usc is forcibly clamped at 2.7V. Whichever voltage is low, the output voltage of the circuit is the lowest voltage plus 0.7V.
Example: Positive Logic Gate Clamper Circuit
The figure shows a thyristor triggering circuit. In this circuit, there are a clamper circuit composed of a positive AND gate, three input terminals of the measurement and control terminal voltage, PID control and trigger pulse circuit.
The normal output of the measurement and control terminal voltage circuit is pulsating DC, and the high-level duty ratio is large, the PID control output is also high-level, and the triggering pulse outputs a high-level alternating positive and negative. It can be seen that under normal conditions, the output of the AND gate is determined by the trigger pulse, because 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
From the measurement and control terminal voltage circuit in the figure above.
0.9×24=21.6V, which belongs to pulsating DC. However, the maximum value must be used in the actual calculation.
We know that the Zener diode is in the reverse breakdown zone, the third quadrant of Figure 1. Its curve characteristic is: the current changes greatly, and the voltage changes very little, this is voltage regulation. But pay attention: the diode is in reverse connection at this time, that is, the Zener diode is working under reverse voltage.
In the figure, say the stable voltage of the Zener diode is 12V, and the maximum stable current is 25mAh. Calculate the value of R1 (R2 is in open circuit).
Therefore, the value of R1 is 820Ω, the power is 0.51W, and the nominal value is 1W.
What is the waveform at both ends of the Zener diode at this time? It is the green part at the bottom of the waveform graph. Here, the Zener diode plays a role of clipping (half-wave DC).
Now, connect R2, so the current flows through the Zener diode becomes smaller. But as long as it is within the stable current range, the voltage stabilization effect can still be effective.
Assuming that the minimum stable current of the Zener diode is 5mAh, the current flowing through R2 and R3 is 25-5=20mAh. Therefore, the value of R2+R3 is:
In fact, as long as the sum of R2+R3 is not less than 600Ω, its actual value will be greater than the calculated value.
We see that the collector of the transistor T1 also has a Zener diode D2, which is also used for clipping, so that the highest value of the pulse output to the subsequent stage is equal to the stable voltage of the Zener diode.