Digital Multimeters (Part 1)

Objectives

  • Inspect and set up a digital multimeter (DMM)
  • Perform common circuit measurements
  • Demonstrate the correct procedure to test solenoids

Common Measurements

Here are the most common measurements made by mechanics who are troubleshooting, ranked in order of how frequently they're used:

  1. Voltage
  2. Resistance
  3. Current

Why this order?

Voltage Measurement

Voltage will be the #1 measurement because it requires no alteration to the circuit before the measurement is made, and does not affect the circuit operation during the measurement. It's safe to say that approximately 75% of all electrical faults can be isolated through voltage measurements.

Resistance Measurement

A resistance measurement is NEVER made in a live circuit; an ohmmeter is a source of voltage and acts much like an additional power supply.

The device being tested must be isolated as the current from the ohmmeter will find all possible paths.

Current Measurement

A current measurement is seldom made because the ammeter must be connected in series with the device being tested. This requires wires to be disconnected or cut.

Be extremely cautious when using an ammeter to measure current because the ammeter acts as a short circuit!

DMM Inspection and Initial Testing

For your own safety, and for the safety and accuracy of the device, here are a few rules to remember before you use a DMM:

  • Inspect the case before using the meter. Look for cracks or missing plastic. Do not use the meter if it is damaged.
  • Pay particular attention to the test leads. Look for damaged insulation or exposed metal.
  • Check the condition of the battery and fuses, if applicable.
  • The test leads can add error to resistance measurements. To test the leads, touch the probe tips together and read the resistance of the leads. Remember to factor this resistance into your measurements.
  • You can use the relative (REL) mode to automatically subtract resistance of the test leads.
  • Make sure that the measurements you are planning are within the operational range of that DMM.

Voltage Measurement

A voltage measurement is made by placing the test leads in parallel with the device or source under test.

Measuring voltage across any electrical or electronic component or device is also referred to as measuring the voltage drop.

Make sure that you dial the mode selector to the proper position to measure volts AC or volts DC.

Be sure to check that your voltmeter is operational before testing a live circuit. This can be done by measuring a known voltage.

A perfect voltmeter has an infinite resistance. This is only hypothetical, of course - in the real world a functioning voltmeter will offer a very high resistance, but it can't be infinite. For example, when you are using a Fluke 87V DMM, the internal resistance (or input impedance) will be 10 MΩ.

Continuity Measurement

A continuity test is the checking of an electrical circuit to see if it is in fact a complete circuit.

Push PEAK MIN MAX with the mode selector at the ohmmeter position to enter the continuity mode.

The meter operates exactly like an ohmmeter when testing continuity.

The continuity test features a beeper that sounds as long as there is a very low resistance path (less than a few hundred ohms). This allows you to perform quick continuity tests without having to watch the display.

The beeping sound only tells you that there is no breaks in the circuitry between the meter probes, not that the resistance reading is correct.

Resistance Measurement

The resistance reading represents the resistance of all paths between the probes.

An ohmmeter applies a small voltage across the resistance under test. The resulting current is dependant upon the circuit resistance between the probes.

It is important to isolate the load or resistance under test to ensure a correct measurement.

Do not touch the leads during the test as the resistance of your body may impact the measurement accuracy, because you have created an additional path for meter current flow.

NEVER place an ohmmeter in a live circuit!

Current Measurement

To measure current, you must break the circuit being tested and place the meter in series with the circuit.

When measuring current less than 400 mA, insert the red lead into the mA/µA terminal.

To measure current above 400 mA, insert the red lead into the A terminal.

If you are about to measure DC current, don't forget to push yellow button to switch the meter to DC.

An ammeter has a very low resistance - close to zero ohms. NEVER place the probes across (in parallel with) any circuit or component. You'll cause a dangerous short circuit!

Testing the Solenoids

The 900 series stroker solenoids from Denison Goldcup Pump.

An example of when you might measure current is to verify correct solenoid operation on a hydraulic pump stroke controller.

Using your multimeter to read the current shouldn't take more then a few moments, and you can determine whether the solenoid is actually receiving the correct current. This will let you know if the problem is the solenoid, or the power supply, without ripping the pump apart and replacing parts unnecessarily.

First, put the multimeter in series with the stroker solenoid to be tested. A test adapter plug is a handy way to do this, or you might need to solder your own adapter.

Set the multimeter to measure DC amps, and take a moment to consider the expected current. Do you need to plug the red lead into the amps, or the milliamps jack on the meter?

If you are unsure, it's always best to start on the highest current range and work your way down.

One popular style of test adapter for Hirschman plugs.

Checking the manual on this hydraulic pump stroker control tells us that the stroker operates with a signal between 180 and 325 mA. If the signal sent is within those parameters, it would make sense to plug into the mA jack on your multimeter.

However...

It's possible that the solenoid may be receiving a higher signal than expected. If this happens, you might blow a fuse on your meter, or, even worse, damage the meter.

The absolute safest scenario for your test meter is to start measuring through the amps jack. Then, once you have verified that the current is within the milliamp range, switch jacks for the finer milliamp resolution.

Since this particular stroker is proportional, you can expect to see a range of values during solenoid operation.

Review Questions

The internal resistance (input impedance) of an ammeter is:

A voltage measurement is made by placing the test leads

Recap

  • The most common electrical measurements are:
    1. Voltage
    2. Resistance
    3. Current
  • It is important to test and inspect a DMM before use.
  • A voltage measurement is made by placing the test leads in parallel with the device or source under test.
  • Resistance is measured by using the ohmmeter to apply a voltage which results in a current that is proportional to the resistance.
  • You must break the circuit being tested in order to measure current, then place the meter in series with the circuit.
  • When measuring current, test at the highest Amperes setting first. Work your way down to the appropriate measurement range as you verify that the current is safe for each setting.

We hope you enjoyed Digital Multimeters (Part 1)

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