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Electrical Isolation Transformer

Away 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 show 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:(Figure below)

Transformer isolates 10 Vac at V1 from 250 VDC at V2.

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, (Figure above) but as you can see there is no result 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 case is often called a common-mode voltage because it is seen at more than one point in the circuit with reference to the general point of ground. The transformer isolates the common-mode voltage so that it is not impressed upon the primary circuit at all, but quite isolated to the secondary side. For the record, it does not subject that the common-mode voltage is DC, either. It can 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 required between two AC circuit without any transformation of voltage or current levels. In this case, transformers called isolation transformers having 1:1 transformation ratios are used. A bench top isolation transformer is shown in Figure below.

Isolation transformer isolates power out from the power line.

Analysis:

* By being able to transfer power from one circuit to another without the use of interconnecting conductors between the two circuits, transformers provide the valuable feature of electrical isolation.

* Transformers designed to provide electrical isolation without stepping voltage and current either up or down is called isolation transformers.