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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