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Electronics for dummies - McComb G.

McComb G., Boes E. Electronics for dummies - Wiley publishing, 2005. - 433 p.
ISBN: 0-7645-7660-7
Download (direct link): electronicsfordummies2005.pdf
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Except for really old analog models that only test voltage or current, all multimeters come equipped with a battery of one type or another. The most convenient multimeters use a standard-size battery, such as a 9-volt or AA cell. Pocket meters typically use a coin-type battery. If your local supermarket or drugstore doesnít carry replacement batteries for your meter, try Radio Shack or a photographic supply store.
The batteries in multimeters tend to last a long, long time ó that is, unless you forget to turn the meter off after using it. The batteries in multimeters can often last a year or longer under typical use. But eventually the battery dies, so be sure to keep a spare battery handy. We prefer alkaline batteries over standard-duty zinc cells, as they last longer.
If your multimeter uses a specialty battery, consider storing the spare in its original packaging in your refrigerator. It lasts longer that way. Take it out of the refrigerator a day before you plan to use it. That allows the battery to slowly come up to room temperature.
Nickel-cadmium and nickel metal hydride rechargeable batteries put out a slightly lower voltage than alkaline batteries of the same size. Most multimeters donít have a problem with this lower voltage. However, some meters may stop working or may give erratic or erroneous results when powered by a rechargeable battery. Check the manual that comes with your multimeter to be sure that your meter can handle rechargeable batteries.
Maximum range: Just how much is enough?
Thereís a limit to what a multimeter can test. You call that limit its maximum range. These days, most consumer multimeters have more-or-less the same maximum range for voltage, current, and resistance. Any meter that has the following maximum ratings (or better) should work just fine for your hobby electronics:
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186 Part IV: Getting Your Hands Dirty
DC volts: 1000 V AC volts: 500 V
DC current: 200 mA (milliamperes)
Resistance: 2 MQ (two megohms, or 2 million ohms)
Home on the automatic range
Most analog multimeters, and many digital ones, require that you select the range (see Figure 9-6) before the meter can make an accurate measurement. For example, if youíre measuring the voltage of a 9-volt transistor battery, you set the range to the setting closest to, and above, 9 volts. For most meters, this means you select the 20 or 50 volt range. You then read the voltage on the meter.
Be sure to read the result from the proper meter scale. If you select the 20-volt range, for example, you must use the 20-volt scale. Otherwise, you end up with inaccurate results.
You shouldnít find manually setting the range of your meter complicated, and the extra effort canít kill you. But these days, automatic ranging, especially for digital multimeters, is all the rage. So-called auto ranging meters donít require you to first set the test range. This feature makes them inherently easier to use and a little less prone to error. When you want to measure voltage, you set the meter function to Volts (either AC or DC) and take the measurement. The meter displays the results in the readout panel. Meters with an automatic ranging feature, like the one in Figure 9-7, donít require a separate range knob.
What if you need to test higher currents?
Most digital multimeters can measure current only less than one amp. The typical digital multimeter has a maximum range of 200 milliamperes. Attempting to measure substantially higher currents may cause the fuse in the meter to blow. Many analog meters, especially older models, support current readings of 5 or 10 amps, maximum.
You may find analog meters with a high ampere input handy if you're testing motors and circuits that draw a lot of current. If you have only a digital meter with a limited milliampere input, you can still measure higher currents indirectly by using a low-resistance, high wattage resistor. You can read more about this kind of resistor in Chapter 7.
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Figure 9-6:
Dial in the proper range before taking a measurement when using a multimeter without an automatic ranging feature.
Chapter 9: Making Friends with Your Multimeter 187
Figure 9-7:
automatic ranging multimeters, setting the desired test function automatically selects the range.
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188 Part IV: Getting Your Hands Dirty_
Whether analog or digital, the meter indicates an over range if the voltage or other measurement is too high for the meter to display. A digital multimeter typically shows over range as a flashing 1 (or OL). An analog meter shows over range as the needle going off the scale. If the meter is auto-ranging, and you see the over range indicator, it means that the value is too high to be measured by the meter. Such an over range indication is common when testing continuity. It simply means the resistance is so high that that meter cannot register it, even at its highest range setting.
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