Electronics All-in-One For Dummies. Doug Lowe
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Название: Electronics All-in-One For Dummies

Автор: Doug Lowe

Издательство: John Wiley & Sons Limited

Жанр: Отраслевые издания

Серия:

isbn: 9781119822134

isbn:

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      FIGURE 8-6: Using an ammeter to measure current flow in the LED circuit.

      Note that it doesn’t matter where in this circuit you insert the ammeter. You’ll get the same current reading whether you insert the ammeter between the LED and the resistor, between the resistor and the battery, or between the LED and the battery.

      1 Set your multimeter’s range selector to a DC milliamp range of at least 20 mA.This circuit uses direct current (DC), so you need to make sure the multimeter is set to a DC current range.

      2 Remove the jumper wire that connects the two terminal strips.The LED should go dark, as removing the jumper wire breaks the circuit.

      3 Touch the black lead from the multimeter to the LED lead that connects to the terminal strip (not the bus strip).

      4 Touch the red lead from the multimeter to the resistor lead that connects to the terminal strip (not the bus strip).The LED should light up again, as the ammeter is now a part of the circuit, and current can flow.

      5 Read the number on the multimeter display.It should read between 12 mA and 13 mA. (The exact reading will depend on the exact resistance value 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.

      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.

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

      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 СКАЧАТЬ