Technical data

61
Just as an open in a circuit will act as an infinite amount of resistance, a short in a circuit will act as a
resistance value of zero. Since a small amount of resistance in a series circuit will have a small voltage
drop, a resistance value of zero will have zero voltage drop. Think of it as measuring the voltage drop
across the ends of a small piece of wire in a circuit.
In Figure T-2, R
1
has become shorted. This creates a situation where more current is flowing in the
circuit than normal. There will be no voltage across the short at R
1
and the total source voltage will be
measured across all the remaining components in the circuit.
Voltmeters are usually the desired tool for detecting problems
Figure T-2
in series circuits. If you noticed,
owever, the voltmeters in Figures T-1 and T-2 indicated the same voltage readings across the same
components in both scenarios. If there were a third resistor connected in series, this would be a much
easier circuit to troubleshoot because it would create a voltage divider that could be used to determine
the open condition in Figure T-1 or the shorted condition in Figure T-2.
With this example circuit, an ammeter could be used in addition to the voltmeter to measure the circuit
current. If a current is detected, then an open condition could not exist and the fault would most likely
be the scenario in Figure T-2. If no current flow is detected in the circuit, then an open will exist
somewhere and would suggest the scenario in Figure T-1.
An ohmmeter could also be used in addition to the voltmeter as long as the individual components or
the power supply can be disconnected from the circuit.
Troubleshooting Parallel Circuits
Parallel circuits can be very
challenging to troubleshoot. This is
arallel voltage
le: “The voltage drop across any
Figure T-3
h
V
A
12.00
mainly because of the p
ru
parallel branch equals the applied
voltage”. For example, in Figure T-3,
R
2
has become open. If a voltmeter
were used to troubleshoot this circuit,
the source voltage would be detected
across all three parallel branches and
the technician would be unable to
determine which of the three parallel
branches is faulted.
DC
V
A
12.
00
V
A
12.00
12 V
R
1
100
R
2
50
R
3

(open)
200

