1. Introduction                                                                                                                     

In school life and study, we often encounter some problems. For example, when we turn off the lights at night, the light switch is at the door. We run over to turn off the lights and then go to bed in the dark. We often kick chairs or mix things with them, or climb up ladders and step on them. Empty, this caused some trouble for us. It would be much more convenient to turn on the desk lamp, but the desk lamp also needs to be turned off manually, which is very inconvenient.

Making a delay-off switch to transform a desk lamp can not only solve this problem, but also apply and practice the knowledge learned to improve knowledge mastery. It combines theory and application, exercises practical skills, and can be given as a gift to friends.

2. Learning objectives

(1) The use and selection of resistor-capacitor voltage reduction.

(2) Necessity and selection of bleeder resistor.

(3) The role and necessity of fuses.

(4) Use of two-way thyristor.

(5) Use of Zener diodes.

(6) The role of varistor on the circuit (anti-surge).

(7) 555 time base IC.

3. Learning content

(1) Resistor-capacitor voltage reduction

Introduction: Using resistance-capacitance step-down, we can easily build an AC-DC step-down circuit with a few components. It is worth noting that resistance-capacitance step-down is a non-isolated power supply! Pay special attention to isolation during use to avoid electric shock.

When the alternating current is rectified but not stabilized after passing through the resistor-capacitor step-down circuit, the DC voltage is generally higher than 30V, and a voltage regulator needs to be used to stabilize the voltage.

Due to the large internal resistance of this type of power supply, the resistance-capacitance voltage reduction is only suitable for small current loads and is not suitable for large current power supply.

When the resistor-capacitor step-down power supply provides current to the load, large voltage fluctuations will occur as the load current changes, and a Zener voltage regulator is required to stabilize the voltage.

principle:

       We all know that capacitors have the property of blocking DC and AC. When connected in series in a DC circuit, no current will flow through the capacitor (unless it is broken down).
　　When the capacitor is connected to the DC circuit, there will be a short charging process. When the positive and negative plates are fully charged (when the voltage at both ends of the capacitor is equal to the power supply voltage), no current will flow anymore, so the capacitor is said to isolate the DC current. .
　　When the capacitor is connected to the AC circuit, there is also a charging process. Charging stops when the capacitor voltage is equal to the power supply voltage. Then the power supply voltage drops and the capacitor begins to discharge. When the supply voltage rises in the reverse direction, the capacitor is charged in the reverse direction and then discharged in the reverse direction. In this way, charging and discharging are the same as AC current actually flowing, so the capacitor can pass AC current.

        However, capacitors will produce capacitive reactance at a certain AC frequency . Capacitive reactance limits the flow of current.

For example, under the AC power supply of 220v/50Hz, the capacitive reactance generated by a 1μF microfarad capacitor is about 3180Ω ohms, and the maximum current flowing is about 70mA. Although it flows through the capacitor, no power consumption is generated, because if the capacitor is an ideal capacitor, the current flowing through the capacitor is the imaginary part current and does no work. When the operating current of the electrical appliance matches its current, it can work normally without burning out. Therefore, using a capacitor to reduce voltage actually uses capacitive reactance to limit the current, and the capacitor actually limits the current and the voltage across the load.

Selection:

1. The selection of capacitor is very important, because it is a constant current power supply. If the capacitor is too large, excess current exceeding the load will flow out through the Zener regulator tube. If the outflow current is too large, the Zener tube will be burned. Accurately calculate the required capacitance size based on the load current.

The calculation formula is as follows: set at 220v 50Hz, 1μF capacitor.

Capacitive reactance:

Current: Ic = U / Xc = 220V/ 3184Ω = 0.069A = 69mA

Usually, the relationship between the capacity C of the buck capacitor C1 and the load current Io can be approximated as: C=14.5 I (at 50Hz), where the capacity unit of C is μF and the unit of Io is A.

It can be seen that the current flowing through the capacitor is 69mA. However, the half-wave rectification we use has only 0.44 times the current after rectification, that is, 69*0.44≈30mA.

Therefore, under half-wave rectification, the relationship between capacitance size and load current is:

I(AV)=0.44*U/Xc=0.44*U*(2 πf C)=0.44*220V*2*3.14*50Hz*0.001F=30.39mA
The capacitor used in this design is 0.47μF, so the load current is 0.47 /14.5*0.44=0.014A=14mA mA.

