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Jul 29 2020

How to Test a Relay with Multimeter?

Introduction

A relay is an electronic control device, which has a control system (also called an input loop) and a controlled system (also called an output loop). It is often used in automatic control circuits. In fact, it is an automated switch using smaller current to control a larger current. Therefore, it plays the role of automatic adjustment, safety protection, and converter in the circuit. Relay has the advantages of fast response speed, stable work, long service life and small size.

Catalog

Introduction

Ⅰ Understanding Relays

1.1 Relay Parameters

Ⅱ How to Test A Relay?

2.1 General Test Ideas

2.2 Types of Relay Test

Ⅲ Relay for Life: Automotive Relay Test

3.1 Automotive Relay

3.2 Common Faults of Automotive Relays

3.3 Detection Method

3.4 Specific Operation

Ⅳ One Question Related to Relay Test and Going Further

4.1 Question

4.2 Answer


Ⅰ Understanding Relays

1.1 Relay Parameters

Main relay parameters include rated working voltage, rated working current, coil resistance, contact load, etc.

1) Rated working voltage refers to the voltage required by the coil when the relay is working normally. For DC relays it refers to DC voltage (Figure a), and for AC relays it refers to AC voltage (Figure b). Relays of the same type often have multiple assessedoperating voltages for circuit requirements, and the specification number is added to the end of the component to distinguish.

relay symbol

Figure 1. Relay Symbol

2) The rated working current refers to the current required by the coil when the relay is working normally.

Coil resistance refers to the DC resistance of the relay coil. When selecting a relay, you must ensure that its rated working voltage and rated working current meet the requirements.

rated working current

Figure 2. Rated Working Current

3) Contact load refers to the load capacity of the relay contact, also known as the contact capacity. For example, the contact load of the jzx-10m relay is: DC 28v×2a or AC 115v×1a. When used, the voltage and current passing through the relay contact should not exceed the rated value, otherwise the contact will be burned out and the relay will be damaged. The load of multiple sets of contacts of a relay is generally the same.

Contact load

Figure 3. Contact Load

Recommended Reading: Basic Knowledge of Relay Electronics Tutorial with Video    The Role of the Relay and Its Working Principle

 

Ⅱ How to Test A Relay?

Relays are widely used in power protection, automation, sport, remote control, measurement and communication devices, so it is very important to check and maintain the normal operation of relays. There are many types of relays. Therefore, the inspection of relays cannot only be judged by measuring the resistance value of the coil. It is necessary to adopt multiple detection methods according to different relay types.

How to Test A Relay (car relay)

2.1 General Test Ideas

1) measuring contact resistance

Apply the specified working voltage to the relay coil, and use a multimeter to detect the on-off condition of the contact at the “R×1k” gear. When the power is not applied, the normally open contact does not work, and the normally closed contact conducts. When the power is turned on, you should be able to hear the pick-up sound of the relay. At this time, the normally open contact is conducting and the normally closed contact is opposite, and the switching contact should be switched accordingly. Otherwise, the relay is damaged. For multi-group contact relays, if some of the contacts are damaged, the remaining contacts can still be used.

relay test

Figure 4. Relay Test

 2) measuring coil resistance

The resistance value of the relay coil can be measured with the multimeter at R×10Ω gear, so as to determine whether the coil is open.

3) measuring of pull-in voltage and current

Use an adjustable regulated power supply to input a set of voltage to the relay, and connect an ammeter in the power supply circuit to monitor. Increase the power supply voltage slowly, and when you hear the pull-in sound of the relay, write down the voltage and current. In order to be accurate, you can try several times to get the average value.

4) measuring the release voltage and current

Same test connection like the above. When the relay pulls in, then gradually reduce the supply voltage. When you hear the relay release sound again, write down the voltage and current at this time. Try several times to get the average release voltage and release current. Under normal circumstances, the release voltage of the relay is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it can't be used normally, which will affect the circuit stability resulting in abnormal operation.

 

2.2 Types of Relay Test

  • Electromagnetic Relay Test

Electromagnetic Relay

Figure 5. Electromagnetic Relay

The multimeter is placed in the “R×100” or “R×1k” gear, and the two test leads (regardless of positive and negative) are connected to the two pins of the relay coil (shown in Figure 5). The indication of the multimeter should basically match the coil resistance of the relay. If the resistance value is obviously too small, it means that the coil is short-circuited locally; if the resistance value is 0, it means that there is a short circuit between the two coil pins; if the resistance value is infinite, it means that the coil is open or the pins are disconnected.

