Isolation Transformers have primary and secondary windings that are physically separated from each other. Sometimes isolation transformers are referred to as "insulated".
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This is because the windings are insulated from each other. In an isolation transformer the output winding will be isolated, or floating from earth ground unless bonded at the time of installation. Secondary neutral to ground bonding virtually eliminates common mode noise, providing an isolated neutral-ground reference for sensitive equipment and an inexpensive alternative to the installation of dedicated circuits and site electrical upgrades.
An isolation transformer allows an AC signal or power to be taken from
one device and fed into another without electrically connecting the
two circuits. Isolation transformers block transmission of DC signals
from one circuit to the other, but allow AC signals to pass. They also
block interference caused by ground loops. Isolation transformers with
electrostatic shields are used for power supplies for sensitive
equipment such as computers or laboratory instruments. Isolation
transformers are different from auto transformers in which the primary
and secondary share a common winding.
Isolation transformers can accomplish a number of tasks :
- The primary and secondary windings may be constructed to step-up or step-down the output voltage. For example, the transformer can accomplish voltage matching between a 120 V load and an electrical system that measures 208 V.
- Isolation transformers constructed with Faraday shields, will improve power quality by attenuating higher frequency noise currents.
- Isolation transformers provide better impendence matching of a critical load to an electrical circuit . Internal low-impedance isolation transformer component offers 100% isolation from the input AC line.
- Hospital Grade Isolation transformers is ideal for the protection of sensitive electronic equipment in patient-care areas.
- Isolation transformer with Faraday shield reduces the cumulative leakage current of the Isolator and connected equipment to levels below 300 microamps.
- Surge suppression components placed at the line input and output combined with full line isolation offers continuous filtering of a full range of power line noise in all modes. Active transformer
filtering offers continuous common-mode noise rejection with no wearable parts, uniquely able to reduce surges in the worst of power environments to harmless levels.
- Isolation Transformer provides a "code-legal" method of re-bonding the electrical system safety ground to the neutral conductor on the transformer secondary. Doing so eliminates neutral-to-ground voltage and noise, which is major cause of reliability problems for microprocessor-based electronics.
- In electronics testing, troubleshooting and servicing, an isolation transformer is a 1:1 power transformer which is used as a safety precaution. Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.
Isolation transformers are commonly designed with careful attention to
capacitive coupling between the two windings. This is necessary
because excessive capacitance could also couple AC current from the
primary to the secondary. A grounded shield is commonly interposed
between the primary and the secondary. Any remaining capacitive
coupling between the secondary and ground simply causes the secondary
to become balanced about the ground potential.
All transformers provide isolation. They are constructed with a
primary and secondary winding closely wrapped around the same ferrous
core. Commercial transformers incorporate a single Faraday shield
between the primary and secondary windings to divert noise, which
would normally be electrically coupled between the primary and
secondary windings to ground . The method through which this
electrical coupling of noise occurs is the capacitance between the
coils of the primary and secondary windings of the transformer, which
does not include a Faraday shield. This same capacitance limits the
upper frequency band pass of the transformer in the same manner as the
mutual and self-inductances of the device determine its low frequency
cutoff. As the frequency of the exciting currents increases, the
reactance caused by the capacitance between the windings, tends to
shunt these currents, thereby limiting high frequency performance.
The single Faraday shield controls all manner of evils which could be
attributed to the electric coupling of noise through a transformer.
However, the problem with a single shield arises when it is bonded to
the ground of either the primary or secondary side of the transformer.
The enclosure of a Faraday shield between the primary and secondary
windings eliminates inter-capacitance, but it also establishes two new
capacitances between the shield and both windings. These two
capabilities allow high frequency currents to flow in the grounding
systems of both the primary and secondary. Bonding the transformer
shield to either the primary or secondary ground establishes current
paths for high frequency noise in the reference conductor of the
circuit to be isolated. The particular choice of ground for connection
of the shield only provides selection of the quieter of the primary
and secondary circuits. In many applications, this current path
defeats any isolating effect, which a transformer might provide.
