Relay control circuit module design and schematic diagram

CMOS integrated circuit circuit that can directly drive the relay to work
In the habit of electronic enthusiasts to understand circuit knowledge, they always think that CMOS integrated circuit itself cannot directly drive the relay to work, but in fact, some CMOS integrated circuits can not only directly drive the relay to work, but also work very stably and reliably. The model of the relay used in this experiment is JRC5M-DC12V miniature sealed relay (its coil resistance is 750Ω). The working principle of CD4066 CMOS integrated circuit driving relay is analyzed as follows:

CD4066 is a four-way analog switch, and the integrated circuit SCR1~SCR4 is the control terminal, which is used to control the on and off of the four-way analog switch. When SCR1 is connected to a high level, the integrated circuit ① and ② feet are turned on, +12V→K1→integrated circuit ① and ② feet→power supply negative pole to make K1 pull in; on the contrary, when SCR1 inputs a low level, the integrated circuit ① and ② feet are open, K1 loses power and releases, and the state of SCR2~SCR4 when inputting a high level or a low level is the same as SCR1.
In this circuit, a diode is connected in parallel at both ends of the relay coil in reverse phase. It is used to protect the integrated circuit itself and must not be omitted. Otherwise, when the relay changes from the energized state to the released state, a high back electromotive force will be generated on the coil due to the effect of the inductance, which can easily cause the integrated circuit to break down. After the diode is connected in parallel, at the moment when the relay changes from the energized state to the released state, the coil will form a short-time freewheeling circuit through the diode, so that the current in the coil will not change suddenly, thereby avoiding the generation of back electromotive force in the coil and ensuring the safety of the integrated circuit.

Relay closure measures under low voltage
Often, the power supply voltage is lower than the relay's closure voltage, which causes it to fail to work properly. In fact, once the relay is closure, it can work reliably at about half of the rated voltage. Therefore, a starting voltage can be given to the relay at the beginning to make it closure, and then let it work at a lower power supply voltage. The circuit shown in the figure can achieve this purpose.
Working principle:

as shown in the figure. V1 is a single junction transistor BT33C, which forms a relaxation oscillator with R1, R2, R3 and C1. SCR is a unidirectional thyristor. After pressing the start button AN1, the circuit is energized. Because the SCR has no trigger voltage, it does not conduct, and the relay J does not operate. The power supply quickly charges the capacitor C2 through R4 and VD1 to a voltage close to the power supply voltage (Vcc-VD1 voltage drop). At the same time, the power supply charges the capacitor C1 through R1. After a few seconds, the voltage on C1 is charged to the trigger voltage of V1, and C1 is immediately discharged through V1, forming a positive pulse on R3. This pulse is added to the base of V2, causing V2 to quickly saturate and conduct, and the collector of V2, that is, the positive electrode of capacitor C2, is close to ground. Since C2 is charged with a positive polarity voltage with a positive polarity at the top and a negative polarity at the bottom, the negative electrode of C2, that is, one end of the J coil, is at a negative potential. The positive pulse on R3 triggers the thyristor to conduct through VD2 and C3, and the cathode of the SCR, that is, the other end of the J coil, is close to the power supply voltage. At this time, the J coil actually withstands about twice the power supply voltage, so J1-1 is closed. After releasing AN1, J1-1 is self-protected. J1-2 cuts off the power supply of V1 and V2, and the relay works at a voltage close to the power supply. In the figure, AN2 is the stop button. Pressing AN2 will cause J to lose power and release, and J1-1 will be disconnected, and the entire control circuit will lose power.
When making this circuit, the rated voltage of the relay can generally be about 1.5 times the power supply voltage. Generally, any type of unidirectional thyristor (or bidirectional thyristor) can meet the needs of this circuit. The withstand voltage of V2, C1, and C3 is selected according to the power supply voltage. The withstand voltage of C2 should not be less than twice the power supply voltage.

