Thumbnail Guide to Digital Multimeters
By Gareth Branwyn
[from Absolute Beginner's Guide to Building Robots]
A digital multimeter (DMM) is an extremely handy tool to have around for any sort of electronics work, from testing to see if the current is actually off in your house, to finding out how much power is left in your MP3 player’s batteries, to finding out the value and state of components for your electronic projects.
A multimeter has “multi” in its name because it contains a number of different measuring instruments for testing electronic components and their processes. Some DMMs are more “multi” than others. The most basic features usually include:
■ Direct Current (DC) Voltage Measuring — Tells you how many volts (or how much electrical potential) are available. Expressed in volts (V).
■ DC Current Measuring — Tells you how many electrons (the little beasties that make electricity possible) are racing through your circuit. Measured in amps (A).
■ Alternating Current (AC) Voltage Measuring — Same as with DC voltage. Also expressed in volts (V).
■ AC Current Measuring — Same as with DC current. Also expressed in amps (A).
■ Resistance — Lets you know the rating of a resistor or how much resistance exists in a circuit. Measured in ohms (Ω).
■ Continuity Test — Lets you know if an electrical connection exists between two points. Measured in ohms (Ω).
■ Diode Check — Enables you to check the health of diodes, transistors, and other semiconductors. You also can check the polarity of diodes (diodes have a negative side and a positive side, and they’re not always clearly marked).
The preceding features cover all the basics and you should make sure that your meter at least handles all of them. A meter with these capabilities can be had for as little as $15 (especially if you look for sales). If you have a few more bucks burning a hole in your cargo pants, these features are worth adding:
■ Capacitance —Can be used to measure storage capacity of capacitors. Measured in farads (F).
■ Frequency — As the name implies, this meter measures how fast (per second) an electrical event is occurring. Measured in hertz (Hz).
■ Duty Cycle — This measures the percentage of “on” time for a given interval of an electronic signal. Measured in percent (%). This feature is often part of a frequency meter.
■ Auto-ranging — Without this feature, you have to guess what the maximum value of the component you’re testing might be (and dial that value on the multimeter's dial). The auto-ranging feature is for those of us with the naïve belief that computers should do this sort of thing for us. Here, you just set the DMM on auto-ranging and it does the guessing for you. What a concept!
■ Data Hold—You’ll find that it can be difficult to hold the probes steady on components being tested and look at the screen at the same time to read the peak value. This feature leaves the results of your test on the screen until you do another test.
■ Auto Off — Turns your meter off after a set time of being idle. Nice to have to save on battery life.
■ Stand — If you can get a DMM with a stand, do it. It’s hard to see the screen when the meter is lying flat on your workbench.
■ Boot/Holster — A boot (or holster) is a rubber jacket used to protect the sides and back of your DMM from shock or damage. Some meters come with them, some are sold as accessories. It’s nice to have one for added protection. Some
boots also incorporate stands.
You’ll also need to get some extra test leads. Your meter will come with a set of leads with straight needle probes on them. To use these, you simply touch the tips of the probes to the appropriate area of the circuit or component you want to test. But you have to continue holding the probes. For situations in which you’ll need hands-free testing, you’ll want to get a set of leads with hook probes, and a set with alligator clips (for times when the little hooks can’t fit on the item to be tested). These three types of leads should cover any testing situation.
