You can buy a multimeter at a well-stocked hardware store, at most Radio Shack retail outlets, and through various websites. A typical multimeter is about the same size as a pocket calculator or small e-book reader. Analog meters have old-fashioned “needle-and-scale” type readouts. Digital meters
actually show you the numbers. The type that you buy depends on your personal preference. I have both an analog multimeter (at left in Fig. 2-10) and a digital multimeter (at right in Fig. 2-10).
FIGURE 2-10 At left, an analog multimeter. At right, a digital multimeter.
Warning!
Whenever you use a multimeter to test any circuit in which you suspect that the voltage will exceed 12 V (the level produced by an automotive battery), wear your rubber gloves and a good pair of rubber-soled shoes. That way, you’ll make absolutely sure that you can’t receive a dangerous shock. You can never tell when high voltage will “sneak in” somewhere and try to zap you. (It’s almost as if gremlins lurk out there in the electrical cosmos, waiting for an opportunity to clobber you the instant you let your guard down.) In addition, by insulatingyourself completely from the circuit under test, you’ll ensure that your body’s internal resistance can’t throw off the meter reading. This phenomenon can occur especially when you measure extremely high resistance values or low-current levels.
My analog multimeter has several graduated scales, and a rotary switch that offers 14 settings.
When the switch points straight up, the meter is turned off. (You should keep the meter turned off when you’re not using it because it contains a battery that will gradually discharge if you leave the meter powered-up, even if it sits idly on a shelf.) Going clockwise from the “OFF” position, the switch allows measurement of the following thirteen quantities, in order:
1. DC volts (DCV) from 0 to 10 2. DCV from 0 to 50
3. DCV from 0 to 250 4. DCV from 0 to 500
5. AC volts (ACV) from 0 to 500 6. ACV from 0 to 250
7. ACV from 0 to 50
8. DC milliamperes (DCmA) from 0 to 250, where one milliampere (1 mA) equals a thousandth of an ampere (0.001 A)
9. DCmA from 0 to 25
10. Battery test (BAT) for 1.5-volt (1.5-V) cells 11. Battery test (BAT) for 9-volt (9-V) batteries 12. Resistance in thousands of ohms (×1 k) 13. Resistance in tens of ohms (×10)
Quick Question, Quick Answer
• When I look at an analog multimeter, I notice that the resistance scale goes
backwards. That is, 0 is on the far right-hand end of the scale, and the left-hand end has a little sideways 8. Why does the resistance scale go backwards?
• When you measure resistance, the meter reading depends on the amount of current flowing through the device that you’re testing. The meter’s internal battery forces a certain current through the component, and that current depends on the resistance. As the resistance goes down, the current goes up. That’s why the scale goes backwards.
The sideways 8 at the left-hand end of the scale means “infinity ohms.” That’s an open circuit, where no current flows at all.
On any analog meter that can measure resistance, you’ll find a little knob that allows you to adjust the meter for the correct zero reading. It will say “0 ADJ” or something like that. The “horseshoe”
symbol is an uppercase Greek letter omega, which stands for “ohms.” To use this control, set the meter switch to the resistance range that you intend to use, short the red and black test probes directly together, and turn the knob until the meter needle goes all the way to the right-hand end of the
resistance scale and comes to rest at the hash mark for 0 ohms. Don’t turn the knob past that point.
The needle should hover exactly over the 0 marker. Use this adjustment control whenever you change the meter from one resistance scale to another, and also if you haven’t used your meter for awhile.
Make it a habit, like adjusting the rear-view mirrors in a car before you drive it. As the battery grows weak, the meter’s accuracy will degrade unless you take advantage of this control before every
resistance test.
Did You Know?
When you want to use any meter to measure a quantity of known type (DC volts, for example), you should start with the highest scale and work your way down. That way, if the quantity that you want to measure exceeds the maximum scale value, the meter’s needle won’t forcibly hit the pin at the top end.
Caution!
When you want to measure the resistance between two points in a circuit, make sure that the device under test is powered down (switched off) before you connect a multimeter to it.Otherwise, you’ll probably get an inaccurate reading. You might even damage your meter, disrupt the operation of the circuit under test, or do both.
My digital multimeter has a rotary switch with 20 positions. As with the analog meter, the top switch position represents “OFF.” Going clockwise from there, the switch allows for measurement of nineteen quantities, in this order:
1. AC volts (V~) from 0 to 500, with a special insert for the red test-probe wire 2. V~ from 0 to 200
3. DC amperes (A—) from 0 to 200 microamperes (the switch says 200μ), where one microampere (1 μA) equals a millionth of an ampere (0.000001 A)
4. A— from 0 to 2000 μA (the switch says 2000μ) 5. A— from 0 to 20 mA (the switch says 20m) 6. A— from 0 to 200 mA (the switch says 200m)
7. A— from 0 to 10, with a special insert for the red test-probe wire (the switch says 10 A) 8. A blank spot that doesn’t do anything as far as I know
9. Diode test (a diode should conduct in one direction but not the other) 10. DC resistance in ohms (Ω) from 0 to 200
11. Ohms (Ω) from 0 to 2000
12. Ohms (Ω) from 0 to 20,000 (the meter says 20k) 13. Ohms (Ω) from 0 to 200,000 (the meter says 200k) 14. Ohms (Ω) from 0 to 2,000,000 (the meter says 2000k)
15. DC voltage (V—) from 0 to 200 millivolts (the switch says 200m), where one millivolt (1 mV) equals a thousandth of a volt (0.001 V)
16. V— from 0 to 2000 mV (the switch says 2000m) 17. V— from 0 to 20
18. V— from 0 to 200
19. V— from 0 to 600, with a special insert for the red test-probe wire
Did You Know?
The red test-probe wire should always go to the more positive point in a DC circuit or system, whether you measure current or voltage. In an AC situation, it doesn’t matter which probe goes where. When you measure resistance, it sometimes matters, and sometimes doesn’t.