1. Design purpose:
- Master the design method of air purifier';
- Master the ability to build LDO with discrete components;
- Consolidate relevant knowledge of transistors, amplifiers, hysteresis comparators, etc. in analog circuits.
2. Functions of air purifiers and how to implement them
- The working mode of the air purifier is divided into manual mode and automatic mode -> Manual: adjustable resistance (voltage signal); automatic: air quality sensor (voltage signal). The voltage signal is then amplified by an amplifier to control its operation.
- Three-position LED indication/buzzer prompt->Use a comparator to distinguish three states, and output signals to three LED lights and the same buzzer.
- Speed adjustment -> generate a triangular wave by building a hysteresis comparator, use an adjustable resistor (manual) or air quality sensor (automatic) to cut the triangular wave, and then use the comparator to generate a PWM wave, and use a push-pull circuit to increase the driving capacity and control the MOS switch to regulate the speed of the motor.
3. Analysis of schematic diagram of each part
1. Build LDO with discrete components
Input 15V power supply to realize 5V voltage supply through triode emitter follower and voltage regulator circuit.
Principle introduction:
①. The Q4NMOS tube realizes the anti-reverse connection function, and the NMOS model can be selected according to the specific power consumption of the subsequent circuit;
②. R5 is used to limit the current while ensuring the normal operation of D3 (generally above 2mA), and the normal conduction of Q3 (1~3mA) amplification;
③, C7 is used for filtering;
④. The voltage on D3 is stabilized at about 5.6V. According to the transistor design following principle (voltage drop is about 0.7V), Vce is about 5V. The current amplification of Ice is generally more than 100 times, which means that the voltage output can be about 5V and about 200mA. of current.
Note: This circuit is not easy to be used in a circuit where the voltage difference is too large and the power consumption of the subsequent stage is too large. Otherwise, it will easily heat up, burn out, and waste power consumption. It is recommended that the voltage difference be below 10V.
2. Automatic and manual mode part
The voltage signal is generated through an adjustable resistor or air quality sensor and amplified by an amplifier to drive the motor, LED indication and buzzer sound.
Principle introduction:
①. The voltage sampled by the air quality sensor is relatively small and needs to be amplified by an operational amplifier and then input to the subsequent circuit;
②. Here we use an adjustable resistor voltage divider to demonstrate;
③. The magnification is adjustable, calculated by the following formula:
④. It is recommended on the Internet to form a voltage follower on the unused channel of the amplifier to reduce interference.
3. Air status switching prompt sound
When going from mode 1 to mode 2/2 to 3/3 to 2/2 to 1, there will be a sound prompt, and the prompt sounds are the same length. The principle is that the charging time is equal to the discharging time.
Principle analysis:
① When switching from good to medium air quality level, B1 is high level (normally low), charging C6 through R13 current limiting, and preventing interference between levels through D2 single-phase conductivity, reaching the Q5 threshold After the voltage is applied, Q5 turns on and the buzzer sounds. The sounding time is determined by R13 and C6 (R*C) until C6 is fully charged (the same as the voltage of B1);
②. When switching from medium to poor air quality level, B2 is the same as ①;
③. When switching from poor to medium air quality, C8 is discharged through R21 and Q5 is turned on until it reaches the threshold voltage and stops beeping. Ensure that the charging resistance and discharge resistance are consistent, and the beeping time is basically the same;
④. When switching from medium to good air quality level, C6 is the same as ③.
4. PWM speed regulation based on air quality
Use a hysteresis comparator to generate a triangular wave, use an adjustable resistor (manual) or an air quality sensor (automatic) to cut the triangular wave, and then use a comparator to generate a PWM wave (frequency around 10K). A push-pull circuit is used to increase the driving capacity and control the MOS. switch to regulate the speed of the motor. (Pay attention to the switching loss of the MOS tube, GS waveform)
Principle analysis:
②. Generate two threshold voltages 5V and 9V through the hysteresis comparator, charge C3 through R27 and R30 in series, discharge it through R30 after reaching the threshold value of 9V, put it to 5V, charge it to 9V, and then put it to 5V. ...and the cycle goes on until the game is over, thus generating a triangle wave oscillating back and forth between 5V and 9V on C3. The frequency is determined by C3 and R30 (RC);
③. Then an adjustable resistor voltage divider circuit generates an adjustable DC voltage, which is tangent to the triangle wave to generate a PWM wave with an adjustable duty cycle;
④. Q9 and Q12 form a push-pull circuit to increase the driving current of the MOS tube;
⑤. To reduce the switching loss of the MOS tube, there are two ways: one is to increase the gate voltage (which cannot exceed its limit), and the other is to increase the gate drive current while also preventing oscillation.
京公网安备 11010802033920号
15KP170CACOX.160
