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Chapter 9: Making Friends with Your Multimeter 179
Taking a Close-Up Look at Multimeters
Multimeters arenít particularly complicated, but the following sections give you some factoids that you should know before you choose one or use one. We cover the basic functions shared by all meters, some of the dials that provide meter readouts, issues related to meter accuracy and the supplies that come with the meter. You also need to know whether the meter automatically adjusts itself to display the most accurate result possible (called auto-ranging) and whether it has special testing features for checking diodes, capacitors, and transistors.
Basic features of every meter
Stripped down to its skivvies, a multimeterís purpose is to take the three basic measurements of electronics: voltage, current, and resistance.
Hello, any voltage or current in there?
You test voltage and current with a circuit powered up. Typical voltage and current tests include
Checking the voltage level of a battery. You can even check the voltage when youíre using the battery. In fact, many consider this test more accurate when the battery is providing power ó what electronics folks call under load.
^ Determining if a circuit or component is drawing too much current.
If the circuit has more current going through it than itís designed to handle, then the components may get overheated and you can permanently damage your circuit.
^ Verifying that the proper voltage reaches a component, such as a light-emitting diode or switch. These kinds of checks can help you pinpoint the location of a problem in your circuit. You use multimeter tests to narrow down the field of suspects until you find the culprit causing all your headaches.
Checking out the resistance movement
You almost always test resistance (measured in ohms, as we talk about in Chapter 1) with the circuit unpowered. Resistance tests may involve an entire circuit or just an individual component. You can check up on wires, resistors, motors, and many other kinds of electronic doodads.
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180 Part IV: Getting Your Hands Dirty
Beep beep goes the continuity test
A feature found on many multimeters, like the one shown here, is audible continuity testing. To use this feature, you turn the meter's control dial
(more about dial turning in the section "Making sense of all the inputs and dials") to Continuity or Tone
You may find this feature handy when you check the wiring of a circuit. If a wire or connection has continuity (a shorted circuit), the meter beeps. If the wire or connection doesn't have continuity (an open circuit), the meter stays
silent. The audible tone gives you a handy way to check a whole circuit without having to keep your eye on the multimeter. Most meters made these days have this feature, and we recommend it.
Resistance, or the absence of it, can reveal short circuits and open circuits; so-called continuity of electrical components. When you perform these tests, a shorted circuit shows zero (or virtually zero) resistance and an open circuit shows infinite resistance. You can use continuity tests to check for breaks in wires.
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Chapter 9: Making Friends with Your Multimeter
Here are some other tests you can perform with a multimeter that rely on resistance:
Fuses: A blown fuse shows an open circuit.
Switches: Flipping the switch should alternate the multimeterís reading between zero (shorted) and infinite (open) resistance.
Circuit board traces: A bad copper trace on a printed circuit board acts like a broken wire and shows up as infinite ohms (open circuit) on the multimeter.
Solder joints: A bad joint may read as an open circuit on the multimeter, showing infinite resistance.
Making sense of all the inputs and dials
Check out Figure 9-3 to see the main points of interest on the typical multimeter. Hereís what they all mean:
Meter face or digital readout: Analog multimeters have a meter face consisting of a set of graduated scales and a precision needle indicator.
A digital multimeter has a numeric readout.
Function knob: Dial the knob to the test that you want to perform: Voltage, Current, Resistance, or whatever. On meters without an autoranging feature, you also typically use the function knob to set the maximum range of the value that you want to test. If you set the maximum range to be just higher than the value you are testing ó whether voltage, resistance, current, or whatever ó you are assured of the most accurate reading possible. If your meter does have an auto-ranging feature, it will automatically adjust itself to give you the most accurate reading.
Test lead inputs: At a minimum, the multimeter has a + (positive) and -(negative or common) lead input. You insert the test leads into these inputs. Some meters have additional inputs for high current testing (usually marked A, for amperage) and special sockets for testing transistors and capacitors, as you can see in Figure 9-4. Note: Many small, pocket multimeters have the leads permanently attached.