Design and implementation of power-off delay time relay

introduction

Time relay is a controller whose delay function is realized by electronic circuit. According to the control occasion, you can choose to use specifications such as: power-on delay type A; power-off delay type F; star-delta delay type Y; power-on delay type C with instantaneous output; interval delay type G; reciprocating delay type R; disconnection delay signal type K to meet the required control occasion.

In the above-mentioned delay type applications, power-off delay type relays are required for control in many occasions. For example, if you need to control a motor and require that the motor restarts after a delay of a period of time after pressing the stop button, then a power-off delay relay is required to achieve the above functions. The so-called power-off delay relay is that when the time relay coil is energized, each delay contact is instantaneously actuated, and after the coil is de-energized, the contact is in a delayed setting working state. When the set delay is reached, the delay contact returns to the initial state. The power-off delay type meets its control requirements due to its working state (no external working power supply is required during the delay process) and the conversion characteristics of the control contacts during the power-off delay process (the normally open contact becomes the on state and should remain in the on state; the normally closed contact becomes the off state and should remain in the off state) (which is exactly the opposite of the working state of the contacts of the conventional power-on delay type time relay).

The power-off delay type time relay is composed of the earliest separated devices (low delay accuracy and short delay time); the corresponding programmable timing integrated circuit or C MOS counting frequency division integration is used to complete the delay. Compared with it, it has the characteristics of high delay accuracy and long delay time. In this way, it can meet the control occasions of long power-off delay.

Typical circuit

The overall structure of the power-off delay relay includes the power supply part of the power-off delay relay (after voltage reduction , rectification, and filtering to provide the power-off delay relay with built-in instantaneous electromagnetic relay and 2-winding latching type R reset coil operation); the secondary power supply part (for the delay part after power failure and the 2-winding latching type S setting coil operation); the delay working part (programmable timing integration or CMOS counting frequency division integration); the driving part; and the execution relay part (Figure 1).

Figure 1 Control block diagram

Figure 2 Schematic diagram of discrete devices

The power-off delay relay composed of V2 P-channel field effect tube, V3, V4 transistor and relay as main components is shown in Figure 2. As follows: After the working power is added to the end, C1~C5 complete the charging process according to their circuits (the charging time should refer to the time specified by the product). At the same time, the internal 2-winding latching relay R reset coil is energized to work (the conversion contacts 4 and 6 in the virtual frame are turned from power on to off state, 4 and 8 are connected), and the corresponding external contacts are connected to the conversion end, which is in a delayed working state).

When the working power supply of the end is off, the corresponding relay enters the delayed working state. For the V2 P-channel field effect tube, as C4 discharges through R6 and RP2, its source S voltage continues to decrease (when powered on, because UGS is small, ID is zero, and V2 is in the cut-off working state). According to the corresponding transfer characteristics of the field effect tube (the relationship curve between the drain current ID and the gate-source voltage VGS), when the VGS voltage reaches VGS (Th) (turn-on voltage), V2 is turned on. As V2 is turned on, the drain current ID generates a corresponding voltage drop through R4, causing the V3 transistor to turn on and work, and finally causing V4 to turn on. When V4 is turned on, the energy stored on the C5 capacitor will energize the 2-winding latching relay setting coil, so that the delay contact returns to its original state, thereby completing the power-off delay work.

The disadvantage of this circuit is that the delay parameters are not easy to set. Usually, it is necessary to adjust RP2 (to control the C4 discharge circuit), RP1 (to determine the V2 gate voltage), and calculate the capacitance parameters of C4 and C3. In addition, the discreteness of the device makes the delay error large and the adjustment is not convenient. Therefore, it is rarely used now.

The delay circuit composed of integrated CD4060 is shown in Figure 3. The core delay of the circuit is composed of CD4060, and the delay setting is set by RP1 and the configured C3. The internal 2-winding latching relay uses DC24V (using a relay with a higher working voltage can reduce its driving current and make the driving part simpler). The working power is added to the end, and the V1 transistor works, so that its R reset coil is attracted and works, and the internal contacts return to the original state. C2 and C4 complete the charging work.

Figure 3 CD4060 integrated schematic diagram

When the working power supply of the [page] end is cut off, the corresponding power-off delay working state is entered. The IC○12 pin generates a level in R3 due to the discharge of C1, which is added to the ○12 pin through R4 and the reset pin is reset to zero, so that the delay starts. The delay time drives V2 to work through Q4~Q14 (according to the required delay time). After the delay is reached, it stops oscillation through VD7. According to the delay situation, the C2 capacitor can be increased or decreased accordingly (through parallel connection to increase or decrease the capacity of C2) and the C4 capacitor completes the work of the S setting coil.

The characteristics of this line are convenient delay setting, high delay accuracy, and simple product adjustment. It is currently widely used.

The delay circuit composed of integrated IC4541 is shown in Figure 4.

Figure 4 IC4541 integrated schematic diagram

The core part of this circuit is composed of IC4541, the delay setting is set by RP2 and C*, and the AB terminal is connected to the corresponding high and low levels (setting terminals) according to the needs. The internal 2-winding latching relay uses DC12V (because the relay working voltage and IC4060 form a delay electrical appliance to be lower, in order to ensure its drive, it is composed of V6, V7, V1, and V3 respectively). Among them, C2 is a secondary energy storage device, which can be adjusted according to the length of the delay, and C4 is to complete the work of the S setting coil.

In short, the power-off delay relay that uses the corresponding integrated circuit to complete the delay should usually consider low power consumption when selecting the integration, and the latching relay should choose a relay with a higher working voltage , so that the power consumption of the relay during the power-off delay process is minimized to ensure accurate and reliable delay operation.

