Definition of a relay : A relay is an automatic control device in which the output changes abruptly when the input quantity (electric, magnetic, sound, light, heat) reaches a certain value.
I. Working principle and characteristics of relays
A relay is an electrical device that turns the controlled output circuit on or off when the input quantity (such as voltage, current, temperature, etc.) reaches a specified value. It can be divided into two main categories: electrical quantity relays (such as current, voltage, frequency, power, etc.) and non-electrical quantity relays (such as temperature, pressure, speed, etc.). It has advantages such as fast action, stable operation, long service life, and small size. It is widely used in power protection, automation, motion, remote control, measurement, and communication devices.
A relay is an electronic control device that has a control system (also known as an input circuit) and a controlled system (also known as an output circuit). It is commonly used in automatic control circuits and is essentially 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.
1. Working principle and characteristics of electromagnetic relays
Electromagnetic relays are generally composed of an iron core, a coil, an armature, and contact springs. When a certain voltage is applied across the coil, a certain current flows through the coil, thereby generating an electromagnetic effect. Under the attraction of the electromagnetic force, the armature overcomes the tension of the return spring and is attracted to the iron core, thus causing the moving contact of the armature to engage with the stationary contact (normally open contact).
When the coil is de-energized, the electromagnetic attraction disappears, and the armature returns to its original position under the spring's reaction force, causing the moving contact to engage with the original stationary contact (normally closed contact). This engagement and release achieves the purpose of connecting and disconnecting the circuit. The "normally open" and "normally closed" contacts of a relay can be distinguished as follows: the stationary contact that is in the open state when the relay coil is not energized is called a "normally open contact"; the stationary contact that is in the closed state is called a "normally closed contact".
2. Working principle and characteristics of thermal reed relays
A thermistor reed relay is a new type of thermal switch that uses a thermistor magnetic material to detect and control temperature. It consists of a temperature-sensing magnetic ring, a constant magnetic ring, a reed switch, a thermally conductive mounting plate, a plastic substrate, and other accessories. Thermistor reed relays do not require coil excitation; instead, the switching action is driven by the magnetic force generated by the constant magnetic ring. Whether the constant magnetic ring can provide magnetic force to the reed switch is determined by the temperature control characteristics of the temperature-sensing magnetic ring.
3. Working principle and characteristics of solid-state relays (SSRs)
A solid-state relay is a four-terminal device with two terminals as inputs and the other two terminals as outputs, with an isolation device used in between to achieve electrical isolation between the input and output.
Solid-state relays can be classified into AC and DC types according to the load power supply type. They can also be classified into normally open and normally closed types according to the switching type. Furthermore, they can be classified into hybrid, transformer-isolated, and opto-isolated types according to the isolation type, with opto-isolated types being the most common.
II. Main Technical Parameters of Relays
1. Rated operating voltage
This refers to the voltage required for the relay coil to operate normally. Depending on the relay model, it can be either AC or DC voltage.
2. DC resistance
This refers to the DC resistance of the coil in a relay, which can be measured with a multimeter.
3. Pull-in current
This refers to the minimum current required for the relay to activate. During normal use, the applied current must be slightly greater than the pull-in current for the relay to operate stably. The operating voltage applied to the coil should generally not exceed 1.5 times the rated operating voltage; otherwise, excessive current will be generated, burning out the coil.
4. Release current
This refers to the maximum current that triggers the relay's release action. When the current in the relay's energized state decreases to a certain level, the relay will return to its de-energized, released state. This current is much smaller than the energized current.
5. Contact switching of voltage and current
This refers to the voltage and current that the relay is allowed to handle. It determines the magnitude of the voltage and current that the relay can control, and these values must not be exceeded during use, otherwise the relay contacts can easily be damaged.
III. Relay Testing
1. Measure the contact resistance
Using the resistance setting on a multimeter, measure the resistance of the normally closed contact and the moving contact. The resistance should be 0 (a more precise method can measure the contact resistance to within 100 milliohms); while the resistance of the normally open contact and the moving contact will be infinite. This allows you to distinguish which is the normally closed contact and which is the normally open contact.
2. Measure the coil resistance
A multimeter set to the R×10Ω range can be used to measure the resistance of the relay coil to determine whether the coil has an open circuit.
3. Measure the pull-in voltage and pull-in current.
Find an adjustable regulated power supply and an ammeter. Input a voltage to the relay and connect the ammeter in series in the power supply circuit for monitoring. Slowly increase the power supply voltage. When you hear the relay click, record the pull-in voltage and pull-in current. For accuracy, you can try several times and calculate the average value.
4. Measure the release voltage and release current.
The connection test is conducted in the same manner as described above. After the relay engages, the supply voltage is gradually reduced. When the relay releases again, the voltage and current at that moment are recorded. This process can be repeated several times to obtain the average release voltage and current.
Under normal circumstances, the release voltage of a relay is about 10 to 50% of the pull-in voltage. If the release voltage is too low (less than 1/10 of the pull-in voltage), it cannot be used normally, which will threaten the stability of the circuit and make the operation unreliable.
IV. Electrical symbols and contact types of relays
In a circuit, a relay coil is represented by a rectangular box symbol. If a relay has two coils, two rectangular boxes are drawn side by side. The letter symbol "J" for relay is marked inside or next to the rectangular box.
There are two ways to represent relay contacts: one is to draw them directly on one side of a rectangle, which is more intuitive. The other is to draw each contact separately in its respective control circuit according to the circuit connection requirements. Usually, the same text symbols are marked next to the contacts and coil of the same relay, and the contact groups are numbered to distinguish them.
There are three basic forms of relay contacts:
1. The normally open (H-type) coil has two open contacts when not energized, and closes when energized. It is represented by the initial "H" of the pinyin for "open".
2. The normally closed (D-type) coil has two closed contacts when not energized and opens when energized. It is represented by the initial "D" of the pinyin for "broken".
3. Changeover Type (Z-type): This is a contact group type. This type of contact group has three contacts: a moving contact in the middle, and one stationary contact above and below. When the coil is not energized, the moving contact is open with one of the stationary contacts and closed with the other. When the coil is energized, the moving contact moves, causing the previously open contact to close and the previously closed contact to open, thus achieving the changeover purpose. This type of contact group is called a changeover contact, represented by the initial "z" of the pinyin for "changeover".
V. Selection of Relays
1. First, understand the necessary conditions:
① The power supply voltage of the control circuit and the maximum current it can provide;
② Voltage and current in the controlled circuit;
③ How many sets and what type of contacts are needed in the controlled circuit? When selecting a relay, the power supply voltage of the control circuit can generally be used as a basis for selection. The control circuit should be able to provide sufficient operating current to the relay; otherwise, the relay's engagement will be unstable.
2. After determining the usage conditions by consulting relevant materials, find the required relay model and specification number. If you already have a relay, check its usability against the available information. Finally, consider whether the size is suitable.
3. Pay attention to the size of the appliance. For general electrical appliances, in addition to considering the chassis size, the main consideration for small relays is the circuit board mounting layout. For small electrical appliances, such as toys and remote control devices, ultra-miniature relays should be selected.
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