Electrical isolation
Aside from the ability to easily convert
between different levels of voltage and current in AC and DC
circuits, transformers also provide an extremely useful
feature called isolation, which is the ability to
couple one circuit to another without the use of direct wire
connections. We can demonstrate an application of this
effect with another SPICE simulation: this time showing
"ground" connections for the two circuits, imposing a high
DC voltage between one circuit and ground through the use of
an additional voltage source:
v1 1 0 ac 10 sin
rbogus1 1 2 1e-12
v2 5 0 dc 250
l1 2 0 10000
l2 3 5 100
k l1 l2 0.999
vi1 3 4 ac 0
rload 4 5 1k
.ac lin 1 60 60
.print ac v(2,0) i(v1)
.print ac v(3,5) i(vi1)
.end
DC voltages referenced to ground (node 0):
(1) 0.0000 (2) 0.0000 (3) 250.0000
(4) 250.0000 (5) 250.0000
AC voltages:
freq v(2) i(v1)
6.000E+01 1.000E+01 9.975E-05 Primary winding
freq v(3,5) i(vi1)
6.000E+01 9.962E-01 9.962E-04 Secondary winding
SPICE shows the 250 volts DC being impressed
upon the secondary circuit elements with respect to ground,
but as you can see there is no effect on the primary circuit
(zero DC voltage) at nodes 1 and 2, and the transformation
of AC power from primary to secondary circuits remains the
same as before. The impressed voltage in this example is
often called a common-mode voltage because it is seen
at more than one point in the circuit with reference to the
common point of ground. The transformer isolates the
common-mode voltage so that it is not impressed upon the
primary circuit at all, but rather isolated to the secondary
side. For the record, it does not matter that the
common-mode voltage is DC, either. It could be AC, even at a
different frequency, and the transformer would isolate it
from the primary circuit all the same.
There are applications where electrical
isolation is needed between two AC circuit without any
transformation of voltage or current levels. In these
instances, transformers called isolation transformers
having 1:1 transformation ratios are used. A benchtop
isolation transformer is shown in the following photograph:
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REVIEW:
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By being able to transfer power from one
circuit to another without the use of interconnecting
conductors between the two circuits, transformers provide
the useful feature of electrical isolation.
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Transformers designed to provide
electrical isolation without stepping voltage and current
either up or down are called isolation transformers.
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