Demonstration device for capacitor charging and discharging process

This capacitor charge and discharge curve demonstrator uses a time-base integrated circuit 555 and a small number of peripheral components to form an oscillator with a duty cycle of about 50%. By controlling the rapid conduction and cutoff of the triode, the capacitor is continuously and rapidly released or replenished. With the help of an oscilloscope, a curve of the capacitor charging or discharging through the resistor can be clearly seen on the screen, thus achieving a vivid and intuitive teaching, which is easy for students to understand and master. When demonstrating the capacitor charging curve, an NPN triode is used to quickly release the capacitor after each RC charging of the capacitor, so that the RC charging can be performed again. When demonstrating the capacitor discharge curve, a PNP triode is used to quickly replenish the capacitor after each RC discharge of the capacitor, so that the RC discharge can be performed again. Since the mushroom frequency of the charging curve or discharge curve sent to the oscilloscope is relatively fast, this curve can be clearly seen. The functions are as follows:

1. Visually demonstrate the law of voltage change on the capacitor when the capacitor is charged;

2. Visually demonstrate the capacitor charging circuit, and the change of the current-seeking curve when the resistance value or capacitance value changes;

3. Shang Guan demonstrated the law of voltage change on the capacitor when the capacitor is discharged;

4. Visually demonstrate the capacitor discharge circuit. When the resistance value or capacitance value changes, the discharge curve changes. The cleverness of this teaching aid circuit is that it only uses one time-based integrated circuit and two triodes. Through the conversion of a double-pole double-throw switch, it can realize the display of the charging and discharging curves: at the same time, through two single-pole double-throw button switches, two resistors with different resistance values ​​and two capacitors with different capacities are selected to visually demonstrate the charging and discharging curves with the change of resistance value and capacity; the light-emitting diode in the circuit is connected to the output end of the time-based integrated circuit, which not only serves as a power indicator, but also serves as a display of whether the time-based integrated circuit is working normally. The entire teaching aid has the characteristics of simple structure, stable performance, low cost, easy material acquisition, convenient use, easy operation, safety and reliability.

See the circuit schematic diagram below. In the circuit diagram, diode D is connected in parallel to both ends of resistor R2. The resistance values ​​of R1 and R2 are equal, so the oscillation ratio of the time base integrated circuit is about 50%. K1 is the switch with the repetitive speed adjustment potentiometer W, which controls the power supply. K2 is the charge and discharge state conversion switch. K3 is a switch for selecting different resistance values. K4 is a switch for selecting different capacities.

1. Materials　　

1 plexiglass outer box of VHS videotape, 1 red and 1 black terminal; 1 double-pole double-throw switch: K2, for switching between charging and discharging states; 2 single-pole double-throw toggle switches: K3 and K4, for selecting different resistance values ​​and different capacities; 1 14.5cm×9.3cm circuit board; 1 555 time-base integrated circuit (IC in Figure 1); 1 470k potentiometer with switch (W in Figure 1, for adjusting repetition speed); φ5 luminous. 1 transistor, to show whether the oscillation circuit is working properly, 1 9012 or A1015 transistor (V1 in Figure 1, to quickly replenish the capacitor when demonstrating the discharge curve); 1 9013 transistor (V2 in the figure, to quickly release the capacitor when demonstrating the charging curve), 1 1N4148 diode (D in the figure, to make the oscillation duty cycle about 50%); 2 low-power 100K resistors: R1, R2; 1 low-power 2.2k resistor: R3; 1 low-power 6.2k resistor: R4; 1 low-power 10k and 33k charging and discharging resistors: R5 and R6; 1 68n oscillation capacitor: Cl; 100μ/16V electrolytic capacitor: C2; 100n and 220n charging and discharging capacitors: C3 and C4; several red and black connecting wires; 1 plexiglass cover for the tape box; 2 copper electrode sheets; 1 9V laminated battery.

