I find transformers fascinating..... the fact that we can convert electrical energy to magnetic energy and then back to electrical energy in itself is amazing. And then, the fact that in such a small volume we can reduce the voltage but increase the current; or conversely decrease the voltage and increase the current it's almost mind-blowing..........so forgive me if I'm a little excited!
It seems like it should take quantum physics and or some kind of black magic to understand or build one..... but it does NOT!
(at least at a level "good enough" to rewind existing transformers and even make your own.)
Thanks to my Dad, an electronics technologist and forever science-minded learner, for giving me a love of the same.
When I was in public school we would set aside time for him to teach me; ohm's law, capacitors, inductors, basic transistors, etc.
He also let me use his basement shop to build projects, rewind transformers, re-charge magnets, build a
Jacob's ladder from a furnace transformer, try electroplating, etc. Luckily my Mom did not see all of it, or some of it would have been shut-down.
With any transformer just remember that there is a certain number of volts per turn of wire.
The winding with the more turns has more volts, whether it is the input or the output.
(The winding with the larger diameter wire has more current.)
So in the case of a step-up transformer (that by definition has fewer volts on the input (primary) than the output (secondary)) there must be fewer turns on the primary winding than the secondary winding.
Just be aware that the volume of the windings is not enough to judge; the two windings can me made of very different gauge of wire. So the coil with fewer windings may actually look bigger!
Sure you can reverse it.
If you have a 120VAC to 12VAC step-down transformer, it is fine to "use it backwards".
You can supply 12VAC to the coil with fewer turns (was secondary, now primary) and generate 120VAC on the coil with more turns (was primary, now secondary).
But just understand a couple things:
Each winding was designed for a specific voltage and current. (affecting the number of turns and the wire gauge.)
You cannot take that same transformer and supply 120VAC on the coil with fewer turns and hope to get 1200VAC on the other side.
Even though it normally has a 10:1 when used "forwards" and 1:10 when used "backwards" there are limits! (to both directions).
In this case the limit is the current thru the coil based on the impedance of the coil.
Don't be scared off.... impedance is just the "AC resistance" of the coil. It is much like the DC resistance, and can be used in ohms law the same way as DC resistance, however the impedance depends on the frequency of the AC voltage.
In this example, if you have not changed the impedance (by changing either the coil inductance or the AC frequency) then by increasing the voltage from 12VAC to 120VAC you have increased the voltage by 10 times.
We know that Ohms law says: I = V/R
but for AC circuits with a coil(inductor) we replace R (resistance) by ZL (inductive impedance), and so get: I = V/ZL
If we increase V by ten times and leave ZL the same, then the current (I) has also increased by ten times.
Remember I said that each coil was designed for a specific voltage and current?
Well a ten times increase in them would very likely violate what that coil was designed for and burn up the winding (unless there was a very carefully sized fuse in the system).
Also, there is no free-lunch....and in fact there are taxes for playing......
All transformers have losses, there is DC resistance in the windings, stray magnetic fields that do not get coupled into the sceondary, there are
Eddy current losses in the core (decreased by the laminations of the transformer core material), etc. So even the best transformer cannot supply 100% of the input power to the output.
I had better quit now, before I start with more equations and put people to sleep.....
Brian