Three additional circuits of relays
Relays are a commonly used component in electronic circuits. In electronic switch drive circuits, which are generally composed of transistors, relays and other components, some additional circuits are often added to change the working characteristics of the relay or to protect it. There are three main forms of additional circuits for relays:

1. Relay series RC circuit:
The circuit form is shown in Figure 1. This form is mainly used in circuits where the rated working voltage of the relay is lower than the power supply voltage. When the circuit is closed, the relay coil will generate an electromotive force due to the self-inductance phenomenon to hinder the increase of the current in the coil, thereby prolonging the pull-in time. The pull-in time can be shortened after the RC circuit is connected in series. The principle is that at the moment the circuit is closed, the voltage across the capacitor C cannot change suddenly and can be regarded as a short circuit. In this way, a power supply voltage higher than the rated working voltage of the relay coil is added to the coil, thereby accelerating the speed of the current increase in the coil and causing the relay to close quickly. After the power supply is stable, the capacitor C does not work, and the resistor R plays a current limiting role.
2. Relay parallel RC circuit:
The circuit form is shown in Figure 2. After the circuit is closed, when the current is stable, the RC circuit does not work. When the circuit is disconnected, the relay coil generates an induced electromotive force due to self-inductance, which is discharged through the RC circuit, slowing down the decay of the current in the coil, thereby extending the release time of the relay armature and playing a delay role.
3. Relay parallel diode circuit:
The circuit form is shown in Figure 3, which is mainly to protect driving components such as transistors. When the transistor VT in the figure changes from on to off, the current flowing through the relay coil will decrease rapidly. At this time, the coil will generate a very high self-induced electromotive force and the power supply voltage will be superimposed and added between the c and e poles of VT, which will cause the transistor to break down. After the diode is connected in parallel, the self-induced electromotive force of the coil can be clamped at the forward conduction voltage of the diode. This value is about 0.7V for silicon tubes and about 0.2V for germanium tubes, thereby avoiding the breakdown of driving components such as transistors. When connecting diodes in parallel, it is important to note that the polarity of the diode cannot be reversed, otherwise it is easy to damage driving components such as transistors.

Non-inductive analog relay
An inductive analog relay is introduced, and its circuit principle is shown in the figure below.

In the figure, the 220V power supply provides bias for Q4 and Q3 in the positive and negative half cycles in turn through the load RL, R1, D1~D4, and ZD1; at the same time, the photocoupler Q1 is provided with power through R3, D5~D8. When the current-stage TTL circuit outputs a high-level signal, the photocoupler is turned on in the positive half cycle of the mains, so a voltage drop is generated across R5, triggering the SCR to turn on, and the load RL is powered on. The function of the entire circuit is like a relay, but no reverse induction voltage is generated, which avoids the possibility of the load being damaged by high reverse voltage breakdown. C1 and R6 are pulse absorption components, and R3 plays a current limiting role.
In order to avoid the 90° phase between the voltage of the thyristor and the power supply of the photocoupler when RL is an inductive load, the power supply of the photocoupler in this circuit is taken from the anode of the SCR instead of directly from the mains power supply.

Small improvement of relay circuit
Relays are often installed inside electrical equipment, and their working state is not intuitive. The author has improved it as shown in the following figure. Connect light-emitting diodes VD1 at both ends of the coil. When the control voltage is positive, the transistor is turned on, the relay J is attracted, and the light-emitting diode is lit, indicating that the relay coil has been powered. The light-emitting diode can be installed in a conspicuous place on the shell.

Correct use of relays
1. Selection of rated working voltage of relays
The rated working voltage of the relay is the most important technical parameter of the relay. When using a relay, the working voltage of the circuit (that is, the circuit where the relay coil is located) should be considered first. The rated working voltage of the relay should be equal to the working voltage of the circuit. Generally, the working voltage of the circuit is 0.86 of the rated working voltage of the relay. Note that the workpiece voltage of the circuit must not exceed the rated working voltage of the relay, otherwise the relay coil will easily burn out. In addition, some integrated circuits, such as the NE555 circuit, can directly drive the relay to work, while some integrated circuits, such as the COMS circuit, have a small output current and need to add a transistor amplifier circuit to drive the relay. This should consider that the transistor output current should be greater than the rated working current of the relay.
2. Selection of contact load
Contact load refers to the bearing capacity of the contact. The contacts of the relay can withstand a certain voltage and current when switching. Therefore, when using a relay, it should be considered that the voltage applied to the contact and the current passing through the contact cannot exceed the contact load capacity of the relay. For example, the contact load of a relay is 28V (DC) × 10A, indicating that the relay contact can only work on a circuit with a DC voltage of 28V and a contact current of 10A. Exceeding 28V or 10A will affect the normal use of the relay and even burn the contacts.
3. Selection of relay coil power supply
This refers to whether the relay coil uses direct current (DC) or alternating current (AC). Usually, beginners use electronic circuits in electronic production activities, and electronic circuits are often powered by DC power, so a relay with a DC voltage coil must be used.