An isolation transformer is designed to address the problems
associated with referencing its internal shields to ground. It is
constructed with two isolated Faraday shields between the primary and
secondary windings. When properly installed, the shield, which is
closest to the primary winding, is connected to the common power
supply ground and the shield closest to the secondary winding is
connected to the shield of the circuit to be isolated. The use of two
shields in the construction of the isolation transformer diverts high
frequency noise, which would normally be coupled across the
transformer to the grounds of the circuit in which they occur. The two
shields provide more effective isolation of the primary and secondary
circuits by also isolating their grounds. The isolation transformer
adds a third capacitance between the two Faraday shields, which may
allow coupling of high frequency noise between the system grounds.
However, increasing the separation between the two Faraday shields
normally minimizes this third capacitance. Additionally, the
dielectric effect of the shields plus the increased separation of the
windings significantly reduce the inter-capacitance between the
Generally, a conductive foil completely enclosing the windings will
provide a ground path for primary circuit noise and has the advantage
that a very much smaller capacitance exists between primary and
secondary coils than in the case of a simple Faraday shield. The
Faraday shield is simply a grounded single turn of conductive
nonferrous foil placed between coils to divert primary noise to
ground. The enclosing shield, if grounded properly, will not
re-radiate the noise signal, and will provide effective
electromagnetic noise reduction. Typically, according to Topaz at a
distance of 18 inches from a transformer's geometric center, the field
strength will be less than 0.1 gauss, and will roughly follow inverse
Since inter-winding capacitances are the primary path by which
significant power line and transient related noise couples to the
system, more information is needed to describe what occurs. During the
time power is being transferred between transformer windings, noise
potentials between the primary circuits and ground is similarly
coupled to the secondary through both capacitive and resistive paths.
This noise appears in three forms normally in a transformer circuit :
- Common Mode
- Transverse Mode
Common - Mode Noise
This noise appears between both sides of a power line and ground.
Since this noise is referenced to the power system ground, the most
obvious method of eliminating this noise is by grounding the
transformer center tap to the system ground via the lowest impedance
path possible. Internal transformer designs, which separate the coils
to reduce capacitive coupling, have some advantage, but it also
increases leakage inductance and reduces the power transfer.
Transverse - Mode
Transverse-mode noise is much more difficult to eliminate than
common-mode noise. The key here is to differentiate between power and
noise, and then reduce the noise.
Noise and power are separated by the difference in their frequencies.
The most effective transformer would be a design exactly opposite to a
audio transformer. The purpose is to transfer the power required by
the load at the fundamental power frequency and to eliminate all
higher and lower frequencies. Sub-harmonic frequencies are attenuated
by operating the transformer at relatively high flux density, which is
effective in reducing or eliminating them. Above the fundamental
frequency, noise is reduced by introducing as much leakage inductance
as possible, consistent with good power transfer to the secondary.
Transverse-mode noise appears as a voltage across both the primary and
secondary windings of an isolation transformer. It occurs when a
common-mode noise signal causes current to flow in the primary winding
(or secondary winding), and from there to ground via capacitance to a
grounded shield. Common-mode noise can also be transformed into
'transverse-mode noise, and thereby, through magnetic coupling,
contaminate the secondary of an isolation transformer. Normally, by
the proper selection of core loss verses primary winding inductance, a
well-designed isolation transformer will eliminate the majority of
this type of noise. Here again, grounding the transformer shield to
the lowest impedance path available, will result in noise currents
using this return path rather than some other higher impedance path to
the noise source ground.
Electromagnetic noise does not constitute a major problem in most
applications, but is sometimes critical in some recording or digital
data systems, and in making electromagnetic interference measurements.
