Jimmie Fouts

Houston County Career Academy

OBJECTIVES

Review individual series and parallel DC circuit calculations and

operation

Determine the equivalent circuit resistance for a given

combination circuit

Determine the voltage drops in a circuit

Determine the current values in a circuit

Apply combination circuit theory to troubleshoot a combination

circuit

REVIEW : DC SERIES CIRCUIT CALCULATION

R1

R2

10

Ω

30

Ω

Rt = R1 + R2 = 10

Ω

+ 30

Ω

= 40

Ω

Es = 20 volts

It = Es / Rt = 20 volts / 40

Ω

= 0.5 amps

I1 = I2 = It

E1 = R1 X I1 = 10 ohm X 0.5 amp = 5 volts

E2 = R2 X I2 = 30 ohm X 0.5 amp = 15 volts

REVIEW: DC PARALLEL CIRCUIT CALCULATION

Es = 15 volts

R1

R2

10

Ω

30

Ω

DC COMBINATION CIRCUITS

•

Often called complex circuits

•

Not complicated when considered as

individual series and parallel circuits

•

E, I, & R solved using Ohm’s Law!

BASIC PROCEDURE FOR DC CALCULATIONS

Reduce the Complex Circuit to a Simple Equivalent

Reduce the circuit to Individual Series and Parallel Circuits

Perform Calculations on the Individual Circuits

Combine Calculations as Appropriate

COMBINATION CIRCUIT EXAMPLE

10

20

20

A 10 ohm resistor is

In series with two 20 ohm

resistors

Reduce to individual series

and parallel circuit

DETERMINE SIMPLE SERIES PARALLEL PARTS

In the parallel portion of the circuit,

the equivalent of two 20 ohm resistors

is 10 ohms

R1

R2

R3

Parallel Circuit Equivalent

Is 10 Ohms

COMBINE CALCULATIONS AS APPROPRIATE

The equivalent circuit is a

10 ohm resistor in series

with the equivalent circuit

resistor of 10 ohms

Rt = R1 + Re = 10 + 10

Rt = 20 ohms

R1

Re

10

10

Rt

USE OHMS LAW FOR CIRCUIT VALUES

Lets now apply a source voltage of

20 volts. The voltage drop across R1

will be 10 volts.

E = 20 volts

CALCULATIONS CONTINUED

CALCULATION OF PARALLEL CIRCUIT VALUES

R2

R3

Is calculated in the same manner

THE RESULTING CIRCUIT VALUES

A summary of the circuit values would therefore be:

ohms

ohms

ohms

= 20 volts

1 amp

10 volts

10 volts

0.5 amps

10 volts

0.5 amp

TROUBLESHOOTING COMBINATION CIRCUITS

Look for obvious damaged components

Take

voltage readings

of

individual

components (

observe safety requirements)

Look for an open or a short circuit condition

Check against

expected values

IMPORTANT SAFETY NOTE!

Before making any resistance measurements in

a circuit, ensure there NO power is being

supplied to the circuit! Failure to follow this

caution will likely result in equipment damage

.

CALCULATE EXPECTED VALUES!

•

It is important to first calculate the expected values before making

measurements using the meter!

•

Using the calculated values, prior to selecting the scale to use, will help prevent

meter damage.

•

Some things to consider:

•

An open across a resistance in a series circuit will result in a measurement of the

source voltage across the defective resistor.

•

An open across a resistance in a parallel circuit will be difficult to identify, without

calculating the expected value. Remember, the voltage across a parallel circuit is

the same on all legs of the circuit. An open on a parallel circuit leg will result in

the circuit exhibiting a greater resistance and larger voltage drop across the

parallel circuit.

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