Electrical Isolation with Transformers: SPICE Analysis & Practical Applications

SPICE Analysis of Electrical Isolation

Transformers not only enable voltage and current conversion in AC and DC circuits but also deliver a critical feature known as electrical isolation. This capability allows two circuits to be magnetically coupled without any direct wire connections, safeguarding sensitive components and enhancing safety.

To illustrate this effect, we run a SPICE simulation that imposes a high DC voltage on the secondary side while grounding the two circuits:

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 that the 250 VDC is present on the secondary circuit relative to ground, yet the primary circuit registers zero DC voltage at nodes 1 and 2. The AC transformation remains unchanged, confirming that the transformer effectively blocks the common‑mode voltage from the primary side.

The voltage applied to the secondary is often called a common‑mode voltage because it is referenced to a common point (ground) and appears at multiple nodes. The transformer’s isolation prevents this voltage from influencing the primary circuit.

This isolation property holds regardless of whether the common‑mode voltage is DC or AC, even at a different frequency.

In applications that require identical voltage and current levels on both sides, 1:1 isolation transformers are used. These devices maintain the same voltage and current while providing electrical isolation. A typical benchtop isolation transformer is shown below.

Isolation transformer isolates power from the mains.

• Transformers enable power transfer between circuits without direct conductors, offering electrical isolation.
• Isolation transformers are specifically designed to provide this isolation without changing voltage or current ratios.

• Step‑up, Step‑down, and Isolation Transformers Worksheet