Using a Multimeter to Troubleshoot and Test
d. bodnar -- revised 03-10-08 & 3-14-14
|Objectives - By the end of this workshop participants should be able to:
|Getting Started with the Meter
Insert the red probe into the center socket (labeled "VΩmA") on the meter and the black probe into the bottom one (COM). Turn the selector to the DCV 20 position and turn the meter on. The display should show 0.00 volts. There may be a leading "-" sign on the display as in this photo.
Touch the tip of the red probe to the positive terminal (the smaller terminal) of a 9 volt battery and the black probe to the negative terminal (the larger one). The display should show about 9 volts depending on the condition of the battery. Repeat the test with a 1.5 volt AA battery. If you inadvertently connect the probes to the incorrect terminals the display will show a negative voltage. This will not damage the meter.
Note: Measuring battery voltage in this way does not really perform a very good test as the batteries are not under load. This is what is called open circuit or open current testing.
|A Better Battery Test
You can perform a better test with 1.5 volt and 9 volt batteries. Do
not use this method with other types of batteries! Move the
meter's selector to the battery test position in the upper right between ACV and
Again touch the red probe to the positive terminal and the black to the
negative. As the meter's label says a 1.5 volt battery should show 4 mA
and a 9 volt battery should show 25 mA.
|Measuring Resistance and Continuity
Set the selector straight down to the 200 Ω setting. Touch the two probes together and the meter should display "00.0" (zero) or a very small resistance of 1 or fewer ohms. Separate the probes and the display shows "1 .". When the meter reads "00.0" (zero) that means that there is no resistance between the probes indicating a good connection.
|Alligator Clips - Test Before Using!
It is frequently easier to do tests using wires with alligator clips on their
ends rather than just touching the pointed probe ends to the test item.
Before using alligator clip jumpers use the meter to make sure that they are
working properly. Inexpensive jumpers are notorious for being faulty as
the connection between the wire and the clip is rarely soldered. Just
leave the meter set as we have it and connect one end of the jumper to each
probe. The reading should be just a little more than we see when touching the probes
directly. Repeat with the other probe or join the two together to test
both at the same time.
|Testing Power Pickups
Let's test the connection between the wheels on a locomotive to see how well its contacts are working. Touch one probe to a wheel that has electrical pickups on it and the other to another wheel with pickups on the same side. You should see a reading of "00.0" or something very close to that, perhaps a few ohms. Try testing between the engine's sliders (if it has them) and a wheel. Again it should show low resistance. In this photo we see a resistance of 13.0 ohms between the sliders and the drive wheel. This is due to the carbon brushes on the wheel having a fairly high resistance . As long as the meter shows that there is a connection the motor should be getting power.
|Repeat these measurements by clipping alligator leads to the test probes. Notice the difference in readings. All indicate that the connections are good.|
|Try measuring from one side's wheels to wheels or sliders on the other side. Now you are seeing the
resistance through the engine's motor. The reading will vary from engine
to engine but should be between 10 and 30
Note that it is not unusual to get an open reading, "1 .", when measuring wheel pickups. Sometimes the wheels stop at a point where there is not a good connection. Try moving the wheel back and forth to see if the connection improves.
|Testing Connecting Wires
Let's test the wires that connect from your power supply to the track. WITH THE POWER SUPPLY OFF, connect one of the meter's probes to one end of the connecting wire. It is OK to test without disconnecting from the power supply's terminals, just make sure the power is off. Touch the other probe to the track. You should see either zero or a very small resistance. Try wiggling the connections. The reading should not change. Test the other wire to the track.
In this photo we are testing the blue wire from the power supply to the near
rail. Note that the power is set to "0"!
|Testing Track Connections
If you suspect that the connection between two pieces of track is poor you
can use the same procedure to check the resistance across that connection.
|Track Test 2 - When measuring across a loose joint the resistance is much higher.|
|Track Test 3 - Pushing the two pieces of track together restores a good connection. Rail clamps, conductive grease or screws would help, too.|
|Testing Power Supplies
Set the selector to the DCV 200 setting. Note that we don't use the DCV 20 setting since most large scale power supplies can put out over 20 volts. Using the 20 volt setting will not hurt the meter but it will stop reporting a voltage when it reaches 20 volts.
Use the alligator clips to connect one probe to one of the power supply terminals and the other probe to the other terminal. Set the speed control to zero or "Stop" and turn on the power supply.
|Slowly advance the throttle and you should see a
gradual increase in voltage.
Note that power supplies that use PWM or PWC will not give an accurate voltage reading as they put out a constant, high voltage but vary the on/off time to control speed.
|Test the AC Auxiliary Output
If the power supply has an auxiliary or accessory connection test that as well. If it is marked AC change the selector to ACV 200. The power supply terminals should show a reading close to the voltage printed on the power supply.
|Measuring Voltage (probes in
paralle with the voltage measured)
When a power supply is connected to a locomotive or other load you can use the meter to measure how much current the load is drawing. Up to now we have put the probes in parallel with the voltage we have been measuring.
In this diagram the probes go across the battery (power supply) or motor. This is the procedure that is used to measure voltage.
Measuring Current (probes in series with the current measured)
When measuring current one must put the probes in series with the power supply and the load .
In this diagram the red probe is moved from the center to the top. The circuit that operates the motor is broken and the two probes are inserted in series with them to complete the circuit. This allows the power to flow through the meter so that current can be measured.
Put an engine on rollers or lay it on its side so that it can run without moving. Use the alligator clips to connect the power supply to the engine and get it running at low speed.
Unplug the red probe from the meter's center connection and reconnect it to the upper connection labeled 10ADC. Put the selector into the 10A position.
|Put the Meter in the Circuit
Disconnect one of the leads between the power supply and the engine. Use the meter's two probes to reconnect the power supply and locomotive.
|Add Some Resistance
The motor should start back up again and a current reading, in amps, should show on the meter's display. Most small engines should draw little more than 1 amp.
Increase the throttle on the power supply. The amperage reading may change a bit.
Push against one of the locomotive's drive wheels with a piece of wood to slow it down. The current reading should go up sharply showing that it needs more power to maintain its speed.
|Measure Current While Running on Track
Although it is convenient to test engines on rollers or other static test stands it is not necessary as the same tests can be made on a train running on a layout. Just disconnect one of the leads to the track and connect the power supply in series as we did in the static test.
|Do Not Exceed 10 Amps!
Make sure that you avoid connecting the meter to loads that have the potential to draw more than 10 amps. There is a fuse in the meter that will "blow" if 10 amps is exceeded.
|Recording Benchmarks for Equipment
You can test the "health" of your model railroad equipment by making periodic tests and noting any changes. When you get a new piece of electrical equipment make a few tests and record the results as benchmarks.
For a power supply record its maximum voltage. For locomotives measure the current drawn at different speeds and when pulling a number of cars.
If you think your equipment is failing test it again and compare the readings with the benchmarks that you made when it was working properly. This can give you a valuable clue in finding the problem.
If you have an engine that will not run you can make a few tests based on what you have learned so far.
(These tests are based on the assumption that the engine is running very poorly or not at all)