5/21/2018 Lecture 3-2

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1Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

Voltage and Current

Division Rules

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2Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

To apply the voltage and current

division rules.

Objectives

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Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

Voltage Division

For example, we know

i = VTOTAL/ (R1 + R2 + R3)

so the voltage over the first resistoris

V1=i R1 = R1 VTOTAL/ (R1 + R2 + R3)

To find the voltage over an individual resistance in series, take the total series

voltage and multiply by the individual resistance over the total resistance.

321

1

1

RRR

RVv++

=

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Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

Example

determine the voltage across each resistor

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Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

Example

determine the voltage acrosseach resistor

21 RRvi s

s

+

=

s

s

vRR

Rv

vRR

Rv

21

2

2

21

1

1

+

=

+

=

iS

iS

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Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

SummaryThe source voltage v is divided among the resistors in direct proportionto their resistancesthe larger the resistance, the larger the voltage drop.

This is called the principle of voltage division,

In generalIf a voltage divider has N resistors (R1,R2, . . . , RN) in series with the

source voltage v, the nth

resistor (Rn) will have a voltage drop of

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7

Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

Current Division

If we know the current flowing into two parallel resistors, we can find out how

the current will divide up in one step.

1

1

R

vi =

+

=

21

21

1

1

1

RR

RRi

Ri

21

2

1

RR

Rii

+

=

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Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

R2 = 0 implies that i1 = 0, i2 = i.R2 = , that is, R2 is an open

R2, then

Req = R1

Note that this differs slightly from the voltage division formula for series resistors

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Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

+

=

+

=

+=+==

21

1

2

21

2

1

2

12

1

2

1

11

2

1

12

RR

Rii

RR

Rii

R

RRi

R

Riii

R

Rii

s

s

s

Example

Find i1 and i2 in terms of is

21 iii

s +=

2211 RiRi =

R1

is i

1 R

2 i2

v1 = v2

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Dr.-Eng. Hisham El-Sherif

Electronics and Electrical Engineering Department

ELCT708: Electronics for Biotechnology

Example

Use Voltage and current division rules to find Vo and io2

36

36=

+

Step 1

Step 2

Step 1