An isolation transformer does not have a direct electrical path from the power input side to the power output side. The term is also used to define how much electrical isolation exists between the input and output windings. For example when using line-voltage input transformers to power low voltage device handled by humans, a high degree of isolation is required for safety. Isolated transformers often use separate bobbins for the primary and secondary coil windings, but usually the windings are just wound on top of each other with insulation in between.There is usually an electrostatic shield between windings which is tied to transformer case. 

Non-Isolated transformers are becoming rare. A common example is the "Variac" which is a non-isolated variable transformer. Usually the term "auto-transformer" is used to describe non-isolated transformers. They are rarely found in consumer products. Although any transformer with a separate primary and secondary winding is an isolation transformer to some extent, The term is usually used to denote a special-purpose transformer built just for that use. It is tested and rated to withstand a very high voltage difference, called the withstand voltage, so that even if thousands of volts are applied to the primary, it will not leak through to the protected side. These transformers are used in the medical industry, to protect patients hooked up to monitoring instruments that are powered by utility mains, as well as other uses.

Light dimming circuits for street lights

Industrial & domestic heating

Induction heating

transformer  tap changing

Speed control of Motors (variable torque)

speed control of winding machines,fans

AC magnet controls [10]

Heatsink

Motors and most other actuation devices are typically indirectly connected to the power supply

through a power transistor which acts as a switch, either allowing energy to flow from the power

supply to the motor, or disconnecting the motor from power. (The CPU, also connected to the

transistor, chooses exactly when to turn it on or off).

When the switch is turned on, most of the power coming from the power supply goes to the

motor. Unfortunately, some of the power is trapped by unwanted parasitic resistance in the power

transistors, heating them up. Often a heatsink is necessary to keep the transistor from

overheating and self-destructing. Practically all modern desktop or laptop PCs require a heat sink

on the CPU and on the graphics chip. (A typical robot requires a heatsink on its power transistors,

but not on its small CPU).

"Application note AN533: thermal management precautions for handling and

mounting" [2] describes "how to calculate a suitable heatsink for a semiconductor device". It looks

like this applies to all power semiconductors -- FET, BJT, Triac, SRC, etc. It gives thermal

resistance for DPAK and D2PAK for FR4 alone, FR4 plus heatsink, Insulated Metallic Substrate

(IMS), and IMS plus heatsink.

The principles behind heatsinking these power semiconductors are the same as the principles

behind PC CPU heatsinking.

What characteristics make a heatsink a good one? There's a number of factors to consider:

High heatsink surface. It's at the surface of the heatsink where the thermal transfer takes place.

Therefore, heatsinks should be designed to have a large surface; this goal can be reached by

using a large amount of fine fins, or by increasing the size of the heatsink itself.

Good aerodynamics. Heatsinks must be designed in a way that air can easily and quickly float

through the cooler, and reach all cooling fins. Especially heatsinks having a very large amount of

fine fins, with small distances between the fins may not allow good air flow. A compromise

between high surface (many fins with small gaps between them) and good aerodynamics must be

found. This also depends on the fan the heatsink is used with: A powerful fan can force air even

through a heatsink with lots of fine fins with only small gaps for air flow - whereas on a passive

heatsink, there should be fewer cooling fins with more space between them. Therefore, simply

adding a fan to a large heatsink designed for fanless usage doesn't necessarily result in a good

cooler.

Good thermal transfer within the heatsink. Large cooling fins are pointless if the heat can't reach

them, so the heatsink must be designed to allow good thermal transfer from the heat source to

the fins. Thicker fins have better thermal conductivity; so again, a compromise between high

surface (many thin fins) and good thermal transfer (thicker fins) must be found. Of course, the

material used has a major influence on thermal transfer within the heatsink. Sometimes, heat

pipes are used to lead the heat from the heat source to the parts of the fins that are further away

from the heat source.

Perfect flatness of the contact area. The part of the heatsink that is in contact with the heat

source must be perfectly flat. A flat contact area allows you to use a thinner layer of thermal

compound, which will reduce the thermal resistance between heatsink and heat source.