Note: Using full-wave rectification will double the load current, but it is not recommended. Although the current of full-wave rectification is slightly larger, the stability and safety are worse than the half-wave rectification type because of the floating ground, so it is used less.
2. To ensure that the capacitor can work normally, the withstand voltage value should be greater than twice the access power supply voltage. For example, a capacitor used on 220V AC requires a withstand voltage of 400V.

Precautions:

1. It is not isolated from 220V AC high voltage, please pay attention to safety and be careful to prevent electric shock!  
2. The current-limiting capacitor must be connected to the live wire, the withstand voltage must be large enough (greater than 400V), and a series of anti-surge and insurance resistors and parallel discharge resistors must be added.  
3. Pay attention to the power consumption of the zener tube, and it is strictly forbidden to disconnect the zener tube.

(2) Bleeder resistor

The resistors connected in parallel at both ends of the energy storage element provide a path for the energy storage element to consume energy, making the circuit safe. This resistor is called a bleeder resistor.

Due to the use of resistor-capacitor voltage reduction, when the power is unplugged, the charge stored on the capacitor will be discharged through the discharge resistor. Generally, it can be discharged in a few seconds (during maintenance, a discharge rod must be used to discharge according to the procedure), and it will not Maintenance personnel may suffer electric shock or damage detection instruments .

A bleeder resistor R1 with a resistance of 470kΩ is connected in parallel with the capacitor C1 to release the charge.

The choice of bleeder resistor is:

Discharge resistance (KΩ) = 500/capacitance (uf). A larger resistance will increase the discharge time. I chose a smaller one.

(3) Fuse

Fuses are also called current fuses , which mainly serve as overload protection.

When a circuit fault or abnormality occurs , the current continues to increase, and the increased current may damage some important components in the circuit, burn the circuit, or even cause a fire. If the fuse is correctly placed in the circuit, the fuse will blow itself and cut off the current when the current abnormally rises to a certain height and temperature, thereby protecting the safe operation of the circuit.

Since we use 220V mains power, for safety reasons, we avoid short circuits that may cause fires or electric shocks . Install a fuse for safety (from the heart/dog head)

Since the triac is 1A, a 2A 250AC fuse tube was selected.

And because the resistance and capacitance reduce the voltage, the specification of the current fuse should be greater than 1.25A. If it is too low, it will easily burn out during the surge test, and it will also burn out during production.

(4) Bidirectional thyristor

A triac is a silicon controlled rectifier device, also known as a triac. This kind of device can realize contactless control of alternating current in the circuit, control large current with small current, and has the advantages of no spark, fast action, long life, high reliability and simplified circuit structure. Therefore, it is widely used in electronic circuits such as speed regulation, light dimming, voltage regulation, temperature regulation, and automatic overload protection of various electrical appliances.

Although the thyristor has no contacts, the insufficient design of the trigger circuit will cause interference and affect the normal operation of other electrical appliances. Although the thyristor can carry large currents, its overload capability is poor, and it is easy to break down and damage if it is slightly overloaded. Therefore, it is impossible for thyristors to completely replace relays. Triacs are mostly used in workplaces with frequent movements and special needs. They are suitable for fixed loads. The price of high-power triacs is higher than that of relays.

Triacs are used as electronic switches, and trigger signals are used to control the on and off of electrical appliances. Relays are generally used by everyone, and additional power is required for the relays to operate normally. Triacs can be used directly, but it should be noted that Never exceed the load of the thyristor, otherwise it will be easily damaged.

What we use here is a 4A/600V triac. As long as it does not exceed 4A, it can be replaced with a larger model.

Triac graphics:

(5) Zener diode

         Zener diode, also called Zener diode . A diode with a voltage stabilizing function is made by taking advantage of the reverse breakdown state of the PN junction, in which the current can change within a wide range while the voltage remains basically unchanged.

principle

　　The forward characteristics of the volt-ampere characteristic curve of the Zener diode are similar to those of ordinary diodes. The reverse characteristics are that when the reverse voltage is lower than the reverse breakdown voltage, the reverse resistance is very large and the reverse leakage current is extremely small. However, when the reverse voltage approaches the critical value of the reverse voltage, the reverse current suddenly increases, which is called breakdown. At this critical breakdown point, the reverse resistance suddenly drops to a very small value. Although the current changes within a wide range, the voltage across the diode is basically stable near the breakdown voltage, thereby achieving the voltage stabilizing function of the diode.

Unlike other diodes, when using a Zener diode you need to connect it in reverse, with the arrow pointing towards the positive terminal, for example:

(6) Varistor

"Varistor" is a resistive device with nonlinear volt-ampere characteristics. It is mainly used to clamp the voltage when the circuit is subjected to overvoltage and absorb excess current to protect sensitive devices.

