Three times and five times three-stage tube into the wood three points to talk. Teacher Zhang Fei takes you to answer questions.
**Question one:**
In the push-pull output driver circuit, the BE terminal of the transistor can withstand a maximum reverse voltage of Veb = 6V (NPN). When the switching signal is low, the gate voltage is 12V during discharge, and the voltage drop is greater than 6V. Isn't it going to burn out the transistor?
**Answer:** From a process perspective, the rising and falling edges of the PWM signal take time, and the diode between the BE terminals has a 0.7V voltage drop clamp function. During normal operation, when driving the PWM signal from 0V to 12V, the NPN transistor will turn on as soon as the voltage exceeds 0.7V, causing the emitter to follow. Meanwhile, the PNP transistor is fully turned off due to the negative voltage of -0.7V. When driving the PWM signal from 12V to 0V, once the voltage drop across the PNP transistor reaches 0.7V, conduction begins. As the base-emitter voltage of the NPN transistor becomes negative at -0.7V, it is fully cut off without exceeding the 6V limit, ensuring safety.
**Question two:**
What is your understanding of the overvoltage protection circuit (partial circuit) shown in the image below?
**Answer:** Assuming the normal output voltage is 12V, when an overvoltage occurs, the Zener diode allows current to flow through it, turning on the transistor, which pulls the output low to shut down the PWM signal. The overvoltage threshold is primarily determined by the Zener diode. Typically, the Zener diode requires about 2mA of current to regulate voltage, allowing adjustments by changing the Zener diode. Resistors R20 and R21 perform voltage division, affecting the threshold voltage within a certain range. Additionally, R20 serves as a current limiter, while capacitor C9 absorbs peak voltages, stabilizes the voltage, and introduces a slight delay.
**Question three:**
When a transistor operates in the switching state, what is the purpose of the pull-up resistor and pull-down resistor?
**Answer:** First, they prevent high-impedance states, which could disrupt the normal operation of the circuit and lead to safety hazards. Second, they provide a discharge path for the transistor's interelectrode capacitances. Third, they serve as bleed circuits for static electricity and lightning, protecting the device. Based on engineering experience, a resistance value of around 2K is generally appropriate.
**Question four:**
As shown in Figure 4, how can a small microcontroller signal control large signals?
**Answer:** Since Q1 is connected to a high voltage of 50V, directly controlling it would require a high driving voltage that a microcontroller cannot provide. Instead, we can introduce a switch on the Q1 control terminal, acting like a large resistance. When the switch is open, the resistance approaches infinity, meaning the drive terminal voltage is 50V, leaving Q1 open. When the switch is closed, the resistance is nearly zero, allowing Q1 to turn on. This setup enables the desired functionality. The R3 resistor limits current.
**Question five:**
Regarding the Zener diode, if it does not reach its regulation voltage (e.g., 5.6V), is there no current flowing?
**Answer:** For semiconductor devices, there is always some leakage current, though it’s usually negligible. Analyzing the circuit in Figure 5, C1 is a bootstrap capacitor designed to charge to 12V using R1 from a 30V input, then power the subsequent circuitry. The Zener diode ensures no current flows until the voltage reaches 11V. However, this theoretical design is impractical since the transistor operates in an amplification state, resulting in a large voltage drop across the CE junction (approximately 12V), leading to significant losses. Additionally, R3 in the circuit can be omitted for simplicity.
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Servo Moto Voltage Stabilizer
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