 

  • Reed Relay Test

Reed relay is also one of the most commonly used relays. It consists of a reed switch and a coil, as shown in the Figure 6. The reed switch is made by sealing two non-interconnected ferromagnetic metal strips in a glass tube, and the reed switch is placed in the coil. When the current passes through the coil, the magnetic field generated by the coil magnetizes the metal strips in the reed pipe, and the two metal strips attract due to opposite polarities to connect the controlled circuit. Several reed pipes can be placed in the coil, and they will act simultaneously under the action of the coil's magnetic field.

reed relay

Figure 6. Reed Relay

 

Reed relay has a pair of coil pins and several pairs of reed switch pins, and there are corresponding marks on the shell for identification.

reed relay

Figure 7. Reed Relay

 

Reed relays can also use a multimeter to detect their coils and contacts, and the detection method is the same as that of electromagnetic relays.

Reed Relay

Figure 8. Reed Relay

 

  • Solid State Relay (SSR) Test

The input end can be tested with a multimeter. The multimeter is placed in the "R×10k" gear, the black test lead (the positive electrode of the battery in the meter) is connected to the positive electrode of the ssr input terminal, and the red test lead (that is, the negative electrode of the battery in the meter) is connected to the negative electrode of the input terminal of SSR. The hands should deflect more than halfway (Figure 9). Re-testing after swapping the two test leads, the hands should not move. If the needle deflects to the top or does not move regardless of the forward or reverse voltage access, the solid state relay has been damaged.

ssr

Figure 9. Solid State Relay  SSR

 

You can also make a test circuit according to Figure 10. When the control voltage of the ssr input terminal is turned on, the light-emitting VD is on; when the control voltage of the ssr input terminal is cut off, the light-emitting diode VD is off.

Solid State Relay

Figure 10. SSR

 

  • Thermal Relay Test

1) heating elements detection

The heating element is composed of electric heating wire or electric heating sheet, and its resistance is very small (close to 0Ω). The detection is shown in Figure 11. The normal resistance of the three groups of heating elements should be close to 0Ω. If the resistance is infinite (the digital multimeter displays the symbol "1" or "OL" for exceeding the range), the heating element is open.

Figure 11. 

Thermal Relay Test

① 200Ω gear is selected.

② The red and black probes are respectively connected to the two ends of a heating element.

③ The resistance is close to 0Ω, indicating that the resistor as a heating element is normal.

 

2) contact detection

Thermal relays generally have a normally closed contact and a normally open contact. This detection includes working and non working conditions. The first picture is the detection of the normally closed contact resistance when it is not in operation. Normally it should be close to 0Ω. Then the detection is taken in the opposite condition. Move the test rod, as showed in the second picture, simulates the over-current heating and bending of the heating element to make the contact action. The normally closed contact becomes an open circuit, and the resistance is infinite.

Figure 12. 

Thermal relay test

Thermal relay test

① 200Ω gear is selected.

② The red and black probes are connected to both ends of the normally closed contact.

③ The resistance is close to 0 Ω, indicating that the normally closed contact is closed.

④ Move the test rod by hand.

⑤ The out of range symbol "1" is displayed to indicate that the normally closed contact is open.

 

  • Intermediate Relay Test

The electrical part of the intermediate relay is composed of coils and contacts, both of which use the resistance gear of a multimeter.

1) The contact is detected when the control coil is not powered. Contacts include normally open contacts and normally closed contacts. When the control coil is power off, the normally open contacts are open and the resistance is infinite, at this time, the normally closed contacts are closed and the resistance is close to 0Ω. The above mentioned detection of the normally open contact is shown in the figure below.

Figure 13.

Intermediate Relay Test

① 200Ω gear is selected.

② The red and black probes are connected to both ends of a normally open contact.

③ The out of range symbol "1" is displayed to indicate that the normally open contact is open.

 

2) Control coil detection of the intermediate relay is shown in Figure 14 . Generally, the greater the rated current of the contact, the smaller the resistance of the control coil. This is because the greater the rated current of the contact, the larger the volume of the contact. Only a small control coil resistance (thicker line diameter) can flow through a larger current to produce a stronger magnetic field suction contact.

Figure 14. 

Control Coil Detection

① 200Ω gear is selected for gear switch.

② Connect the red and black lead to the two pins of the control coil.

③ The display of "6.60" indicates that the resistance of the control coil is 6.6kΩ.

3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then use a multimeter to detect the resistance of the normally open and normally closed contacts. The normally open contact should be closed and the resistance should be close to 0Ω; the normally closed contact should be open and the resistance is infinite.

 

  • Time Relay Test

The detection of time relay mainly includes contact normal state detection, coil detection and coil energization detection.

1) Normal-state detection of contacts. It refers to the detection of the resistance of the contact when the control coil is not energized. The normally open contact is open and the resistance is infinite, while the normally closed contact is closed, and the resistance is close to 0Ω. Normal detection processes are shown in the figure below.

Figure 15. 

Time Relay Test

① 200Ω gear is selected for gear switch.

② The red and black lead is connected with two pins of a normally closed contact.

③ The resistance is close to 0Ω, indicating that the normally closed contact is closed.

 

2) Detection of control coil. It is shown in the Figure 16.

