What is a delay circuit? 6 kinds of delay circuit principle explanation
“As we all know, when it comes to delay, many people will think of using software to achieve, such as timers and the like. Today, let’s talk about the way to use hardware to achieve timing. Although it is not so accurate, it can still be used in some occasions. Today we will introduce the working principle of 6 kinds of delay circuits.
As we all know, when it comes to delay, many people will think of using software to achieve, such as timers and the like. Today, let’s talk about the way to use hardware to achieve timing. Although it is not so accurate, it can still be used in some occasions. Today we will introduce the working principle of 6 kinds of delay circuits.
1. Precise long delay circuit diagram
The circuit consists of CD4060 to form the time base circuit of the timer. The timing time base pulse generated by the circuit is divided by the internal frequency divider to output the time base signal. The required timing control time is obtained by dividing the frequency by the frequency dividing circuit of the peripheral.
After power-on, the time base oscillator oscillates and outputs the time base signal after frequency division. IC2 as a frequency divider starts counting and dividing. When the count reaches 10, Q4 outputs a high level, which is reversed to a low level by D1 to make VT cut off, the relay is powered off and released, and the working power supply of the controlled circuit is cut off.
At the same time, the low level output of D1 is reversed to a high level by D2 and then added to the CP terminal of IC2, so that the high level output by the output terminal is maintained.
After the circuit is powered on to reset IC1 and IC2, the four output terminals of IC2 are all low level. The low level output by Q4 is reversed to high level by D1, VT is turned on through R4, and the relay is energized and absorbed. This working state is the power-on and timing-off state.
2. RC delay circuit
The RC delay circuit is shown in the figure. The delay time of the circuit can be adjusted by the size of R or C. However, due to the simple delay circuit, there are disadvantages of short delay time and low precision. For occasions that require a longer delay time and require accuracy, a time relay should be selected.
In automatic control, sometimes in order to make the controlled object work within a specified period of time or to issue the next operation command at an appropriate time, a relay delay circuit is often used. The figure shows several relay delay circuits.
The circuit shown in Figure (a) is a slow and slow suction circuit. When the circuit is turned on and off, the charging and discharging function of RC is used to realize the delay of pulling in and releasing. This circuit is mainly used when a short delay is required to pull in the occasion. Sometimes according to the needs of control, the relay is only required to be released slowly, and the slow pull-in is not allowed. At this time, the circuit shown in Figure (b) can be used.
When the power is just turned on, since the contact KK-l is in a normally open state, the RC delay circuit will not have a delay effect on the pull-in time, but when the relay K is used. After the pull-in, its contact Kk-1 is closed, so that the release of the relay kk can be carried out slowly. Simply calculate the time delay generated by the RC delay circuit, such as R=470K, C=0.15UF time constant, just use R*C directly.
3. Simple long-delay circuit composed of 555
When the button SB is pressed, the 12V power supply charges the capacitor Ct through the resistor Rt, so that the potential of the 6-pin keeps rising. When the potential of the 6-pin rises to the potential of the 5-pin, the circuit reset timing ends.
Because a diode VD1 is connected in series with the 5-pin to make the 5-pin potential rise, it has a longer timing than the general connection method (floating or grounding through a small capacitor).
4. Long delay circuit composed of two 555 time base circuits
The IC1 555 time base circuit is connected to a self-excited multivibrator with adjustable duty cycle. When the button SB is pressed, the 12V DC voltage is added to the circuit. Since the voltage of the capacitor C6 cannot change abruptly, the 2 pin of the IC2 circuit is low level, the IC2 circuit is in the set state, the 3 pin output high level, the relay When K is energized, the contacts K-1 and K-2 are closed. After the K-1 contact is closed, a self-locking state is formed. The K-2 contact is connected to the electrical equipment to control the on and off of the electrical equipment.
At the same time, the time base circuit of IC1 555 begins to oscillate, so pin 3 outputs high and low levels alternately. When pin 3 outputs a high level, the capacitor C3 is charged through the diode VD3 and the resistor R3.
When pin 3 outputs a low level, the diode VD3 is turned off and C3 is not charged, so C3 is charged only when pin 3 is at a high level, so the charging time of capacitor C3 is longer.
When the potential of capacitor C3 rises to 2/3VDD, the time base circuit of IC2 555 is reset, pin 3 outputs low level, the relay K loses power, the contacts K-1 and K-2 are disconnected, and it returns to the initial state, which is the next Be prepared every time.
5. Monostable delay circuit composed of single op amp
In normal state, the IC output remains low, and this state is stable. When the negative pulse is input to the inverting terminal through C1, the potential of the inverting terminal is lower than the potential of the non-inverting terminal, and the output terminal is turned from low level to high level, this state is unstable.
This high level is added to the non-inverting terminal of the IC after being divided by R1 and R2, so that the potential of the non-inverting terminal is higher than that of the inverting terminal, thereby maintaining a high-level output. At the same time, the high level is charged by R3 and C2. When the voltage on C2 is charged to make the potential of the inverting terminal higher than the potential of the non-inverting terminal, its output terminal is turned to a low level again.
At this time, the potential of the non-inverting terminal is about zero, and the voltage on C2 is rapidly discharged to the output terminal through VD1, so that the circuit is accelerated to return to the initial state.
After the circuit is stable, the potential of the inverting terminal is still higher than the potential of the non-inverting terminal, so that the output low level can be maintained.
The delay time T of this circuit depends not only on R3, C2, but also on the voltage divider ratio of R1 and R2.
Therefore, it is very convenient to adjust the delay time. You can adjust C2 and R3 for coarse adjustment, and adjust R2 for fine adjustment.
However, the state of the circuit is random when it is powered on. To make the circuit have a unique output state after it is powered on, there are two methods:
One is to add R4 in the circuit. In this way, when the power is turned on, since the voltage on C1 cannot be abruptly changed, the power supply voltage is added to the inverting terminal through R4 and C1, and the output can be set to a low level;
The second is to connect a diode VD2 and a switch S between the non-inverting terminal and the ground (as shown by the dotted line).
If the output is high level when powered on, although this state is unstable, as mentioned above, the output will be low level after time T, and in practice, it is often necessary to reset the circuit immediately when it is powered on.
For this reason, S can be turned on first when the power is turned on. If the output is a high level, the circuit can be reset by charging C2 to 0.7V, which greatly shortens the power-on reset time of the circuit. Disconnect S after reset, the circuit can work normally.
6. Transistor delay circuit
The delay part is composed of BG1, BG2 and capacitor C to form a Miller integrator circuit. Before the power is turned on, the terminal voltage of C is zero. After the power is turned on, BG3 and BG4 are turned on, the relay J is pulled in, and the capacitor C is charged at the same time. The potential of the point drops, and the drop of the potential of the point b limits the rise of the potential of the a point.
As a result of the mutual compensation of the potentials of the two points a and b, the rise of the potential of point a is very small, and the charging current is approximately constant.
When the potential of point b rises to about 10V, BG3 and BG4 are close to cut-off, relay J is released, and the delay process ends. Press the button AN once, the capacitor C is rapidly discharged through D1, the relay J is pulled in, and the next delay process starts.
Delay circuits are often used, and RC circuits are relatively simple circuits. Of course, by changing the parameters of each component of the circuit, different delays can be achieved.
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