Relay identification and testing

Relay is a commonly used control device. It can use a small current to control a large current, use a low voltage to control a high voltage, use direct current to control alternating current, etc. It can also achieve isolation between the control circuit and the controlled circuit. It is widely used in automatic control, remote control, protection circuits, etc. Commonly used relays are shown in Figure 1.

Conventional detection methods for relays

Commonly used relays include power relays, electromagnetic relays, solid-state relays, time relays, temperature relays, etc. Relays are devices that use the effect of current to close or open circuits for active maintenance and active control. In most cases, a relay is an electromagnet whose armature can close or open one or more contact points. When current flows through the winding of the electromagnet, the armature is attracted by the electromagnet, thereby changing the state of the contact.

Now we briefly introduce the detection method of relays:

Detect the relay coil: adjust the multimeter to "R*100" or "R*1K" position, connect the two test leads (regardless of positive or negative) to the two pins of the relay, and the multimeter indication should be basically consistent with the coil resistance of the relay. If the resistance is significantly smaller, it means that the coil is partially short-circuited. If the resistance is zero, it means that the two coil pins are short-circuited. If the resistance is infinite, it means that the coil is open-circuited. The above three conditions all indicate that the relay is damaged.

检测继电器接点：给继电器线圈接上规则的作业电压，用万用表“R*1K”挡检测接点的通断状况，未加上作业电压时，常开接点应不通，常闭接点应导通，，当加上作业电压时，应听到继电器吸合声，此刻常开点应导通，常闭点应不通，转换接点应随之转换，不然阐明该继电器损坏，对於多组接点继电器，假如有些接点损坏，其余接点动作正常则仍可运用。

1. Measure contact resistance

Use the resistance range of the multimeter to measure the resistance between the normally closed contact and the moving point. The resistance should be 0, while the resistance between the normally open contact and the moving point is infinite. This can distinguish which is the normally closed contact and which is the normally open contact;

2. Measure coil resistance

The resistance of the relay coil can be measured with a multimeter at R×10Ω to determine whether the coil is open circuit.

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

Find an adjustable voltage-stabilized power supply and an ammeter, input a set of voltages to the relay, and connect an ammeter in series in the power supply circuit for monitoring. Slowly increase the power supply voltage. When you hear the relay closing sound, write down the closing voltage and closing current. For accuracy, you can try several times and calculate the average value.

4. Measure release voltage and release current

Also connect and test as described above. When the relay is energized, gradually reduce the supply voltage. When you hear the relay release sound again, write down the voltage and current at this time. You can also try several times to obtain the average release voltage and release current. In general, the release voltage of the relay is about 10% to 50% of the energizing voltage. If the release voltage is too small (less than 1/10 of the energizing voltage), it cannot be used normally, which will threaten the stability of the circuit and make the operation unreliable.

Illustration of the identification and detection methods of relays

The symbol for a relay is "K", as shown in Figure 2. In a circuit diagram, the contacts of a relay can be drawn next to the relay coil or away from the relay coil, and numbers are used to indicate their relationship to each other.

Relay contacts are diverse and can be divided into two categories: single-group contact relays and multi-group contact relays. Among them, single-group contact relays are divided into three types: normally open contacts (make contacts, referred to as H contacts), normally closed contacts (break contacts, referred to as D contacts) and transfer contacts (referred to as Z contacts) (Figure 3). Multi-group contact relays can include multiple groups of contacts of the same form or multiple groups of contacts of different forms.

Relay parameters include rated operating voltage, rated operating current, coil resistance, contact load, etc. Rated operating voltage refers to the voltage required by the coil when the relay is working normally. For DC relays, it refers to DC voltage (Figure 4a), and for AC relays, it refers to AC voltage (Figure 4b). The same model of relay often has multiple rated operating voltages to choose from, and the specification number is added after the model to distinguish them.

The rated operating current refers to the current value required by the coil when the relay is working normally. The coil resistance refers to the DC resistance of the relay coil. When selecting a relay, its rated operating voltage and rated operating current must meet the requirements (Figure 5).

