Transformerless Power Suppply

[T.Jackson]

You will have a standing current... ie you are sucking energy in, but not taking any out. All this is doing is heating up the core, and it's worse the smaller the Transformer becasue of the smaller amount of iron. So from this you can gather, that the less current we draw from a Transformer from it's optimum design point, the more inefficient it will be. If a Transformer is designed to be most efficient at say 100mA, then any current draw less than that is heading towards the ZERO efficiency point when it is completely off-load. Likewise if you exceed the optimum design point (which is usually just below the Magnetic Saturation Point of the core), you then risk driving the core into saturation and again your efficiency plumets as all you end up doing is pumping more current in which just heats up the core.

I think you've have described the term "Eddy loss current" ?

Just having a quick flick through some of my old college notes, I have %95 written down as a typical rate of efficiency...

Trent Jackson

[T.Jackson]

Extract of taken from college notes 16/06/98

Transformers are very efficient components. Typically they boast efficiency levels of better than 95%. This means that only a mere 5% of the total power is lost. Consumed / wasted in the form of heat. Transformers work on the principle of self-induction, and because of this, there is almost perfect isolation between the primary & secondary windings. With this infinite impedance between the two windings, power transformers are deemed very safe.

By Faraday's law of magnetic induction, when an alternating current is applied to the primary windings, a voltage will be self-induced in the secondary. These windings must be close wound. Secondary power is introduced from the primary because of the changing magnetic field between the two.

When a voltage is applied to the primary a magnetic field is set up, and if the frequency is high enough the primary windings will begin to radiate its power to the secondary. The voltage that is produced on the secondary is directly proportional to the turns ratio.

In short, minus all the math, magnetism theory, that's how a transformer works.

Trent Jackson

[Melanie]

Time college lecturers got out a bit more and got dose of the real world.

In theory everything is lovely.

In practice I challenge you to take ANY transformer you've got, and plot Input Current & Output Current from off-load, thru to full rated load in say 5% steps. From that you can calculate and plot a graph of efficiency to prove my point. Enjoy the experience. Then take the efficiency graph and tell your college that life in the real world is far from theory - so far that it's not even close!

For example... let's try to prove the theory of the International Ampere (you know Amps... it's that unit of measurement all of us use every day of our lives... so really we should try to prove the definition before we use it)...

Quote from my University Notes "That constant current which when flowing down two straight parallel conductors of infinite length and negligible cross section, separated by a distance of one meter in a vacuum, produces a force between them of 2 × 10-7 newtons per meter of length."

You get full marks for reciting this like a Parrot in your exam... but let's try to prove it...

1. You go down to your local store and ask for two drums of cable of infinite length and negligible cross-section... you lug them home...

2. You borrow the space shuttle and unravel them in the (supposed) vaccum of space from planet Earth out towards infinity.

3. When you've reached infinity, you join the far ends together and come home.

4. Then you hook up the wire to your PSU with a DVM in series switched to the famous "Newtons per Metre Length" setting on the dial.

5. Take a second DVM in series and switch it to Amps.

6. When DVM No.1 shows 2x10-7 Newtons the other should be showing 1.0000 Amps (assuming it's in calibration).

Worked when I tried it...

[T.Jackson]

Unfair game here. Someone has a UNI degree...

Where's the referee?

I'm in the process of doing a degree in Applied Science In Information Technology. Only done two years of electronics in college. I'm only at trade level
with most of the theory.

Cheers

Trent Jackson