Figure 16. 

Detection of control coil

① 20kΩ gear is selected for gear switch.

② Connect the red and black lead to the two pins of the control coil.

③ The display of "4.93" indicates that the resistance of the control coil is 4.93kΩ.

3) Power on the control coil to detect the contacts. Apply a rated voltage to the control coil, then check whether the contact status has changed according to the characteristics of types of the time relay. For example, for a delay time relay, after a period of time delay, check whether the delay contact is closed (resistance is close to 0Ω) and whether the delay contact is disconnected (resistance is infinite).

 

 

Ⅲ Relay for Life: Automotive Relay Test

3.1 Automotive Relay

Relays are widely used in automotive circuits, such as starting system circuits, wiper circuits, and rear window heating circuits. When the vehicle starts, a larger starting current is required. If the ignition switch is used for direct control, the starting contacts will ignite and burn, which will affect the service life of the ignition switch and even cause serious consequences such as line ablation and fire. Using a relay to control a large current with a small current will not cause the above problems.

When a certain voltage or current is applied to both ends of the electromagnetic relay coil, the magnetic flux generated by the coil passes through the magnetic circuit composed of the core, yoke, armature, and the working air gap of the magnetic circuit. Under the action of the magnetic field, the armature attracts the pole face of the iron core, making the normally closed contact to open and the normally open contact to close. When the voltage or current at both ends of the coil is less than a certain value, the mechanical reaction force is greater than the electromagnetic attraction force, and the armature returns to the initial state: the normally closed contact is on and the normally open contact is off. One of the automobile relays functions is a switch; the other is  load overload protection; the third is a fault protection.

 

3.2 Common Faults of Automotive Relays

Including coil burnt, short circuit, insulation part aging, contact ablation, etc.

1) Relay Malfunction

When the controlled circuit is required to be closed, the relay will not act, on the contrary, when the controlled circuit is not required to be closed, the relay will act. This kind of problems occurs mainly because the interference voltage in the circuit exceeds the allowable range of the drive circuit of the relay. When designing the circuit, pay attention to the factors that can cause interference (such as chip command errors, short circuits, grid fluctuations, etc.).

 

2) Relay Burned

There are many reasons for burnout. For example, the actual switching current exceeds the rated switching current of the relay, and the actual inrush current exceeds the rated switching current of the relay. According to design experience, in order to avoid these problems, the rated current should be selected to be 2-3 times the actual switching current, and the impact current of the relay is 2-3 times the actual current.

 

3) Contact Welding

Generally speaking, the temperature rise of the AC conversion relay coil is higher than that of the DC conversion relay. This is because of the eddy current loss and hysteresis loss in the magnetic circuit. In addition, when the AC conversion relay is operating at a voltage lower than the rated voltage, a bounce phenomenon may occur. This will cause burnout, welding of contacts and damage to the relay or disconnection of the self-protection circuit. Therefore, measures must be taken to prevent fluctuations in the power supply voltage.

In addition, regardless of the length of the fluctuation time, it will cause the failure of the relay. So ensure that there is a power supply with sufficient capacity.

 

4) Coil Temperature Rise is Too High

The loss of magnetic materials such as copper wires and iron cores or the heat transfer of the contacts will cause the temperature rise. Therefore, the heat resistance of the insulating material and the distance between the relay and the heat-generating device should be paid special attention in the circuit design.

 

3.3 Detection Method

Static detection: check the resistance of the coil and the resistance of the normally closed contact.

Dynamic detection: energize the coil and detect the resistance of the normally open contact.

 

3.4 Specific Operation

  • Turn on the ignition switch and hear whether there is a pull-in sound in the control relay or feel the relay with your hands for vibration. If so, it means that the relay is basically inroutine The failure of the circuit may be caused by other reasons. On the contrary, it means that the relay is faulty.
  • Replace the relay to be tested with an identical working relay. Turn on the switch, and if the electrical equipment is working normally, it can be determined there is a problem with the relay to be tested.
  • Use the multimeter Rx100Ω gearand combine the resistance of each pin of the circuit to analyse. If the conduction and disconnection are normal, it means that there is no problem with the relay, otherwise it means the relay is faulty.
  • Open the relay shell to checkwhether the contacts are ablated or oxidized. If there are bumps and rust on the contact, it means that the contact is ablated or oxidized and does not work properly.
  • Check whether the coil is ablated or discolored. If the coil is ablated with jelly, the coil is black or has a gluey smell, whichmeans the coil is short-circuited by ablation.

auto relay

4.1 Question

What are the symptoms of a bad car relay?

4.2 Answer

Car suddenly stalls while operating. One of the most common symptoms of a failed ignition relay is a car that suddenly stalls while operating. 
Car not starting. Another symptom of a faulty ignition relay is a no power condition.
Dead battery. A dead battery is another symptom of a faulty ignition relay.
Burned relay.

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