Electronics All-in-One For Dummies. Doug Lowe
range, you can easily tell differences of a tenth of a volt, such as the difference between 1.6 and 1.7 V. But when the range is set to the 500 V range, you’ll be lucky to pick out differences of 10 volts.
On/off switch: Some multimeters don’t have an on/off switch. Instead, one of the positions on the selector dial is Off. Other multimeters have a separate on/off switch. If your meter doesn’t give you any readings, check to make sure the power switch is turned on or that the battery doesn’t need to be replaced.
Test leads: The test leads are a pair of red and black wires with metal probes on their ends. One end of these wires plugs into the meter. You use the other end to connect to the circuits you want to measure. The red lead is positive; the black lead is negative.
What a Multimeter Measures
A meter is a device that measures electrical quantities. A multimeter, therefore, is a combination of several different types of meters all in one box. At the minimum, a multimeter combines three distinct types of meters (ammeter, voltmeter, and ohmmeter) into a single device, as described in the following sections.
Ammeter
As you learn in Chapter 2 of this minibook, current is the flow of electric charge through a conductor. Current is measured in units called amperes. It should come as no surprise, then, that a meter that measures amperage is called an ammeter.
Don’t ask me why the p is dropped to form the word ammeter, rather than ampmeter. After all, the short form of the word ampere is amp, not am. Go figure.
Very few electronic circuits have currents so strong that they can be measured in actual amperes. So ammeters usually measure current in milliamperes, also called a milliamp and usually abbreviated mA. One mA is one-thousandth of an ampere; in other words, there are 1,000 milliamps in an amp.
After fooling around with it a bit, Hans discovered that the more current he ran through the wire, the farther the needle strayed away from north. It didn’t take him long to figure out that this discovery could be utilized to measure the amount of current flowing through a circuit. Analog ammeters work by this very same principle even today.
Hans Christian Oersted wasn’t the famous writer of children’s stories; that was Hans Christian Andersen. In a strange twist of history, though, Hans Christian Oersted was close friends with Hans Christian Andersen. It’s entirely possible that they were the founding members of some secret society of Evil Mad-Scientist Children’s Book Writers. Perhaps we’ll read about them in Dan Brown’s next novel.
Voltmeter
In Chapter 2 of this minibook, you learn about a second fundamental quantity of electricity, voltage, a term that refers to the difference in electric charge between two points. If those two points are connected to a conductor, a current will flow through the conductor. Thus, voltage is the instigator of current.
The unit of voltage is, naturally, the volt, and a device that measures voltage is called a voltmeter.
It turns out that, all other things being equal, a change in the amount of voltage between two points results in a corresponding change in current. Thus, if you can keep things equal, you can measure voltage by measuring current, and you already know of a device that can measure current: It’s called an ammeter.
The basic difference between an ammeter and a voltmeter is that in an ammeter, you let current run directly through the meter so that you can measure the amount of current. In a voltmeter, the current is first run through a very large resistor and then through the ammeter, and the device makes the necessary calculations as follows.
You haven’t learned it yet (unless you’re eagerly skipping around the book), but in Book 2, Chapter 1, you find out that there’s a direct relationship between voltage, resistance, and current in an electrical circuit. In particular, if you know any two of these quantities, the third one is easy to calculate. In a voltmeter, a large fixed resistance is used, and the ammeter measures the current. Because you know the amount of the fixed resistance and the amount of current, you can easily calculate the amount of voltage across the circuit.
Don’t worry; you don’t have to do any math to calculate this voltage. The voltmeter does the math for you. In an analog voltmeter, the calculation is built in to the scale that’s printed on the meter, so all you have to do is look at the position of the needle on the scale to read the voltage. In a digital voltmeter, the voltage is automatically calculated and displayed digitally.
Ohmmeter
As you know, a resistor is a material that resists the flow of current. How much the current is restricted is a function of the amount of resistance in the resistor, which is measured in units called ohms. The symbol for ohms is the Greek letter omega (Ω). A device that measures resistance is called an ohmmeter.
Like voltage, resistance can also be measured with an ammeter. Remember how I say in the previous section that there’s a direct relationship between voltage, resistance, and current in any circuit, and that if you know any two of these quantities you can easily calculate the third? To recap, to measure voltage, a voltmeter provides a fixed resistance, uses an ammeter to measure the current, and then uses the resistance and current to calculate the voltage.
To measure the resistance of a circuit, an ohmmeter provides a fixed amount of voltage across the circuit, uses an ammeter to measure the current that flows through the circuit, and then uses the amount of voltage provided by the meter and the amount of current read by the meter to calculate the resistance.
As with voltage, you don’t have to do this calculation; the calculation is automatically made by digital multimeters and is built in to the meter scale for analog multimeters. Thus, all you have to do is read the display or the needle on the meter to determine the resistance.
Other measurements
All multimeters can measure current, voltage, and resistance. Some multimeters can perform other types of measurements as well. For example, some meters can measure the capacitance of capacitors, and some meters can test diodes or transistors. These features are handy, but not essential.
Schematic symbols for meter functions
Ammeter, voltmeter, and ohmmeters are often included in schematic diagrams. When they are, the following symbols are used:
| Symbol | Meaning |
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