Using a multimeter

Volt/amp meters or multimeters are one of the most useful tools in model railroading after a soldering iron, yet many modelers are mystified by them. Before we take a look at some of the most common uses for them in model railroading let me go over some options.

Although you can still purchase inexpensive analog models I don’t recommend them. These usually have a needle type readout with multiple scales on the display and some kind of control knob to select functions. These can be difficult for a new user to figure out. Consequently I suggest a digital meter. These have a control knob for selecting the functions and a digital display screen so the direct readout leaves no guesswork. They are available in varying degrees of quality and price, just remember you get what you pay for. I think I paid about $39 for the Extech DM110 shown a couple years ago.

With the digital models you have a choice of autoranging and manual ranging readouts. Autoranging means that the electronics in the device automatically displays the measured values within a given range and will usually display the correct units. Manual meters on the other hand require that the user select an expected readout range from among several possible choices. The autoranging devices take care of the mental gymnastics required to select a range and make the devices more user friendly.

For example you might be measuring the resistance of some wiring and not know whether to expect a result in the milliohms  or ohms range–an autoranging device will automatically select that for you. They are a little more expensive but in my opinion are worth the extra money. There are also units that offer the ability to measure and test transistors and diodes, among other things. I can personally say that I never used that feature on my old unit so skipped it  when I bought a new autoranging one a couple years ago. Ok, let’s take a look at what you can measure with a multimeter.

Voltage

One of the most common measurements is voltage–either AC or DC. Multimeters have two probe tips for making measurements. By convention the red one should be positive and the black one negative. Using the control knob you select volts and touch the probes to the contacts on the device or circuit you want to measure and it should display the value. If a negative value is displayed it means you have connected the red and black probes backwards. This is a quick way to determine which wire in a circuit is positive and negative and provides a good way to check polarity. Also some units will automatically decide whether you are measuring AC or DC and display the correct value whereas on others you may need to make the selection manually.

Voltage measurements can be useful to test a transformer to confirm it is putting out the correct voltage or even working at all. It also can be useful for making sure you are getting full voltage on tracks, and determining how much voltage drop is occurring in a wiring bus. The simplest measurement might be to check the voltage of a battery–just touch the probes to the battery contacts and read the voltage.

Amperage

Measurements of how much current is being drawn by a circuit or electrical device are also often required. For example you may want to know how much current a motor is drawing to see if it is operating properly or simply if it pulls too much current for a decoder. To measure amperage the probes are attached in a different manner than with voltage. The meter must be connected in one leg of the circuit instead of across it as with voltage. For example to measure the current draw of a motor you would connect one probe to a wire to the motor and the other probe to the power pack. A second wire from the motor would go to the other contact on the power pack completing the circuit. Units of measurement for current typically are in the milliamps range although for a larger application you might get into the full amps range.

Resistance

Resistance measurements in ohms are very common especially when you want to know how much resistance a length of wire or track might have. It can also be useful to determine whether you have a good solder connection or a high resistance track connection. I also find that when I need to sort a bunch of resistors it is a lot faster to just measure their resistance instead of deciphering the color bands on them.

Measurements of resistance are made across the device or wire or piece of track. For example to measure the resistance of the points in a switch put one probe tip on the stock rail and the other probe tip on the point rail–the resulting measurement will tell you whether the points might be poorly conductive due to corrosion or dirt, or highly conductive. Similarly to measure the value of a resistor put one tip on one side of the resistor and the other tip on the other side.

Continuity is another resistance measurement. Basically with continuity you either have a continuous circuit or none at all. So a resistance measurement will immediately tell you which you have–either you will have infinite resistance or no resistance at all. Some meters have a special continuity setting and will beep or buzz if there is a good electrical connection and no sound if there is none.

A multimeter is also a useful tool around the house. I have used mine to keep appliances running, fix broken lamps, and diagnose all kinds of household problems. For example to determine whether a heating element in your water heater is blown you can do a resistance measurement to check for continuity (turn the power off first). I have saved thousands of dollars in repair bills with multimeters over the years. So now you have an excuse when your significant other questions that charge on your next credit card bill for a multimeter.

4 comments

  • Dwaine Armentrout

    Great overview of what can be checked with a multimeter and the meaning of autoranging. Thanks for explaining it clearly.

  • Larry,

    Cheap multimeters should be used for measuring low voltage circuits such as track rails in a layout. They are only “safe” for lower voltage conditions. They are seldom designed with circuits to handle higher voltage loads safely. Cheap multimeters are potentially a hazard. The probes supplied with cheap multimeters can also cause hazards in higher voltage conditions.

    When we enter into measurement of mains circuits, your multimeter better be a good one. It should be approved by UL, CSA or other such safety agencies. Choose a CAT III multimeter for mains measurements; and CAT IV multimeter for measurements close to the mains source.

    There should be no fear concerning the use of a multimeter. However, we must be aware of the potential hazards in working with household current. Much safety information is available on the internet.

    Ron Pope

    • Ron–that is a good warning for all. I can’t remember ever having used any of my multimeters for AC mains work. However they are great for looking for open circuits in small appliances and just about everything related to model railroading.

  • Larry as you know I am challenged with using a multimeter so I greatly appreciate this post. It was very helpful. For instance I did not know the difference between an auto and manual ranging meter. Thanks for the clarification.