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You can also test current that flows through a portion of the circuit, or you can even test a single component. Figure 9-10 shows how to test the current through an LED. Make this test with the meter dialed to the Milliamperes setting.
All current measurements use this setup. You insert the meter in series with the circuit, as you can see in Figure 9-10. Connect the black lead either to ground, if testing the current draw of the entire circuit, or to the more negative side of the circuit. If you find that you get no reading at all, reverse the connections of the leads to the multimeter and try again.
After making a current test, return the meter dial to Off. This habit helps prevent damage to the meter.
Testing current involves connecting the meter in series with the circuit or component.
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194 Part IV: Getting Your Hands Dirty
Don't blow your fuse!
Remember that hooking up a circuit or component that draws more current than your multimeter is rated to handle can lead to big problems. You run the risk of blowing the fuse in the meter, and then you have to replace the fuse before you can use the multimeter again.
Many analog and digital meters provide a separate input for testing current. If your multimeter has this input, it's usually marked as A (for amps)
or mA (for milliamps). Be sure to use this input when testing current. Some multimeters have a separate input for testing higher currents, typically up to 10 amps. Typically, this input is marked as 10A.
Be sure to select the proper input before making any current measurement. Forgetting to do this step may either damage your meter (if you're unlucky) or blow a fuse (if you're lucky).
Testing wires and cables for continuity
Continuity tests whether a circuit is complete or not. We can describe continuity most clearly using a wire as the circuit:
A short circuit shows that your circuit has continuity between two points of the same wire. The meter shows this state as 0 (zero) ohms.
^ An open circuit means that your circuit doesn’t have continuity. There is a break somewhere inside the wire in the circuit. The meter shows this situation as infinite ohms, which means so many ohms that the meter can’t register them.
When testing a cable with many wires, you also want to determine if any of the individual wires are touching each other. When this situation happens, the wires short out. If a short happens, your circuit fails, so you want to make this test every time things go wrong.
Follow the diagram in Figure 9-11 for testing a wire by using these procedures:
^ Test for continuity in a single wire. Connect the multimeter probes to either end of the wire. You should get a reading of 0 (zero) ohms, or very low ohms. A reading of more than just a few ohms indicates a possible open circuit.
^ Test for a short between different wires that shouldn’t be electrically connected. Connect the multimeter probes to any exposed conductor of the two wires. You should get a reading other than zero ohms. If the reading shows zero or very low ohms, it indicates a possible short circuit.
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Chapter 9: Making Friends with Your Multimeter 195
TESTING FOR CONTINUITY
Connect the meter probes to the points that this figure indicates to test a wire.
TESTING FOR A SHORT
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196 Part IV: Getting Your Hands Dirty
Even wire resists the flow of electrons
Why don't you always get zero ohms when testing wire, especially a long wire? All electrical circuits have a resistance to the flow of current; the ohms measurement tests this resistance. Even short lengths of wire have a resistance, but it's usually well less than 1 ohm and so not an important test subject for continuity or shorts.
However, the longer the wire, the more the resistance, especially if the wire has a small diameter. Usually, the larger the wire, the lower its resistance per foot. Even though the meter doesn't read exactly zero ohms, you can assume proper continuity in this instance if you get a low ohms reading.
If you’re testing two different wires that shouldn’t be electrically connected in a circuit, you get a reading on the multimeter of infinite ohms, showing an open circuit, right? Most of the time that statement is true. But it’s not always the case. Here’s the reason: Even though the wires may not be directly joined, they’re both connected to the circuit. This connection, whatever it is, may show a certain resistance when tested on the multimeter. So when you’re looking for shorts across wires, don’t be too worried if you get a reading other than infinite ohms.
Mechanical switches can get dirty and worn, and they can sometimes just plain break. When your switch becomes a bit worse for wear, it may no longer pass electrical current when you want it to.
Testing a wide Variety of switches
Follow the diagram in Figure 9-12 to test a switch. As we discuss in Chapter 5, the most basic switch is the single-pole, single-throw, or SPST. You can readily identify such a switch by its two terminals: One acts as an inlet for the electrical current coming into the unit, and the other acts as an outlet. The switch passes or interrupts the current, depending on its position.