The varistor is mainly used to discharge surges. When the input voltage suddenly increases and exceeds the varistor threshold value, the varistor is short-circuited, sacrificing itself to protect the subsequent circuit. The high current caused by the short-circuit of the varistor will blow the fuse. Further protects the circuit and provides safety. Therefore, the varistor is generally placed together with the fuse:

Selection:

The so-called varistor voltage is the breakdown voltage or threshold voltage. Refers to the voltage value under a specified current. In most cases, the voltage value is measured when 1mA DC current is passed into the varistor. The varistor voltage range of its products can range from 10-9000V. The correct choice can be made according to specific needs. Generally V1mA=1.5Vp=2.2VAC, where Vp is the peak value of the rated voltage of the circuit. VAC is the effective value of the rated AC voltage. The selection of the voltage value of the varistor is crucial, as it is related to the protection effect and service life. For example, the rated power supply voltage of an electrical appliance is 220V, then the voltage value of the varistor is V1mA= 1.5Vp=1.5×1.414×220V=476V, V1mA=2.2VAC=2.2×220V=484V, so the breakdown voltage of the varistor can be Choose between 470-480V. Therefore, the varistor selected in the picture is 07D471K, where 07D is the package: 07 represents the size, the diameter of the varistor chip is 7mm, and d represents the round shape of the valve piece. 471 is the nominal voltage, and the calculation method is like resistance: 471=47*10^1=470V voltage, K refers to the error of 10%.

(7) 555 time base IC

The 555 time base circuit is a combination integrated circuit that cleverly combines analog functions and logic functions on the same silicon chip. It has novel design, ingenious conception and wide range of uses. It is favored by professional electronic designers and electronic enthusiasts. People nickname it as a great little IC.

This design uses its monostable circuit for delayed triggering:

principle

        The characteristic of a monostable trigger is that the circuit has a stable state and a temporary stable state . Under the action of the trigger signal, the circuit will flip from the steady state to the temporary stable state. The temporary stable state is a state that cannot be maintained for a long time. Due to the role of the RC delay link in the circuit, the circuit will automatically return to the steady state after a period of time. And get a rectangular wave with pulse width tw at the output end. In a monostable flip-flop, the output pulse width tw is the temporary stable state maintenance time, and its length depends on the parameter value of the circuit.
        In steady state, the output Uo is low level, that is, when there is no trigger signal (Ui is high level), the circuit is in a stable state - outputting low level. Under the action of Ui negative pulse, the low-level trigger terminal gets lower than (1/3) Vcc, the trigger signal, the output Uo is high level, the discharge tube VT is cut off, the circuit enters a temporary stable state, and the timing starts. 
        During the transient steady state, the power supply +Vcc→R→C→ground charges the capacitor, the charging time constant T=RC, and Uc rises exponentially. When the voltage Uc across the capacitor rises to (2/3) Vcc, terminal 6 is high level, the output Uo becomes low level, the discharge tube VT is turned on, and the charging of the timing capacitor C ends, that is, the temporary steady state ends. The circuit returns to the steady state Uo is low level. When the second trigger pulse arrives, the above process is repeated.

By adjusting the values ​​of R and C, the duration of the transient steady state can be changed, and the duration is 1.1RC . However, due to the positive error of the capacitor and the influence of leakage, the actual value is generally greater than the calculated value.

Design explanation:

          When the delay switch is not used, simply press the boat-shaped switch to control the off and on of electrical appliances. The circuit changes when using a delay switch as follows.

          When the 220V AC power is connected, a DC voltage of about 12V is obtained at both ends of C2, which serves as an energy storage capacitor to power the 555 time base IC. When the IC is in a steady state, pin 3 is low level, and the triac is G-rated Low level, so it is turned off, and the electrical appliances are turned off. When you touch the touch spring (or other metal part) with your hand, the negative half cycle of the clutter signal induced by the human body is transmitted to pin 2 of the IC through R2, triggering the IC to flip into a transient state, and pin 3 becomes high level to trigger a bidirectional Silicon control, electrical power supply conduction. At this time, it enters the delay state, and capacitor C3 begins to charge through R1. When the level of pin 6 rises to 2/3VCC, the transient state ends, the IC flips to a steady state, and pin 3 becomes low level, causing the thyristor to turn off. Outage, electrical appliances are out of power. Each touch delays approximately 150 seconds.

Production process

Finished product