Box Level Applications
Isolation transformers are often used to protect high gain circuits,
or prevent noisy ground paths in instrumentation. Shielding at the
instrument level is difficult and often ineffective. Since most
commercial instrumentation has single shielding in its power
transformer, designers sometimes hope that by adding a isolation
transformer ground problems can be eliminated. This approach often
results in no benefits to the system unless all other ground paths in
the instrument can be totally isolated. An isolation transformer is
not a substitute for the proper shielding or grounding of individual
The amount of ground isolation provided by the transformer at the
box level is limited by the use of a single chassis shield enclosing
the box. High frequency noise currents generated by the box circuitry
can be coupled onto the circuit reference conductors through the
connection of both transformers' shields to the circuit reference.
Additionally, any potential difference between the power system ground
at the isolation transformer primary input and the power system ground
at the equipment and the power system ground at the equipment chassis
will cause currents to flow in the reference conductor of circuitry.
Rack Level Applications
The most effective application of isolation transformers is with racks
of equipment. A rack acts as an outer shield for internal instruments,
while serving as the zero-signal reference for system output signals.
Isolation transformers are used to control shield currents, and to
break up the mutual capacitance between rack instrumentation and an
unknown power ground.
The main benefit of using an isolation transformer with a rack of
equipment is the enhanced control of currents in the equipment
shields. Any potential differences between the utility power ground
and the rack's ground will cause currents to flow in the loop. The
isolation transformer allows these "ground" currents to be directed
through a portion of the rack's shielding which will not effect the
operation of sensitive circuits and completely isolates these currents
from the internal equipment reference conductors.
Room Level Applications
It is often necessary to isolate EMC test enclosures from noisy
building grounds. Not only can isolation transformers be used to
effectively decouple building power, but also since they also act as
tuned circuits; they reduce the differential noise from external
equipment, which reaches your screen room. While it is recognized as a
second isolation transformer inside the test room will greatly reduce
power line ambient, this section will only consider using transformers
on the power lines to a typical screen room.
As with any transformer, isolation transformers radiate magnetic
fields. Physically locating the transformer adjacent to, or connected
to, a screen room may increase rather than decrease ambient noise.
Since the physical case of a transformer, as well as the primary
winding shield, are normally connected to the third-wire power ground
of the supplied power, the secondary winding shield must be isolated
from the transformer case and connected only to the conduit shield
going to the shielded room to achieve proper ground isolation. The
conduit acts as an RF shield for the room's power and completes the
connection between the shielded room and the secondary winding shield
in the transformer.
If the transformer is three phase and supplies more than one room, the
best application for isolation between rooms is to use only one phase
for each room, with a limit of three rooms per transformer. With this
approach, power line filters will effectively isolate the room while
providing practical noise attenuation.
Proper transformer design, wiring, and, above all, grounding, are the
only effective means of reducing the three types of noise problems.
Grounding should be controlled and use the lowest impedance path
possible (i.e., bonding) to the central reference ground system to
insure maximum attenuation of noise sources. To achieve the maximum
protection from a transformer, not only must it be applied properly,
but also the transformer should be one specially designed for
Three Phase Isolation Transformers
Three phase Isolation transformers are used for many applications
ranging from grain dryer, saw mills, conveyer belt systems,
refrigeration and air conditioning. Three phase have 3 primary and 3
secondary windings that are physically separated from each other. Each
of these windings are insulated from each other. The output windings
will be isolated, or floating from earth ground unless bonded at the
time of installation .
The Shielded three phase isolation transformers have all the feature
of the standard 3 phase plus they also incorporate a full metallic
shield (usually copper or aluminum) between the 3 phase primary and 3
phase secondary windings. This electrostatic shield or Faraday
Shield, is connected to earth ground and performs two functions :
Its attenuates (filters) voltage transients (voltage spikes).
These shielded 3 phase isolation transformers have an attenuation
ratio of 100 to 1.
It filters common mode noise, Attenuation of approximately 30 decibels.
The shield three phase isolation transformer is preferred over the
standard three phase isolation transformer because it provides
protection to sensitive and critical equipment. When more that one
shielded 3 phase isolation transformer is used between the source and
the load, it is referred to as a " cascading" and greatly improves
Technical Information of Power Isolation Transformers