Ⅰ. What is Relay?

Definition of relay:

Relay is an autonomous electrical appliance used in electric drive systems for control, protection, and signal conversion. It is suitable for remote connection and disconnection of AC and DC small-capacity control circuits. The control relay's input quantity is normally an electric quantity like current or voltage, but it can also be a non-electric quantity like temperature, pressure, or speed. The output quantity is the electric signal generated by the contact action or the output circuit's parameter change. The relay's property is that as the input changes to a specific program, the output changes step by step. In automobile control circuits, a relay is a frequent component. It controls the on/off of a circuit using the principle of electromagnetic induction, allowing small currents to control large currents, reducing the current load on the control switch contacts, and protecting them. The connections on the switch are not ablated. In autos, electromagnetic relays are commonly employed. Power supply relays, starter relays, horn relays, fog lamp relays, wiper relays, and other types of relays are all common.

The structure of the relay

Iron core, coil, armature, return spring, contact, and other components make up an electromagnetic relay. The internal structure of a typically open and normally closed hybrid relay is depicted in the diagram below.

The classification of relay

Iron core, coil, armature, return spring, contact, and other components make up an electromagnetic relay. The internal structure of a typically open and normally closed hybrid relay is depicted in the diagram below.

Ⅱ. How does Relay Works?

The relay's functioning principle is depicted in the diagram below. When the switch is closed, a voltage is provided to both ends of the coil, and a current flows through the coil, producing an electromagnetic effect. The armature will overcome the return spring's pulling force and attract to the iron core due to electromagnetic force. As a result, the armature's moving contact is attracted to the static contact (usually open contact), and the red bulb turns on. When the coil is de-energized, the electromagnetic attraction dissipates, and the armature returns to its original position under the reaction force of the spring, attracting the moving contact and the static contact (usually closed contact). The green light bulb then illuminates. This pulls in and releases to meet the circuit's goal of conducting and switching off.

The relay contacts that are "usually open" and "typically closed" can be identified as follows: The "normally open contact" is the static contact that is in the off state when the relay coil is not activated; the "normally closed contact" is the static contact that is in the on-state when the relay coil is energized. It's what's known as a "typically closed contact." The control circuit and the operating circuit are the two circuits found in most relays.

Ⅲ. Technical parameters of Relay

1. Rated working voltage

It is the voltage that the coil requires when the relay is in regular operation. The voltage of the relay might be AC or DC, depending on the model.

2. DC resistance

Refers to the DC resistance of the coil in the relay, which can be measured by a universal meter.

3. Pull-in current

The minimum current that the relay can provide for the pull-in motion is referred to as this value. In normal operation, the provided current must be somewhat higher than the pull-in current in order for the relay to function properly. When it comes to the coil's working voltage, it shouldn't be more than 1.5 times the rated working voltage; otherwise, it will produce a bigger current and burn the coil.

4. Release current

The maximum current generated by the relay to release the action is referred to as this value. The relay will return to the unpowered release state when the current in the pull-in state is lowered to a specified threshold. The current is substantially smaller than the pull-in current at this moment.

5. Contact switch voltage and current

This is the maximum voltage and current that the relay can load. It specifies the magnitude of the voltage and current that the relay can manage; this value must not be exceeded during operation, otherwise, the relay's contacts will be damaged.

Ⅳ. How to Test Relay? 

1. Measure contact resistance

Measure the resistance of the normally closed contact and the moving point using the universal meter's resistance profile. The resistance value should be zero (the contact resistance can be measured more precisely within 100 milliohms); the normally open contact and the moving point should both be open. A point's resistance is limitless. It is possible to tell which is a generally closed contact and which is a normally open contact based on this.

2. Measure coil resistance

The resistance value of the relay coil can be determined using the universal meter R10 to determine whether the coil is an open circuit.

3. Measure the pull-in voltage and pull-in current

Obtain an adjustable regulated power supply and an ammeter, then program the relay with a set of voltages and monitor the power supply loop with an ammeter. Increase the power supply voltage gradually. Write down the pick-up voltage and current when you hear the relay pick-up sound. You can try numerous times to determine the average value for accuracy.

4. Measure the release voltage and release current

The connection test is similar to the last one. The power supply voltage is progressively lowered when the relay is closed. Write down the voltage and current at the time you hear the relay release sound again. You can also attempt a few times to acquire a good result. Current release and voltage. The relay's release voltage is typically 10-50 percent of the pull-in voltage under normal conditions. It cannot be utilized regularly if the release voltage is too low (less than 1/10 of the pull-in voltage), putting the circuit's stability at risk. The work is insecure.

Ⅴ. The Electrical Symbol and Contact Form of Relay

In the circuit, the relay coil is represented by a rectangular box symbol. Draw two parallel rectangular boxes if the relay has two coils. Mark the relay's text symbol "J" in or beside the long box at the same time. The contacts of the relay can be represented in one of two ways: directly on the side of the long box, which is more intuitive, or indirectly on the side of the long box, which is less intuitive. The other option is to separate each contact into its own control circuit based on the circuit connection requirements. The contacts and coils of the same relay are usually labeled with the same text symbols, and the contact groups are numbered. To demonstrate the distinction.

There are three basic forms of relay contacts:

1. When the moving-on (H-type) coil is not electrified, the two contacts are unconnected, and when the power is energized, the two contacts are closed. The pinyin prefix "H" of the ligature represents it.

2. The two contacts are closed when the movable-break (D-type) coil is not electrified, and the two contacts are detached following energization. The hyphenation pinyin prefix "D" is used to indicate it.

3. The type of conversion (Z type) This is a form of the contact group. This type of contact group has three contacts in total: a moving contact in the middle, a static contact in the upper and lower, and a moving contact in the middle. The moveable contact and one of the static contacts are separated and the other is closed when the coil is not powered. Following the energization of the coil, the moveable contact moves to convert the original open state to closed, and the original closed state to open, resulting in the converted state. Purpose. A changeover contact is a type of contact group. The pinyin prefix "z" in the word "Zhuan" indicates this.

Ⅵ. The Selection of Relay

1. First understand the necessary conditions

① The control circuit's power supply voltage and maximum current;

②The voltage and current in the regulated circuit;

③How many sets and types of contacts are required for the controlled circuit. When choosing a relay, the general control circuit's power supply voltage can be used as a starting point. The control circuit must be able to deliver enough operating current to the relay; otherwise, when the relay is closed, it will be unstable.

2. You can search for relevant information to find out the kind and specification number of the required relay after consulting the relevant information to identify the circumstances of use. If you have a relay, you can see if it can be used based on the information. Finally, examine the appropriateness of the size.

3. Pay attention to the appliance's volume. Small relays, in addition to cabinet volume, take into account the circuit board installation arrangement when utilized for general electrical appliances. Ultra-tiny relay products should be utilized for small electrical gadgets like toys and remote control devices.