Contact load refers to the load capacity of the relay contact, also known as contact capacity. For example, the contact load of the JZX-10M relay is: DC 28V×2A or AC 115V×1A. The voltage and current passing through the relay contact should not exceed the rated value during use, otherwise the contact will burn out and cause damage to the relay. The loads of multiple groups of contacts of a relay are generally the same (K-1 in Figure 6).

Electromagnetic relay is one of the most commonly used relays, and its structure is shown in Figure 7. It uses electromagnetic attraction to push the contact to move, and is composed of iron core, coil, armature, moving contact, static contact and other parts. Normally, the armature is tilted upward under the action of the spring. When the working current passes through the coil, the iron core is magnetized, and the armature is attracted and moves downward, pushing the moving contact to connect with the static contact, thus realizing the control of the controlled circuit. According to the different working voltages of the coil, electromagnetic relays are divided into DC relays, AC relays and pulse relays.

Electromagnetic relays have a pair of coil pins and several groups of contact pins. Sealed electromagnetic relays usually have the lead-out terminal diagram marked on the relay, as shown in Figure 8.

Electromagnetic relays can be tested with a multimeter. Set the multimeter to "R×100" or "R×1k", connect two test leads (regardless of positive or negative) to the two pins of the relay coil (as shown in Figure 9), and the multimeter indication should be basically consistent with the coil resistance of the relay. If the resistance value is significantly smaller, it means that the coil is partially short-circuited; if the resistance value is 0, it means that the two coil pins are short-circuited; if the resistance value is infinite, it means that the coil is broken or the pins are unsoldered.

Check the relay contacts: Add the specified working voltage to the relay coil, and use the multimeter "R×1k" to check the on/off status of the contacts (as shown in Figure 10). When not powered on, the normally open contacts are not connected, and the normally closed contacts are connected. When powered on, you should be able to hear the relay pull-in sound. At this time, the normally open contacts are connected, the normally closed contacts are not connected, and the switching contacts should switch accordingly. Otherwise, it means that the relay is damaged. For relays with multiple contacts, if some contacts are damaged, the remaining contacts can still be used if they operate normally.

Reed relay is also one of the most commonly used relays. It consists of a reed switch and a coil, as shown in Figure 11. The reed switch is made by sealing two ferromagnetic metal strips that are not connected to each other in a glass tube, and the reed switch is placed in the coil. When the working current passes through the coil, the magnetic field generated by the coil magnetizes the metal strip in the reed switch, and the two metal strips are attracted due to opposite polarities, connecting the controlled circuit. Several reed switches can be placed in the coil, and they act simultaneously under the action of the coil magnetic field.

A reed relay has one pair of coil pins and several pairs of reed switch pins, all of which are marked on the housing for easy identification (Figure 12).

The coil and contacts of a reed relay can also be tested with a multimeter in the same way as for an electromagnetic relay (Figure 13).

Solid-state relay (abbreviated as SSR) is a new type of relay that uses electronic circuits to realize the function of relays and relies on photocouplers to achieve isolation between the control circuit and the controlled circuit. Solid-state relays can be divided into two categories: DC and AC. The DC solid-state relay is shown in Figure 14. The control voltage is input from the IN terminal, and the control signal is coupled to the controlled terminal through the photocoupler. After amplification, it drives the switch tube VT to turn on. The output terminal OUT of the solid-state relay is connected to the controlled circuit loop, and the output terminal OUT has positive and negative poles.

The circuit principle of AC solid-state relay is shown in Figure 15. Different from the DC type, the switching element adopts bidirectional thyristor VS, so the output terminal OUT of AC solid-state relay has no positive or negative pole, and can control the on and off of AC circuit.

The input terminal of the solid-state relay can be tested with a multimeter. The method is: set the multimeter to the "R×10k" position, connect the black test lead (i.e. the positive pole of the battery in the meter) to the positive pole of the SSR input terminal, and the red test lead (i.e. the negative pole of the battery in the meter) to the negative pole of the SSR input terminal. The needle should deflect more than half (Figure 16). Swap the two test leads and test again. The needle should not move. If the needle deflects to the end or does not move regardless of whether it is connected in the forward or reverse direction, the solid-state relay is damaged.