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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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Putting the Logic Probe to Work
No doubt, you’re dying to see the logic probe in action. In the following sections, we run down some safety issues that you need to be aware of before you start, take you through the steps of using a logic probe to test a circuit, and tell you just what the readings you get from the probe may mean.
Observe the usual safety precautions, please
The same safety precautions that you use with a multimeter apply when you use a logic probe, only more so. We won’t repeat those precautions here, but you should take a quick look at Chapter 9 before you actually start using a logic probe.
Safety is even more important with a logic probe than with a multimeter because the logic probe is an active-circuit tester. You have to turn the circuit on in order to test it. This requirement is not always true of the multimeter, with which you can conduct certain tests, such as continuity (testing whether a circuit is complete), without applying any power to the circuit.
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212 Part IV: Getting Your Hands Dirty
Take special care if the circuit that you’re testing runs off AC power and you need to expose the power supply components to perform the test. You may find yourself in this situation, for instance, if you’re trying to figure out why your VCR is on the fritz. Always consider that you may expose dangerously high voltages when you remove the cover from any AC-operated equipment. If you’re working close to equipment that conducts these voltages, cover the equipment with insulating plastic to prevent accidental shock.
Connecting the probe to the circuit
The logic probe has four connections, as you can see in Figure 10-4. The red and black leads use alligator clips so that you can securely attach them to ground and the power supply of the circuit that you’re testing.
Be sure to first determine if the supply voltage of the circuit falls within the acceptable range for the logic probe. Most probes work with a minimum supply voltage of about 3 volts and a maximum of no more than 15 volts (sometimes more, sometimes less). For the exact voltage range of your logic probe, check the manufacturer’s instruction booklet.
RED LEAD
R2
R1
C1
Figure 10-4:
The logic probe requires four connections to the circuit.
BLACK LEAD
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 213
You need to make these four connections:
^ You clip the black power lead to circuit ground.
^ Clip the red power lead to the circuit voltage supply. Be sure that this supply doesn’t exceed about 15 volts, or you can damage the logic probe.
^ You connect a second black ground lead to circuit ground. This separate ground is important; if you fail to solidly connect the probe to circuit ground, the probe may fail to work, or it may yield erratic results.
^ Place the tip of the probe against the part of the circuit that you’re testing.
When you’ve made these connections, observe the probe’s reaction. Indicator lights and audible tones (on most logic probes) help you determine the logic level at the test point:
^ Low indicator (accompanied by the low buzz tone): This reaction tells you that the test point has a logic low (at or about 0 volts).
^ High indicator (accompanied by the high buzz tone): This result indicates that the test point has logic high (usually at or about 5 volts).
^ Quickly toggling Low and High indicator: This reaction means that the logic signal is pulsing (changing between low and high at a fast pace). Note: Most logic probes have a separate indicator that tells you when a circuit is pulsing.
^ No indicator: If you get nothing from your probe, the test point has no discernable high, low, or pulsing signal.
What if the indicator doesn't indicate?
You may find logic circuits bewildering beasts, especially if you’re new to working with them. In some instances, the output of a logic circuit may yield no indication of a signal. When you get this indication from your probe it doesn’t necessarily mean that you have a faulty circuit. (But remember that, in many cases, no signal means that your circuit does have a problem.) When the logic probe gives no indication of a signal, the lack of signal can mean that you have either a bad circuit or an incorrectly connected logic probe, or both.
When you’re trying to figure out why the probe doesn’t react to your circuit, having a wiring diagram or some type of schematic for that circuit comes in handy.
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214 Part IV: Getting Your Hands Dirty
You can help rule out the chance that you’ve incorrectly connected the probe by conducting this quick test:
1. Touch the test lead of the probe to the power supply of the circuit.
The probe should indicate a high value.
2. Next, touch the test lead of the probe to the ground of the circuit.
Now, the probe should indicate a low value.
3. If either or both tests fail, examine the connection of the probe to the circuit and make corrections as necessary.
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