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When using an analog multimeter, avoid over range conditions because these conditions can damage the precision needle movement. For this reason, always dial in the highest scale that you believe you need when using an analog meter and then work your way down. This approach avoids the needle slamming against its stops (the upper limit reading) in an over range.
Extra nice-to-haVe functions
As we discuss in the section ďThe Basics of Multimeters,Ē earlier in this chapter, all standard multimeters let you measure AC volts, DC volts, current, and resistance. Beyond these functions, digital multimeters vary in the number and type of functions that they provide. Here are some extra functions that can make the testing process a little easier and a bit more accurate:
Test the operation and value of capacitors. Because test leads can influence capacitance readings, most multimeters with a capacitor-testing feature provide separate input sockets. Plug the capacitor into these sockets and take the reading.
Test whether or not a diode is operational. Digital meters with this feature have a special Diode test setting. Note that most analog meters can also test the proper operation of diodes using a low resistance scale. See the section titled ďTesting diodes,Ē later in this chapter, for details on how to do this test.
Test whether or not a transistor is operational. Both analog and digital multimeters can perform simple testing of bipolar transistors. When using an analog meter, you can usually test the transistor in the same way that you test a diode. When using a digital meter, you test the transistor by using specially marked transistor input sockets.
Auto-zero a multimeterís reading. For digital multimeters only, the meter automatically sets a proper zero point before taking a measurement. For analog multimeters, and some digital models, you have to first set the meter to zero. Your meterís manual outlines the precise method that you need to use.
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Chapter 9: Making Friends with Your Multimeter 189
Setting Up the Meter
Before using your meter, you must make sure that itís working properly. Any malfunction gives you incorrect testing results, and you may not even realize it.
Modern meters, especially the digital kind, require batteries. Check and replace the batteries as needed. If your meter comes with a low battery indicator or light, note when it activates and replace the meterís battery (or batteries) right away. Use only fresh alkaline batteries. Most meters arenít designed to run from rechargeable nickel-cadmium (NiCad) batteries, which deliver slightly lower voltage than their alkaline counterparts. Unless the instruction manual indicates otherwise, donít use NiCads to power your meter.
To test your multimeter, follow these steps:
1. Turn on the meter and dial it to the Ohms (Q) setting.
If the meter isnít auto-ranging, set it to low ohms.
2. Plug both test probes into the proper connectors of the meter and then touch the ends of the two probes together, as Figure 9-8 shows you.
Touch the test probes of the meter to test proper continuity and
3. The meter should read 0 (zero) ohms or very close to it.
If your meter doesnít have an auto-zero feature, press the Adjust (or Zero Adjust) button. On analog meters, rotate the Zero Adjust knob until the needle reads 0 (zero). Keep the test probes in contact and wait a second or two for the meter to set itself to zero.
Here are some important points to keep in mind when youíre testing a multimeter:
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790 Part IV: Getting Your Hands Dirty
Avoid touching the ends of the metal test probes with your fingers while youíre performing the test. The natural resistance of your body can throw off the accuracy of the meter.
^ Check to be sure that the test probes at the end of the test leads are clean. Dirty or corroded test probes can cause inaccurate results. Clean the probes with electronic contact cleaner, available at Radio Shack. Clean both ends of the test probes and, if necessary, the connectors on the meter.
^ Double-check the dial setting of the meter. Make sure that you have it set to Ohms. If you donít have an auto ranging multimeter, set the range dial to the lowest Ohms setting.
You can consider the meter calibrated when it reads zero ohms with the test probes shorted together (held together so that theyíre touching each other). Do this test each time you use your meter, especially if you turn off the meter between tests.
Testing the resistance of good ol' water
You can use your multimeter for a simple science experiment that not only demonstrates the process of measuring resistance, but also how much crud your drinking water contains. (Yuck!) Here's how:
1. Get two clean glasses.
2. Rinse out both glasses with distilled water.
You can get distilled water at the supermarket.
3. Fill one glass with the distilled water and the other glass with tap water.