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Neg ativ
DMM Set to Read Current
FIGURE 52 A digital multimeter can be used to measure the voltage across a component as well as the current through it. Negativ e
ti Posi ve
DCV 20 DCA 200m
Find: i = From Ohm's Law Triangle: Result: i = V R
V i R
FIGURE 53 Using the Ohm s law triangle, voltage, current, or resistance can be calculated when the other two values are known. 5.2 WIRE GAUGE
V R T = R1 + R2 + R3 R1 R2 R3 VR1= VR2= VR3= V x R1 R1 + R2 + R3 V x R2 R1 + R2 + R3 V x R3 R1 + R2 + R3 FIGURE 54 When resistors are wired in series, the total resistance of the circuit is proportional to the sum of the resistances. R1 Vout R2
Vout =
Vin x R2 R1 + R2
FIGURE 55 Two resistors can be used to change an input voltage to another, lower value.
1 1 1 + R1 R2
1 Rn
FIGURE 56 Placing resistors in parallel will reduce the total resistance of the circuit.
ELECTRONIC COMPONENTS
different components in the circuit. As the electrons pass through a component they lose some of the pressure, just as water loses pressure due to friction when it moves through a pipe. The initial voltage applied to the electrons is measured with a volt meter or a multimeter set to measure voltage and is equal to the voltage drops through components in the circuit. The label given to voltage is V. The second measurement that can be applied to electricity is current, which is the number of electrons passing by a point in a given time. There are literally several billion, billion, billion, billion, billion, billion electrons flowing past a point at a given time. For convenience, the unit Coulomb (C) was specified, which is 6.25 1018 electrons and is the basis for the ampere (A), which is the number of electrons moving past a point every second. The label given to current is the nonintuitive i. The voltage across a component and the current through it can be measured using a digital multimeter as shown in Fig. 52. It is important to remember that voltage is the pressure change across a component, so to measure it you have to put a test lead on either side of the component. Current is the volume of electrons moving past a certain point every second, and to measure it, the circuit must be broken and the tester put in line, or in series, with the component being measured. The current flowing through a component can be calculated if the voltage change, or drop, is known along with the resistance of the component using Ohm s law. This law states that the voltage drop across a resistance is equal to the product of the resistance value and the current flowing through it. Put mathematically, Ohm s law is: V=i R Where V is voltage across the component measured in volts, i is the current through the component measured in amperes or amps, and R is the resistance measured in ohms, which has the symbol . Using algebra, when any two of the three values are known, the third can be calculated. If you are not comfortable using algebra to find the missing value, you can use Ohm s law triangle (Fig. 53). This tool is quite simple to use. Just place your finger over the value you want to find, and the remaining two values along with how they are located relative to one another shows you the calculation that you must do to find the missing value. For the example in Fig. 53, to find the formula to calculate current, put your finger over i and the resulting two values V over R is the formula for finding i (divide the voltage drop by the resistance of the component). Resistances can be combined, which changes the electrical parameters of the entire circuit. For example, in Fig. 54 resistances are shown placed in line or in series and the total resistance is the sum of the resistances. Along with this, the voltage drop across each resistor is proportional to the value of the individual resistors relative to the total resistance of the circuit. The ratio of voltages in a series circuit can be used to produce a fractional value of the total voltage applied to a circuit. Fig. 55 shows a voltage divider, which is built from two series resistors and outputs a lower voltage than was input into the circuit. It is important to remember that this circuit cannot source (provide) any current any current draw will increase the voltage drop through the top resistor and lower the voltage of the output. Finally, resistances can be wired parallel to one another as in Fig. 56. In this case, the total resistance drops and the voltage stays constant across each resistor (increasing the total

