Using the multimeter, we can perform three types of tests on common low voltage relays:
• Coil and contact continuity
• Sensitivity
• Maintenance current verification
With the tests described we can not only check the condition of a relay coil and its contacts, but we can also analyze its electrical behavior by discovering how its contacts are. For some tests we will need additional material to assemble simple test circuits through which we will determine some important characteristics of this component.
Additional Material
1 Variable direct current source with maximum voltage according to the relay under test.
Procedure
1. Coil Continuity Test
a) Set the multimeter to a low resistance scale of OHM x 1 or OHM x 10 for analog types and 200 or 2000 ohms for digital types.
b) Zero the multimeter if it is analog.
c) Touch the test leads to the terminals of the relay coil. The relay must be out of the circuit. Note the resistance reading.
Interpretation
• Low resistance: between 10 and 500 ohms for low voltage types - the relay is in good condition (this test does not detect shorts in the coil, which will also be indicated by low resistance).
• High resistance (above 100 k ohms) - the relay has an open coil.
Reading Condition
Low resistance Good
High resistance Open
Note:
a) High voltage relays (above 48 V) may have high resistances, above 1,000 ohms.
b) In this case we can easily determine the resistance that should be measured from the knowledge of the trip voltage and the current when the manufacturer does not directly provide the resistance.
To calculate the resistance that should be measured we use the formula:
R = V/I
Where: R is the coil resistance in ohms
V is the operating voltage in volts
I am the operating current in amperes.
For example, a 6 V relay that triggers at 100 mA must have a resistance of:
R = 6/0.1
R = 60 ohms
2. Contact Actuation Test
Procedure:
a) Set the multimeter to the lowest resistance scale OHMS x1 or OHMS x10 for analog devices, and 200 or 2000 ohms for digital devices.
b) Zero the multimeter if it is an analog device.
c) Assemble the circuit in Figure 2 to trigger the relay using an appropriate power supply.
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d) Identify the contacts to be tested and connect the multimeter as shown in the same figure. Use clamps to facilitate this test. e) Note the resistances indicated by the multimeter when the power is off (relay de-energized) and on (energized). Pay attention to the clicking sound of the contacts that can be heard in this test on most relays.
Interpretation
• The reading before tripping is of high resistance and after tripping it drops to zero - the relay is in good condition (inverse if the NC contacts - normally closed - are being tested).
• The reading is of high resistance before and after tripping - the relay is having problems and is not operating.
• The reading is of low resistance before and after tripping - the relay is with the contacts "sticking".
• In the low resistance reading the resistance oscillates between low and high values - intermittent contact (worn or faulty contacts).
Reading Condition
High resistance before and low after triggering Good.
High resistance before and after triggering Open (does not operate)
Low resistance before and after triggering Stuck contacts.
Resistance varying when closing Contact problems.
Observations:
a) This test is also valid for reed relays
b) In theory, the resistance of the closed contact should be zero. In practice, it is a fraction of an ohm, but in general it is too low to be detected by common multimeters.
CHECKING THE SENSITIVITY OF A RELAY
Type of Test
• Current sensitivity in direct current relays
• For relays outside the circuit with voltages from 3 to 48 volts.
The current at which the contacts of a relay close can be measured with the help of a multimeter in this test that requires the use of an auxiliary source of direct voltages according to the relay being evaluated.
Additional material
1 direct current source capable of triggering the relay being tested.
Procedure:
1. Triggering current
a) Set the multimeter to an intermediate current scale according to what is expected for the relay to trigger. Common relays trigger with currents between 10 and 200 mA.
b) Assemble the test circuit shown in figure 4. The polarity of the multimeter test leads must be observed.
c) Starting from zero, gradually increase the source voltage until the relay contacts close. You can hear this by the clicking sound of the contacts at this point. If the relay is silent and this cannot be heard, use a second multimeter to test the contact or use a load such as an LED or other circuit to detect this.
2. Trigger voltage
d) If the source has a voltmeter connected to its output, we can read the trigger voltage directly. Otherwise, we can remove the multimeter, connect the relay directly to the source (without touching its adjustment) and read the voltage at its output (using the appropriate Volts DC scale).
Notes:
a) If the source has a fixed voltage, we can also test it using a 470-ohm wire potentiometer connected to its output, as shown in figure 5. This potentiometer allows you to vary the voltage applied to the relay to trigger it.
b) Manufacturers usually specify their relays for the nominal operating voltage, at which the relay will certainly close its contacts firmly. With the indicated test, we establish the minimum closing current that normally occurs with voltages slightly below the nominal voltage. For a 6 V relay, it is typical to find the closing voltage around 4.5 V, when a current up to 25% lower than the current specified by the manufacturer is required. Of course, this indication should serve as a basis for a project, in the sense that we should have values greater than the one found and never equal or lower, because, otherwise, the relay activation may be subject to failures. This same type of test is also valid for reed relays.
