26-11-2012, 02:05 PM
Parallel dc Circuits
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Introduction
There are two network configurations – series and parallel.
In Chapter 5 we covered a series network. In this chapter we will cover the parallel circuit and all the methods and laws associated with it.
Parallel Resistors
Two elements, branches, or circuits are in parallel if they have two points in common as in the figure below
For resistors in parallel, the total resistance is determined from
Note that the equation is for the reciprocal of RT rather than for RT.
Once the right side of the equation has been determined, it is necessary to divide the result into 1 to determine the total resistance
Kirchhoff’s Current Law
Most common application of the law will be at the junction of two or more paths of current.
Determining whether a current is entering or leaving a junction is sometimes the most difficult task.
If the current arrow points toward the junction, the current is entering the junction.
If the current arrow points away from the junction, the current is leaving the junction.
Current Divider Rule
The current divider rule (CDR) is used to find the current through a resistor in a parallel circuit.
General points:
For two parallel elements of equal value, the current will divide equally.
For parallel elements with different values, the smaller the resistance, the greater the share of input current.
For parallel elements of different values, the current will split with a ratio equal to the inverse of their resistor values.
Open and Short Circuits
A short circuit can carry a current of a level determined by the external circuit, but the potential difference (voltage) across its terminals is always zero volts.
Voltmeter Loading Effects
A good practice is to always check the meter resistance against the resistive elements of the network before making a measurement.
Most DMMs have internal resistance levels in excess of 10 MW on all voltage scales.
The internal resistance of a VOM depends on the scale chosen.
Internal resistance is determined by multiplying the maximum voltage of the scale setting by the ohm/volt ( / V) rating of the meter, normally found at the bottom of the face of the meter.