Diagnostic Tools

You can have all the best and most expensive diagnostic tools on the planet but if you don't use them properly, the diagnostic information they provide may be meaningless or worse, completely misleading which can easily send you into erroneous solutions to non-problems.

DVM/VOM Meter

The digital volt meter or volt-ohm-meter is is primarily used as a device to measure voltages, and 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. Things to know about them:

• 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.
• 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 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. In this example, 1/2 volt isn't that important but when you get down to low voltage DC, a half of a volt can mean the difference to whether a part works or not.

• 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.

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