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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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Testing diodes
A diode is the simplest form of semi-conductor. Diodes perform a lot of odd jobs in electronics circuits, including changing AC current to DC, blocking
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Chapter 9: Making Friends with Your Multimeter 203
voltages, limiting voltage, and lighting up your life. You can test whether or not the diode functions correctly if you have a digital multimeter that has a diode-check setting.
To test a diode using a multimeter with a diode-check feature, perform these steps:
1. Dial the meter to the diode-check setting.
2. Apply the test probes of the meter to the diode.
Observe proper polarity: Attach the red test lead to the anode (negative terminal) of the diode, and the black test lead to the cathode (the positive terminal; the cathode has a stripe so that you can identify it). Remember to avoid touching the test probes with your fingers.
3. Observe the reading.
4. Reverse the probes and test again.
Table 9-2 shows you how to interpret your test results. Although this test works for most diode types, it doesn’t give a proper reading for light-emitting diodes. But you can often test light-emitting diodes visually.
Table 9-2 Display Value
1st Test 2nd Test Condition
About 0.5* Over range Good
Over range Over range Bad — open
Zero Zero Bad — short
* The exact reading isn't critical, as long as its fairly low but not zero.
Diode testing with
If you have an analog multimeter, you can test most types of diodes by using the resistance setting and following these steps:
1. Set the meter to a low-value resistance range.
2. Connect the black lead to the cathode (striped end) and the red lead to the anode.
an analog meter
The multimeter should display a low resistance.
3. Reverse the leads.
The multimeter should display infinite resistance.
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204 Part IV: Getting Your Hands Dirty
Testing capacitors
You use capacitors to store electrons for a short period of time. Capacitors can die an early death for a number of reasons, so use your multimeter to find out which ones you need to bury because of
Old age: Certain types of capacitors, mainly those with a liquid electrolytic, can dry out over time. When they’re dry, they stop working.
Too much voltage: All capacitors are rated for a specific working voltage; apply voltage beyond what the capacitor is rated for and you can damage the capacitor.
Reversed polarity: A polarized capacitor, which has a + or - sign marked on it, can literally blow apart if you connect it to the circuit backward.
You can check a capacitor using a multimeter that doesn’t have a special capacitor-testing feature. You don’t always get conclusive results, but the results you do get can help point the way to whether you should replace a component. Follow these steps to test without a capacitor-testing feature:
1. Before testing, use an insulated bleeder jumper (see Figure 9-16) to short out the terminals of the capacitor. You can make this jumper yourself. A bleeder jumper is simply a wire with a 1 or 2 megohm resistor attached. The resistor prevents the capacitor from being shorted out, which makes it unusable.
This step discharges the capacitor. You need to short out the terminals because large capacitors can retain a charge for long periods of time, even after you remove power.
Figure 9-16:
Purchase or make a bleeder jumper, useful for draining excess charge from a capacitor.
BLEEDER JUMPER
2 MQ
2. Dial the meter to Ohms.
3. Touch the meter probes to the terminals of the capacitor. Wait a second or two and then note the reading.
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Chapter 9: Making Friends with Your Multimeter 205
A good capacitor shows a reading of infinity when you perform this step. A reading of 0 (zero) may mean that the capacitor has shorted out. A leaky capacitor, one that is losing its ability to hold its charge, gives an ohms reading somewhere between infinity and zero.
If you are working with a polarized capacitor, connect the black lead to the - (negative) terminal of the capacitor and the red lead to the + (positive) terminal. For unpolarized capacitors, it doesn’t matter how you connect the leads.
This test doesn’t tell you if the capacitor is open, which can happen if the component becomes structurally damaged inside or if its dielectric (insulating material) dries out or leaks. An open capacitor will read infinite ohms.
For a conclusive test, use a multimeter with a capacitor-testing function.
If your multimeter has a capacitor-testing feature, by all means use it rather than the method that we give you here. Refer to the manual that came with your meter for the exact procedure because the specifics vary from model to model. Be sure to observe proper polarity when connecting the capacitor to the test points on the meter.
You get another advantage by using a multimeter with a capacitor testing feature because the meter displays the value of the capacitor. You may find this measurement handy if you need to determine whether a capacitor falls within the tolerance range for your circuit. This feature also helps to verify the value markings on the component because not all capacitors follow the industry standard identification schemes.
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