2. Production Method

1. Solder the electronic components to the circuit board L as shown in the figure above.

2. Drill holes on the plexiglass case of the videotape to mount a potentiometer, a light-emitting diode, two toggle switches, two wiring rods, and a charge-discharge switch.

3. Solder the red and black connecting wires to the two electrode copper sheets.

4. Saw off a 3cm×5cm piece of the tape plexiglass box cover, and glue it and the two electrode copper sheets to the lower right corner of the tape plexiglass box to make a laminated battery box for installing 9V laminated batteries.

5. Fix the entire circuit board to the video tape plexiglass box panel with nuts and washers through a potentiometer and two toggle switches, and extend the LED into the hole of the panel.

6. Solder the red connecting wire of the battery box to one leg of the switch and connect the other leg of the switch to the positive pole of the power supply on the circuit board.

7. Solder the black connecting wire of the battery box to the negative terminal of the power supply on the circuit board, which is the ground terminal.

8. Spray or paste "repetition speed", "charge", "discharge", "output", "⊥", "10K", "33K", "1OOn", "220n" and other words and graphics on the demonstrator panel.
　　
III. Usage
　　
1. Choose an oscilloscope with a relatively large fluorescent screen, such as the J2458 teaching oscilloscope.　　

2. Install a 9V laminated battery in the capacitor charge and discharge curve player.

3. Use the signal line to connect the red terminal on the demonstrator panel to the "Y input" terminal of the oscilloscope, and the black terminal to the "ground" terminal of the oscilloscope's Y axis. At the same time, set the oscilloscope's "Y axis attenuation" to "100" and the Y axis AC/DC input selection to "DC".

4. Adjust the X-axis "Scan Range" on the oscilloscope to "10-100" and turn the "Scan Fine Adjustment" counterclockwise to the minimum.

5. Turn on the oscilloscope power supply, and adjust the "X-axis gain" and X-axis shift knob of the oscilloscope so that the bright spot sweeps the entire screen in the left and right directions.

6. First, demonstrate the capacitor charging curve and turn the charge and discharge selection switch on the demonstrator panel to the "charge" position.

7. Turn on the power of the demonstrator by turning the "repetition speed" potentiometer on the demonstrator clockwise. The repetition speed is at the slowest position when the power is just turned on. Adjust the "Y-axis gain" and Y-axis shift knob on the oscilloscope so that the charging curve is displayed between the highest and lowest scale lines on the screen.

8. Fine-tune the "scan fine adjustment" of the X-axis on the oscilloscope and the "repeat speed" potentiometer on the demonstrator to make the curve stable and only one complete curve waveform appears.

9. Toggle the two buttons on the demonstrator to change the resistance of the charging resistor and the capacity of the charging capacitor, and observe the changing pattern of the charging curve.

10. Demonstrate the capacitor discharge curve again and turn the charge and discharge selection switch on the demonstrator panel to the "discharge" position.

11. Similarly, flip the two button switches on the demonstrator to change the discharge resistance and the discharge capacitance respectively, and observe the change pattern of the discharge curve.

12. Similarly, fine-tune the "scan fine adjustment" of the X-axis on the oscilloscope and the "repeat speed" potentiometer on the demonstrator to make the curve stable and only one complete curve waveform appears.

13. When the resistance value of the charge and discharge resistor and the capacity of the capacitor change, the "repetition speed" of the demonstrator and the X-axis scanning speed of the oscilloscope should also be changed appropriately. When the resistance value of the charge and discharge resistor and the capacity of the charge and discharge capacitor are small, the "repetition speed" of the demonstrator can be increased. Otherwise, the "repetition speed" of the demonstrator should be decreased.

14. To stabilize the displayed charge and discharge curve, after increasing the "repetition speed" on the demonstrator, the scanning speed of the X-axis on the oscilloscope should also be increased. You can adjust the "scanning fine-tuning" of the X-axis clockwise.

15. In order to make the displayed charge and discharge curve flicker less obvious, the repetition speed of the demonstrator should not be set too slow, so the additional value and capacity value used for charge and discharge should not be too large.