What is a relay?
A relay is an electrical control device that causes a predetermined step change in the controlled quantity in the electrical output circuit when the change in the input quantity (excitation quantity) reaches a specified requirement.
It features an interactive relationship between the control system (also known as the input loop) and the controlled system (also known as the output loop). Commonly used in automated control circuits, it essentially functions as an "automatic switch" that uses a small current to control a large current. Therefore, it plays roles in automatic adjustment, safety protection, and circuit switching within the circuit.
How to wire a relay
The primary purpose of relays is to control high-voltage electricity with low-voltage electricity (including low voltage controlling high voltage, and small current controlling large current). This is achieved by controlling the energization of the relay's own coil, thus opening and closing its contacts. (Simply put, it controls whether a magnet is attracted or not, and the magnet in turn controls the contact switch.)
Therefore, the connectors of a relay include the contact connectors that control its circuit and the connectors that control its coil.
Intermediate relays have a certain load-carrying capacity. When the load capacity is relatively small, they can be used to replace small contactors, such as in the control of electric roller shutters and some small household appliances. The advantage of this is that it not only achieves the control purpose but also saves space, allowing for a more compact control section in electrical appliances.
This is the most common use of intermediate relays. For example, in a circuit control system, when the contacts of one contactor need to control multiple contactors or other components, an intermediate relay is added to the circuit. We know that although the contact capacity of an intermediate relay is not very large, it still has a certain load-carrying capacity, and the current required for its operation is very small. Therefore, intermediate relays can be used to expand the contact capacity.
For example, you generally can't directly use the output of an induction switch or transistor to control electrical components with large loads. Instead, you use an intermediate relay in the control circuit to control other loads, thereby expanding the control capacity.
In industrial control circuits, it is common to encounter situations where the control requirements necessitate the use of normally closed contacts of a contactor to achieve the control objective, but the normally closed contacts of the contactor itself have been exhausted and the control task cannot be completed.
At this point, an intermediate relay can be connected in parallel with the original contactor coil, and the normally closed contacts of the intermediate relay can be used to control the corresponding components. By changing the contact type, the desired control purpose can be achieved.
In some control circuits, the on/off state of certain electrical components is often controlled by intermediate relays, which use the opening and closing of their contacts. For example, in the automatic degaussing circuit commonly found in color TVs or monitors, transistors control the on/off state of intermediate relays, thereby controlling the on/off state of the degaussing coil.
Eliminating interference in circuits: In industrial control or computer control circuits, although there are various types of intermediate relays, their function is to transmit signals or control multiple circuits simultaneously. They can also be used directly to control small-capacity motors or other electrical actuators. Their structure is basically the same as that of an AC contactor, except that the electromagnetic system is smaller and there are more contacts.
How to select a relay
Most electrical technicians are familiar with the thermal relay, an electrical component. Its main function is to protect motors or other equipment from overload in motor control circuits. When an overload fault occurs in the circuit or equipment, the thermal relay will disconnect the circuit to achieve the protection effect.
Many beginners in electrical work find thermal relays complicated and confusing. Today, we'll explain them in the simplest terms, focusing on how to determine if a thermal relay is good or bad.
Key parameters of a thermal relay: Simply put, there are three: the thermal element, the normally open contact (NO) and normally closed contact (NC), and the manual/automatic reset knob. When an overload fault occurs in a circuit or device, the thermal element deforms, causing the normally open contact (NO) of the thermal relay to close and the normally closed contact (NC) to open. Typically, we use the normally open contact (NO) as a fault alarm signal and the normally closed contact (NC) as a disconnection control circuit. After the thermal relay trips, you can choose between manual or automatic reset.
We mainly judge the quality of thermal relays based on the three attributes mentioned above. Let's take a closer look:
1. The thermistor should always be conductive. Set the multimeter to diode or buzzer mode; it should measure continuity. The thermistor measurement is as follows:
2. Measurement of normally open (NO) and normally closed (NC) contacts
Precautions: The multimeter should be set to either diode or buzzer mode. The normally open (NO) contact should be open under normal conditions, and the normally closed (NC) contact should be closed under normal conditions.
This is the normal state of the thermal relay. Next, we will manually simulate the thermal relay's protection state by switching the overload protection switch to the activated state:
3. After the thermal relay performs overload protection, you can choose manual reset or automatic reset. Taking manual reset as an example, we need to measure the changes in the contacts after reset:
In summary: If all the above measurements are normal, we can generally assume that the thermal relay is working properly. Please note that the thermal relay can be manually or automatically reset. If it's set to automatic reset, you don't need to do anything in practice; the thermal relay will automatically return to its original operating state and perform the next protection cycle. If it's set to manual reset, you must manually press the reset button (the red button in the image above) to perform the next overload protection cycle!
What is a relay? How are relays wired? By now, you should have a basic understanding. A relay is an electronic control device with a control system (input circuit) and a controlled system (output circuit). It is commonly used in automatic control circuits. Essentially, it's an "automatic switch" that uses a smaller current to control a larger current. Therefore, it plays a role in automatic adjustment, safety protection, and circuit switching in circuits.
