Voltage dividers are devices that make it possible to obtain more than one voltage from a single power source. A voltage divider usually consists of a resistor, or resistors connected in series, with fixed or movable contacts and two fixed terminal contacts. As current flows through the resistor, different voltages can be obtained between the contacts.
Series circuits are used for voltage dividers. The voltage divider rule allows the technician to calculate the voltage across one or a combination of series resistors without having to first calculate the current in the circuit. Because the current flows through each resistor, the voltage drops are proportional to the ohmic values of the constituent resistors.
A typical voltage divider is shown in Figure 10-83.
To understand how a voltage divider works, examine Figure 10-84 carefully and observe the following:
Each load draws a given amount of current: I1, I2, I3. In addition to the load currents, some bleeder current (IB) flows. The current (IT) is drawn from the power source and is equal to the sum of all currents.
The voltage at each point is measured with respect to a common point. Note that the common point is the point at which the total current (IT) divides into separate currents (I1, I2, I3).
Each part of the voltage divider has a different current flowing in it. The current distribution is as follows:
Through R1 — bleeder current (IB)
The voltage across each resistor of the voltage divider is:
90 volts across R1
The voltage divider circuit discussed up to this point has had one side of the power supply (battery) at ground potential. In Figure 10-85 the common reference point (ground symbol) has been moved to a different point on the voltage divider.
The voltage drop across R1 is 20 volts; however, since tap A is connected to a point in the circuit that is at the same potential as the negative side of the battery, the voltage between tap A and the reference point is a negative (-) 20 volts. Since resistors R2 and R3 are connected to the positive side of the battery, the voltages between the reference point and tap B or C are positive.
The following rules provide a simple method of determining negative and positive voltages: (1) If current enters a resistance flowing away from the reference point, the voltage drop across that resistance is positive in respect to the reference point; (2) if current flows out of a resistance toward the reference point, the voltage drop across that resistance is negative in respect to the reference point. It is the location of the reference point that determines whether a voltage is negative or positive.
Tracing the current flow provides a means for determining the voltage polarity. Figure 10-86 shows the same circuit with the polarities of the voltage drops and the direction of current flow indicated.
The current flows from the negative side of the battery to R1. Tap A is at the same potential as the negative terminal of the battery since the slight voltage drop caused by the resistance of the conductor is disregarded; however, 20 volts of the source voltage are required to force the current through R1 and this 20-volt drop has the polarity indicated. Stated another way, there are only 80 volts of electrical pressure left in the circuit on the ground side of R1.
When the current reaches tap B, 30 more volts have been used to move the electrons through R2, and in a similar manner the remaining 50 volts are used for R3. But the voltages across R2 and R3 are positive voltages, since they are above ground potential.
Figure 10-87 shows the voltage divider used previously. The voltage drops across the resistances are the same; however, the reference point (ground) has been changed. The voltage between ground and tap A is now a negative 100 volts, or the applied voltage.
The voltage between ground and tap B is a negative 80 volts, and the voltage between ground and tap C is a negative 50 volts.
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