Electronics All-in-One For Dummies. Doug Lowe
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A SNEAK PEEK AT OHM’S LAW
In Book 2, Chapter 2, you take a closer look at resistors and also take an in-depth look at Ohm’s law, which is one of the most important mathematical relationships in electronics. But without going into all the details of Ohm’s law here, I want to at least give you a sneak preview.
Ohm’s law describes a fundamental relationship between current, voltage, and resistance in an electrical circuit. Remember from Chapter 2 that voltage is a difference in electrical charge between two points, and that if those two points are connected by a conductor, current will flow through the conductor.
Other than exotic superconductors (which exist only in laboratory experiments), no conductor is perfect. All conductors have a certain amount of resistance that inhibits the flow of current. The greater this resistance, the less the current will flow. The less this resistance is, the more the current will flow.
Ohm’s law is a mathematical formula that formalizes the relationship between current, voltage, and resistance. The formula is this:
In other words, the amount of current running through a circuit is equal to the amount of voltage across the circuit divided by the amount of resistance in the circuit. The amount of current in amperes is represented by the letter I (don’t ask why; it just is). V represents voltage in volts, and R represents resistance in ohms.
Using basic math, you can use this equation to calculate the voltage if you know the current and the resistance. Then, the formula becomes:
In other words, voltage is equal to current times resistance.
Similarly, you can calculate resistance if you know the current and the voltage. Then, the formula becomes:
In other words, resistance equals the voltage divided by the current.
Using Your Multimeter
In the following sections, I show you how to use your multimeter to measure current, voltage, and resistance in a simple circuit. The circuit being measured consists of just three components: a 9 V battery, a light-emitting diode (LED), and a resistor. The schematic for this circuit is shown in Figure 8-4.
FIGURE 8-4: A simple circuit with a battery, a resistor, and an LED.
If you want to follow along with the measuring procedures detailed in the following sections, you can build this circuit on a solderless breadboard. You’ll need the following parts:
Small solderless breadboard
470 Ω, ¼ W resistor
Red LED, 5 mm
9 V battery snap connector
9 V battery
Short length of jumper wire (1 inch or less)
Figure 8-5 shows the circuit installed on the breadboard. Here are the steps for building this circuit:
1 Connect the battery snap connector.Insert the black lead in the top bus strip and the red lead in the bottom bus strip. Any hole will do, but it makes sense to connect the battery at the very end of the breadboard.
2 Connect the resistor.Insert one end of the resistor into any hole in the bottom bus strip. Then, pick a row in the nearby terminal strip and insert the other end into a hole in that terminal strip.
3 Connect the LED.Notice that the leads of the LED aren’t the same length; one lead is shorter than the other. Insert the short lead into a hole in the top bus strip, and then insert the longer lead into a hole in a nearby terminal strip. Insert the LED into the same row as the resistor. In the figure, both the LED and the resistor are in row 26.
4 Use the short jumper wire to connect the terminal strips into which you inserted the LED and the resistor.The jumper wire will hop over the gap that runs down the middle of the breadboard.
5 Connect the battery to the snap connector.The LED will light up. If it doesn’t, double-check your connections to make sure the circuit is assembled correctly. If it still doesn’t light up, try reversing the leads of the LED (you may have inserted it backwards). If that doesn’t work, try a different battery.
FIGURE 8-5: The LED circuit assembled on a breadboard.
Measuring current
Electric current is measured in amperes, but actually in most electronics work, you’ll measure current in milliamps, or mA. To measure current, you must connect the two leads of the ammeter in the circuit so that the current flows through the ammeter. In other words, the ammeter must become a part of the circuit itself.
The only way to measure the current flowing through the LED circuit that’s shown in Figure 8-4 is to insert your ammeter into the circuit. Figure 8-6 shows one way to do this. Here, the ammeter is inserted into the circuit between the LED and the resistor.
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.
To measure the current in the LED circuit, follow these steps:
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,