Voltage Dividers and Current Dividers · 2012. 2. 25. · Voltage Dividers and Current Dividers Topics Covered in Chapter 7 7-1: Series Voltage Dividers 7-2: Current Dividers with

Voltage Dividers Voltage Dividers

and Current Dividersand Current Dividers

Topics Covered in Chapter 7

7-1: Series Voltage Dividers

7-2: Current Dividers with Two Parallel Resistances

7-3: Current Division by Parallel Conductances

7-4: Series Voltage Divider with Parallel Load Current

7-5: Design of a Loaded Voltage Divider

ChapterChapter

77

© 2007 The McGraw-Hill Companies, Inc. All rights reserved.

77--1: Series Voltage Dividers1: Series Voltage Dividers

VT is divided into IR voltage drops that are proportional

to the series resistance values.

Each resistance provides an IR voltage drop equal to its

proportional part of the applied voltage:

VR = (R/RT) × VT

This formula can be used for any number of series

resistances because of the direct proportion between

each voltage drop V and its resistance R.

The largest series R has the largest IR voltage drop.

McGraw-Hill © 2007 The McGraw-Hill Companies, Inc. All rights reserved.


The Largest Series R Has the Most V.

V2

=R

2

RT

× VT

999 kΩ

1000 kΩ= × 1000 V = 999 V

V1

=R

1

RT

× VT

=1 kΩ

1000 kΩ× 1000 V = 1 V

KVL check: 1 V + 999 V = 1000 VFig. 7-2a: Example of a very small R1 in

series with a large R2; V2 is almost

equal to the whole VT.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


Voltage Taps in a Series Voltage Divider

Different voltages are available at voltage taps

A, B, and C.

The voltage at each tap point is measured with respect to ground.

Ground is the reference point.

Fig. 7-2b: Series voltage divider with voltage taps.



Note: VAG is the sum of the voltage across R2, R3, and R4.

VAG is one-half of the applied voltage VT, because R2+R3+

R4 = 50% of RT.

VCG =

1 kΩ

20 kΩ

× 24 V = 1.2 V

VBG =

2.5 kΩ

20 kΩ

× 24 V = 3 VVAG = 12 V

Voltage Taps in a Series Voltage Divider


77--2: Current Dividers with 2: Current Dividers with

Two Parallel ResistancesTwo Parallel Resistances

IT is divided into individual branch currents.

Each branch current is inversely proportional to the

branch resistance value.

For two resistors,

R1 and R2, in parallel:

Note that this formula can only be used for two branch

resistances.

The largest current flows in the branch that has the

smallest R.

IR

R RI

T12

1 2

=+

×

77--2: Current Dividers with 2: Current Dividers with

Two Parallel ResistancesTwo Parallel Resistances

Current Divider

Fig. 7-3: Current divider with two branch

resistances. Each branch I is inversely

proportional to its R. The smaller R has

more I.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

I1 = 4 Ω/(2 Ω + 4 Ω) × 30A = 20A

I2= 2 Ω /(2 Ω + 4 Ω) × 30A = 10A

77--3: Current Division by 3: Current Division by

Parallel ConductancesParallel Conductances

For any number of parallel branches, IT is divided into

currents that are proportional to the conductance of the

branches.

For a branch having conductance G:

I = × IT

G

GT

77--33:: Current Division by Current Division by


Fig. 7-5: Current divider with branch conductances G1, G2, and G3, each equal to 1/R. Note that

S is the siemens unit for conductance. With conductance values, each branch I is directly

proportional to the branch G.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

G1 = 1/R1 = 1/10 Ω = 0.1 S

G2 = 1/R2 = 1/2 Ω = 0.5 S

G3 = 1/R3 = 1/5 Ω = 0.2 S

77--3: Current Division by 3: Current Division by


KCL check: 5 mA + 25 mA + 10 mA = 40 mA = IT

The Siemens (S) unit is the reciprocal of the ohm (Ω)

GT = G1 + G2 + G3

= 0.1 + 0.5 + 0.2GT = 0.8 S

I1 = 0.1/0.8 x 40 mA = 5 mAI2 = 0.5/0.8 x 40 mA = 25 mAI3 = 0.2/0.8 x 40 mA = 10 mA

77--4: Series Voltage Divider with 4: Series Voltage Divider with

Parallel Load CurrentParallel Load Current

Voltage dividers are often used to tap off part of the

applied voltage for a load that needs less than the total

voltage.

Fig. 7-6: Effect of a parallel load in part of a series voltage divider. (a) R1 and R2 in series without

any branch current. (b) Reduced voltage across R2 and its parallel load RL. (c) Equivalent circuit

of the loaded voltage divider.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

77--4: Series Voltage Divider with4: Series Voltage Divider with


V1 = 40/60 x 60 V = 40 V

V2 = 20/60 x 60 V = 20 V

V1 + V2 = VT = 60 V

(Applied Voltage)

Fig 7-6

77--4: Series Voltage Divider with 4: Series Voltage Divider with


The current that passes through all the resistances in the voltage

divider is called the bleeder current, IB.

Resistance RL has just its load current IL.

Resistance R2 has only the bleeder current IB.

Resistance R1 has

both IL and IB.

Fig. 7-6

77--5: Design of a 5: Design of a

Loaded Voltage DividerLoaded Voltage Divider

Fig. 7-7: Voltage divider for different voltages and currents from the source VT.Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

77--5: Design of a 5: Design of a


I1 through R1 equals 30 mA

I2 through R2 is 36 + 30 = 66 mA

I3 through R3 is 54 + 36 + 30 = 120 mA

V1 is 18 V to ground

V2 is 40 − 18 = 22 V

V3 is 100 V (Point D) − 40 = 60 V

77--5: Design of a5: Design of a


R1 = V1/I1 = 18 V/30 mA = 0.6 kΩ = 600 Ω

R2 = V2/I2 = 22 V/66 mA = 0.333 kΩ = 333 Ω

R3 = V3/I3 = 60 V/120 mA = 0.5 kΩ = 500 Ω

NOTE: When these values are used for R1, R2, and R3 and connected in a voltage divider across a source of 100 V, each load will have the specified voltage at its rated current.

Voltage Dividers and Current Dividers · 2012. 2. 25. · Voltage Dividers and Current Dividers Topics Covered in Chapter 7 7-1: Series Voltage Dividers 7-2: Current Dividers with

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