Product Review

Feb 10, 2011

Laminated core                                                Overview from Wikipedia

This is the most common type of transformer, widely used in appliances to convert mains voltage to low voltage to power electronics

  • Widely available in power ratings from 1.2w to several kilowatts
  • Insulated laminations minimize eddy current losses
  • Most use a split bobbin, giving a high level of insulation between the windings
  • Rectangular core
  • Core laminate stampings are usually in EI shape pairs. Other shape pairs are sometimes used.
  • Mumetal shields can be fitted to reduce EMI (electromagnetic interference)
  • A screen winding is occasionally used between the 2 power windings
  • Many such transformers have a thermal cut out built in, many don't
  • 4 turns per volt is typical for continuous use
  • Occasionally seen in low profile format for use in restricted spaces
  • laminated core made with silicon steel with high permeability


Doughnut shaped toroidal transformers are used to save space compared to EI cores, and sometimes to reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring (and thus threaded through the ring during winding), and tape for insulation.

Toroidals compared to EI core transformers:

  • Lower external magnetic field
  • Smaller for a given power rating
  • Higher cost in most cases, as winding requires more complex & slower equipment
  • Less robust
  • Central fixing is either
    • bolt, large metal washers & rubber pads
    • bolt & potting resin
  • Overtightening the central fixing bolt may short the windings
  • greater inrush current at switch-on.

Rectangular toroids are also sometimes encountered. These use a rectangular version of a toroidal core, usually wound as a multilayered single flat steel strip. Windings are placed on 2 of the 4 sides of the core.

Ferrite Core

Ferrite core power transformers are widely used in switched mode power supplies (SMPSUs). The powder core enables high frequency operation, and hence much smaller size to power ratio than laminated iron transformers. Widely used in IF stages in radio receivers. These are mostly tuned transformers, containing a threaded ferrite slug that is screwed in or out to adjust IF tuning. The transformers are usually canned for stability and to reduce interference.

Ferrite transformers are not usable as power transformers at mains frequency.

Isolating Transformer

Most transformers isolate, meaning the secondary winding is not connected to the primary. But this isn't true of all transformers.

However the term 'isolating transformer' is normally applied to mains transformers providing isolation rather than voltage transformation. They are simply 1:1 laminated core transformers. Extra voltage tappings are sometimes included, but to earn the name 'isolating transformer' it is expected that they will usually be used at 1:1 ratio.

Pulse transformers

A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a relatively constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other high energy pulsed power applications.Modern meters eliminate the need of VT's since the voltage remains constant and it is measured in the incoming supply.

To minimize distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors.

The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.

Pulse transformers by definition have a duty cycle of less than 1, whatever energy stored in the coil during the pulse must be "dumped" out before the pulse is fired again.

Transmission line transformers

For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique.

The core material increases the inductance dramatically, thereby raising its Q factor. The cores of such transformers help improve performance at the lower frequency end of the band. RF transformers sometimes used a third coil (called a tickler winding) to inject feedback into an earlier (detector) stage in antique regenerative radio receivers.


Baluns are transformers designed specifically to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar or coaxial cable and are similar to transmission line transformers in construction and operation.

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