Source Resistance.

Basic Electronics
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64

resistance of the battery (the opposition to current offered by the

electrolyte in the battery). All sources of electromotive force have some

form of internal resistance which causes a drop in terminal voltage as

current flows through the source.

This principle is illustrated in figure 40 (on the following page) where the

internal resistance of a battery is shown as Ri.

In the schematic, the

internal resistance is indicated by an additional resistor in series with

the battery. The battery, with its internal resistance, is enclosed within

the dotted lines of the schematic diagram.

With the switch open, the

voltage across the battery terminals reads 15 volts.

When the switch is

closed, current flows causing voltage drops around the circuit. The circuit

current of 2 amperes causes a voltage drop of 2 volts across Ri. The 1 Ohm

internal battery resistance thereby drops the battery terminal voltage to 13

volts.

Internal resistance cannot be measured directly with a meter; an

attempt to do so would damage the meter.

The effect of the source resistance on the power output of a dc source may

be shown by an analysis of the circuit in figure 41 on page 69. When the

variable load resistor (RL) is set at the zero Ohm position (equivalent to a

short circuit) current (T) is calculated using the following formula:

This is the maximum current that may be drawn from the source. The terminal

voltage across the short circuit is zero volts and all the voltage is across

the resistance within the source.

If the load resistance (RL) were increased (the internal resistance

remaining the same), the current drawn from the source would decrease. At

the same time, the terminal voltage applied across the load would increase

and approach a maximum as the current approaches zero amps.