Voltage Meter (DVM/VOM)

The digital volt meter or volt-ohm-meter is is primarily used as a device to measure voltages, currents and resistance, although some models have additional functions for measuring capacitance, transistors and diodes and even more. The DVM/VOM will likely become the most-used diagnostic tool in your toolkit, especially when you're building electronic projects. You don't need to spend a lot of money on one, but understand that a $75 DVM would probably be more accurate and have more capabilities than a $7 unit. Also know that these devices use batteries and batteries don't last forever. As the battery discharges the meter will become less accurate, which can easily cause the misdiagnosis of a problem. Replace the battery at least annually. They also don't bounce when you drop them -- at least they don't bounce very well or very many times. Take care of your DVM.

Selector Dial

Most meters have a dial that is used to select a specific mode for the meter to be in. Typical meter selections are to measure A/C voltage, DC voltage, resistance (in Ohms) and current (in either milliamps or full amps). When measuring something for the first time, select the HIGHEST range first. This gives you more margin for error and will help prevent damaging the meter. For example, if you were measuring the A/C voltage of portable generator's A/C outlet, you would select the highest range for A/C testing which might be 600 volts, and then make your first test. You can always dial down to a lesser level once you get an initial reading. Meters are more accurate when the measuring range is closer to and just slightly larger than the item being tested, so instead of seeing 120 on the DVM, you might get 120.5 volts instead. It doesn't seem that a half volt is very important and much of the time, it isn't. But when you get down to low voltage DC, a half of a volt can mean the difference to whether a part functions or not.

• In the above example, the top white portions of the dial on either side of the "OFF" mark are voltage ranges; the left side is DC (V---) while the right side is for AC current (marked with the V~). Most meters have more range options for measuring DC current than AC. The selector dial is pointed to 20 DC which means the meter is set to measure voltages up to 20vdc. Good meters have built-in protection to prevent meter damage in case the range selected isn't high enough for the voltage being measured, but it's always best to start high and work your way lower as needed. But as you gain experience, you'll find that most DC voltages on controller circuit boards fall into the <20vdc range.

• The lower left green section of this example dial shows the Greek omega symbol, which stands for "Ohms." This is the measure for resistance. Setting the dial to the 20k mark would set the meter to measure from 0-20,000 ohms of resistance. The K represents thousands of ohms; M is millions.

• The green section on the right side marked A--- stands for current. Setting the dial to the 20m mark would set the meter to measure up to 20 milliamps of current. The bottom setting, 10A would measure up to 10 Amps, a very high value. Milliamps are thousandths of an amp (.001 = 1ma). At the very top is 200u, which is microamps (.000001 = 1ua). Most digital electronics on a controller circuit board is usually in the milliamps ranges while measuring the current a light string might use is usually in the amps range or at least higher than the 200ma setting on this meter example.

• This meter dial also has a setting for hFE (for measuring transistors) and another curious red setting down near the bottom of the dial. That's a continuity tester that emits a beep if you're testing something just to see if there's a completed circuit. Remember, for electricity to work there must be a source and return to have a completed circuit. A continuity tester can be helpful for testing plugs or wires to see if there's a break in the wire.

• There's a "backlight" button next to the dial on this meter which illuminates the display for a few seconds -- a handy feature for times when you might be outside at night or in other low-light situations.

Safety

You'll be measuring live circuits. Sometimes those circuits carry only a handful of volts which is harmless. Other times, they might be carrying a lethal voltage that can knock you across the room if you're not careful. The test leads on your meter have insulated handles. Never touch the metal part of the lead to anything but the items to be tested, and never touch the leads with your finger. The leads generally have sharp points for a reason -- to help you zero-in on a specific point. If you wear glasses for better vision, always wear them when you're using your meter so you don't accidentally miss the proper test points. Missing a test point can cause a short circuit and damage something -- maybe even you. Also remember to use the proper kinds of test leads. Meters usually come with a pair of "pencil-type" leads which require you to usually hold one with each hand. If you need to attach a pair of leads to a circuit yet have one or both of your hands free, use leads that have spring clips so you can clamp them on the parts so the leads won't slip and possibly touch some other "hot" parts of the circuit and cause a short.