Electronics All-in-One For Dummies. Doug LoweЧитать онлайн книгу.

of the resistor. Resistor values aren’t exact, so even though you’re using a 470 Ω resistor in this circuit, the actual resistance of the resistor may be anywhere from 420 Ω to 520 Ω. For more about this effect, see Book 2, Chapter 2.)

6 Congratulate yourself!You have made your first official current measurement.

7 After a suitable celebration, replace the jumper wire you removed in Step 2.If you forget to replace the jumper wire, the procedure described in the next section for measuring voltage won’t work.

Warning There are two places in this circuit that you should not connect the ammeter. First, don’t connect the ammeter directly across the two battery terminals. This effectively shorts out the battery. It will get real hot, real fast. Second, don’t connect one lead of the ammeter to the positive battery terminal and the other directly to the LED lead. That will bypass the resistor, which will probably blow out the LED.

If you want to experiment a little more, try measuring the current at other places in the circuit. For example, remove the battery snap connector from the battery, and then reconnect it so that just the negative battery terminal is connected. Then, touch the red meter lead to the positive battery terminal and the black lead to the lead of the resistor that’s connected to the bus strip (not the lead that’s connected to the terminal strip). This measures the current by inserting the ammeter between the resistor and the battery. You should get the same value that you got when you measured between the LED and the resistor.

You can use a similar method to measure the current between the LED and the negative battery terminal. Again, the result should be the same.

Measuring voltage

Measuring voltage is a little easier than measuring current because to measure voltage, you don’t have to insert the meter into the circuit. Instead, all you have to do is touch the leads of the multimeter to any two points in the circuit. When you do, the multimeter displays the voltage that exists between those two points.

For example, Figure 8-7 shows how you can insert a voltmeter into the LED circuit so that you can measure voltage. In this case, the voltage is measured across the battery. It should read in the vicinity of 9.3 V. (9 V batteries generally provide a bit more than a full 9 V unless you’ve placed a load on the circuit.)

FIGURE 8-7: Using a voltmeter to measure voltage in the LED circuit.

To measure voltages in the LED circuit, first put the circuit back together (assuming you took it apart to measure currents). Then spin the multimeter dial to a range whose maximum is at least 10 V. Now just touch the leads to different spots in the circuit. To measure the voltage across the entire circuit as shown in Figure 8-7, touch the black lead to the LED lead that’s inserted into the negative bus strip, and touch the red lead to the resistor lead that’s inserted into the positive bus strip.

Tip If you connect the leads backward when measuring voltage, you’ll read a negative voltage. On a digital voltmeter, the meter will display a negative number. On an analog meter, the needle will move in the wrong direction. (Unless the voltage is large, this won’t damage your meter.)

Here’s an interesting exercise. Write down the following three voltage measurements:

Across the battery: Connect the red meter lead to the resistor lead that’s inserted into the positive bus strip and the black meter lead to the LED lead that’s inserted into the negative bus strip.

Across the resistor: Connect the red meter lead to the resistor lead that’s inserted into the positive bus and the black meter lead to the other resistor lead.

Across the LED: Connect the black meter lead to the LED lead that’s inserted into the negative bus and the red meter lead to the other LED lead.

What do you notice about these three measurements? (It’s a little bit of a puzzle, so I won’t give the answer here. But you find it in Book 2, Chapter 2.)

Measuring resistance

Measuring resistances is similar to measuring voltages, with a key difference:

Remember You must first disconnect all voltage sources from the circuit whose resistance you want to measure. That’s because the multimeter will inject a known voltage into the circuit so that it can measure the current and then calculate the resistance. If there are any outside voltage sources in the circuit, the voltage won’t be fixed, so the calculated resistance will be wrong.

Here are the steps for measuring resistance in the LED circuit:

1 Remove the battery.Just unplug it from the battery snap connector and set the battery aside.

2 Turn the meter selector dial to one of the resistance settings. If you have an idea of what the resistance is, pick the smallest range that’s greater than the value you’re expecting. Otherwise, pick the largest range available on your meter.

3 If you’re using an analog meter, calibrate it.Analog meters must first be calibrated before they can give an accurate resistance measurement. To calibrate an analog meter, touch the two meter leads together. Then, adjust the meter’s calibration knob until the meter indicates 0 resistance.

4 Touch the meter leads to the two points in the circuit for which you wish to measure resistance.For example, to measure the resistance of the resistor, touch the meter leads to the two leads of the resistor. The result should be in the vicinity of 470 Ω.

You can learn much more about measuring resistances in Book 2, Chapter 2. But until then, here are a few additional thoughts to tide you over:

When you measure the resistance of an individual resistor or of circuits consisting of nothing other than resistors, it doesn’t matter what direction the current flows through the resistor. Thus, you can reverse the multimeter leads and you’ll still get the same result.

Some components such as diodes pass current better in one direction than in the other. In that case, the direction of the current does matter. You can learn more about this effect in Book 2, Chapter 5.

Resistors aren’t perfect. Thus, a 470 Ω resistor rarely provides exactly 470 Ω of resistance. The usual tolerance for resistors is 5 percent, which means that a 470 Ω resistor should have somewhere between 446.5 Ω and 495.5 Ω of resistance. For most circuits, this amount of imprecision doesn’t matter. But in circuits where it does, you can use the ohmmeter function of your multimeter to determine the exact value of a particular resistor. Then, you can adjust the rest of your circuit accordingly. (More on this in Book 3, Chapter 2.)

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