Step up tranformers?

Question

in a step up transformer, the out put voltage is say 10 times the input. So the output current would be around one tenth of the input current. but, plugging in the values into ohms law, we get that the resistance in the second coil/wire is 100 times less than in the first one. What exactly has caused this change in resistance. Is it by magnetism or something?

Answer 1

To make a step up transformer, the secondary coil would have more windings than the primary coil. This would increase resistance in the secondary coil, decrease current and increase voltage (or step it up). So bottom line is the the resistance change in the secondary coil is by design, increasing the number of turns around the coil).

The fact that voltage and current get “stepped” in opposite directions (one up, the other down) makes perfect sense when you recall that power is equal to voltage times current, and realize that transformers cannot produce power, only convert it. Any device that could output more power than it took in would violate the Law of Energy Conservation in physics, namely that energy cannot be created or destroyed, only converted.

Answer 2

ohms law really doesn't apply to a source of power.

Look at a battery that delivers 12 volts at up to 10 amps. you can calculate 12/10 = 1.2 ohms but it doesn't mean anything. There is no 1.2 ohm resistor anywhere. It means if you want to load up the battery so it delivers 10 amps, then you put a 1.2 ohm resistor on it.

Same with a transformer.  Assume it is 10v/10 amp primary and 100v/1 amps secondary.

The secondary, if you load it up, will take a 100 ohm resistor to load it fully. Now if the primary is supplied with 10 volts, the secondary will be delivering 100 volts at 1 amp, or 100 watts.

The primary will be using that same amount of power (perfect transformer) 100 watts, but as 10 volts 10 amps. But there is no one ohm resistor around, that is just what the load of the transformer looks like to whatever is delivering the 10 volts.

Answer 3

Transformers are AC devices; so impedance dictates current flow. Impedance is resistance + capacitance + inductance.

For a transformer, the inductance is largely what limits the current, not the resistance. And yes, the inductance is caused by the number of turns of wire and by the magnetic properties of the core.