Here is something I have on my website as an answer to an argument with a moderator on a forum:
HOW THE TRANSFORMER WORKS
The first item you need to study is the TRANSFORMER.
It's the main component of the power supply and appears to be the simplest. But there is a lot to know about how it works and the main reason why it is present in a power supply has NEVER been explained.
Of course it provides ISOLATION and converts a high voltage to a low voltage and can deliver a current higher than the mains will deliver, but all this can be done without the magnetic core - that makes the transformer so heavy.
So, why is the core needed?
Let's explain it this way.
If you get a length of wire, say 120 metres, it will have a resistance, say 120 ohms and if you connect it to a 240v AC supply, a current of 2 amps will flow, according to Ohm's Law.
For this discussion, the voltage must be AC. In other words it must be alternating, such as from the mains, at a frequency of 50 or 60 times per second. A transformer only works on alternating voltage - commonly called alternating current.
This will produce a wattage of 2 amps x 240 volts = 480 watts and the wire will get very HOT.
But if we wind the wire on a plastic pipe with lots of layers, to produce a closely bunched coil, each turn will produce magnetic flux and this flux will cut all the other turns of the coil.
As the flux cuts each of the other turns, it produces a tiny voltage in each turn that is opposite to the incoming voltage.
It might produce a "back voltage" of say 20 volts and so the effective incoming voltage is 240v - 20v = 220v. This means less current will flow and the coil will not get as hot.
If we now wind another coil of half the number of turns (or any number of turns) and place it beside the first coil and connect the ends of this coil to a low value resistor, we will see what happens.
As the applied voltage increases, the magnetic flux from the primary winding increases and cuts the turns of the new coil. Voltage is produced in each turn and this is added by each turn and appears on the output leads. This voltage allows a current to flow in the LOAD resistor and the magnetic flux from the primary winding is converted to current in the secondary winding and thus it is not available to cut the turns of the primary winding. In other words, all the flux produced by the primary is "used up" by the secondary winding.
If the LOAD resistor is a very low value, the secondary winding will be able to accept and convert all the flux it receives and convert it to current.
But what controls the current in the primary?
The current flowing in the secondary produces magnetic flux and this flux produces a back voltage in the primary as the effective forward voltage is reduced. This reduced voltage allows a smaller current to flow so that the total watts delivered by the primary is equal to the watts required by the LOAD.
This means the two coils will work perfectly without the need for a magnetic core.
But if the load resistor is a higher value, the secondary winding will not require a high current and it will produce less flux and the back voltage will not be as high. Thus the forward voltage increases and produces a higher current in the primary winding and the winding will get hot.
So, what is the purpose of the magnetic core?
When the secondary is not accepting all the magnetic flux produced by the primary, the excess flux passes into the magnetic core and and the tiny metal particles in the core produce a current just like the secondary winding and this current produces magnetic flux that opposes the flux produced by the primary winding. This reduces the effective incoming voltage and thus the current in the primary winding is reduced.
In other words, the core takes the excess flux and converts it to an opposing voltage to prevent any increase in primary current, over that required by the secondary winding.
If we take this effect to the extreme, and remove the secondary load, ALL the flux produced by the primary winding is passed to the core and the core sends back an opposing voltage that can be as high as 99% of the incoming voltage. This is why a transformer will consume very little current when the output is not connected.
Not loading the secondary is EXACTLY the same as removing the secondary winding and this fact has escaped many moderators on the web, who have not studied the concepts of the operation of a transformer.
This fact has never been covered in any text books and that is why many engineers do not have a firm understanding of the concepts.
. . . . more on my website . . . . . .