The delay t in the working timing diagram (Figure 5) is the delay time of the delayed breaking contact after the working power supply is disconnected; if a reset signal is added during the delay process, the delay ends.

Devices used

Due to the requirement of switching of the electric contact controlled by the power-off delay relay, a bistable polarized electromagnetic relay (also known as a 2-winding latching relay) is usually used to complete and meet its contact switching requirements. Its internal coil and contacts are shown in the figure. The relay has a set coil S and a reset coil R inside, and is a latching structure relay that can maintain the set state or reset state. When there is a current flowing through the set coil S , a magnetic flux is generated in the magnetic circuit composed of the internal iron core, magnet, armature coil and the working air gap, and a magnetic field is established in the working air gap, generating electromagnetic attraction to attract the armature. When the current in the coil reaches a certain value (i.e., the action value), the electromagnetic attraction generated is sufficient to overcome the magnetic attraction and the resistance generated by the contact spring, driving the armature group to move, and the push cards at both ends of the armature group push the contact spring, so that the make contact group is closed and the break contact group is disconnected, thereby completing the contact switching and maintaining the set state. When the power-off delay ends, the reset coil R has current flowing through it (the set coil has no current), and the working state is the same as the set coil, and finally the closed make contact is opened and the open break contact is closed again. When using it, you should pay attention to the polarity of the set coil S and the reset coil R.

In view of the application of power-off delay relays, when selecting internal latching relays, the following conditions should be considered as selection criteria: low power consumption, high sensitivity, large load, high insulation withstand voltage, and resistance to vibration and impact; only in this way can the power-off delay work be reliable. Especially in terms of vibration and impact resistance, due to the special internal structure, the power-off delay relays that match them should be installed and used with care to avoid incorrect contact switching caused by vibration or impact of the latching relay contacts.

In the selection of relay working voltage, just like the power consumption relay, in principle, an electromagnetic relay with a higher coil working voltage is selected. This can reduce the current action value added to the set coil S and the reset coil R, thereby ensuring that the action current added to the reset coil of the electromagnetic relay after the delay meets its rated action value, and also fully guarantees the reliable conversion of the contact operation.

In some power-off delay time relays, the internal execution relay also uses a 1-winding latching relay, which has a coil (S, R) and is a latching relay that can switch and maintain the set or reset state according to the polarity of the applied voltage. However, because the polarity of the applied voltage of its own working coil must be switched, the control circuit is relatively complicated and is rarely used at present. Usually, a 2-winding latching relay is used to make the internal control circuit simple and reliable.

[page]Control circuit analysis

The circuit for motor braking is shown in Figure 7.

Figure 7 Motor control circuit

Normal starting process: press the start button SB2 → KM1 is energized, the main contact KM1 is closed, the motor works, the auxiliary normally open contact of KM1 is self-locked, and the auxiliary normally closed contact of KM1 is disconnected; KT is energized, the KT normally open delayed open contact is closed, and KM2 is de-energized.

Braking process: Press the stop button SB1 → KM1 is powered off, the main contact of KM1 is disconnected, the motor is disconnected from the power supply, the auxiliary normally closed contact of KM1 is closed again, KM2 is powered on, the DC power supply is connected, and the braking starts. At the same time, the auxiliary normally open contact of KM1 is disconnected again, and the KT coil is powered off. The delay starts. After the set delay time is reached, the delayed disconnected normally open contact of KT is disconnected again, KM2 loses power, the DC power supply is cut off, and the braking ends.

In the time relay control circuit, its working power supply and the contacts involved in the corresponding control circuit should be standardized to better reflect its use in work.

Precautions for use

The relay power supply voltage should operate within the allowable voltage fluctuation range, usually 85%~110% of the rated value; the DC voltage peak ripple factor should not exceed 5%. If the relay working power supply has a strong inductive load and works frequently, you should consider adding and using a surge absorption device at the relay working power supply end to withstand the higher (1500V) surge voltage to prevent the relay power supply from breaking down and burning.

When the relay is in use, the power-on time must be greater than 1s so that the secondary power supply inside the relay has sufficient energy reserve to ensure that the load is connected or disconnected at the preset time after the power is disconnected; if the external reset signal function of the relay is required, the connection duration must be no less than 50ms to ensure that its reset function works normally. It is strictly forbidden to connect power, active signal or ground to the reset signal end, otherwise the relay will be damaged.

The power supply circuit of the relay is generally high impedance, so in actual use, the leakage current should be as small as possible after the power is cut off to avoid the generation of corresponding induced voltage and false shutdown to cause malfunction (the delay time is up after the power-off delay but the relay does not release). To avoid the above situation, the residual voltage at the power supply end of the power-off delay type should be less than 7% of the rated voltage, and the residual voltage allowed for the power-on delay type is less than 20% of the rated voltage.

The power-off delay relay uses a 2-winding latching relay internally. Compared with ordinary relays, this relay has poorer environmental adaptability, especially in strong magnetic fields and high impact and vibration environments. Therefore, it should be avoided as much as possible when using it in the above environment.

When controlling loads, do not use it to directly control large-capacity loads (the 2-winding latching relay used internally usually has a weak load capacity). It should be used within the rated allowable conditions, taking the load form into consideration and leaving corresponding margins.

The relay should operate in accordance with the requirements specified by the relay in terms of operating environment, installation form, temperature, humidity, pollution level, etc.

Conclusion

With the wide application of time relays in the field of automatic control, not only the conventional power-on delay time relays are used more and more, but also the power-off delay relays are frequently used in specific occasions to meet the needs of control. Therefore, the power-off delay relays with the characteristics of high delay precision, long power-off delay, large load control, strong anti-interference ability, etc. will definitely play a more important role in the field of automatic control.