# What and how does a step up transformer work that converts a 120V outlet to a 240V outlet?

I've been fixing electronic equipment for some time and I've never dived into what a transformer really is.

In my shop I have a 240V reflow machine that I run for at most 5-10 minutes at a time off of a step up transformer from 120V to 240V. At most the reflow machines consumes around 2000W. My breakers haven't popped yet, but I want to understand what is actually happening in the transformer. Is it just drawing more energy into the step up transformer from a 120V outlet and storing it inside this circular copper device inside the transformer to make it 240V?

• Do you know how a step-down transformer works? Do you know how a 1:1 transformer works? Do you know how any transformer works? Do you understand about coil-coupling? Have you heard of faraday's laws of induction? Oct 24, 2022 at 15:20
• Basically, the power input to any transformer will be equal to the power output, plus a little for losses in the transformer. If your reflow machine consumes 2000 watts at 240 V, the transformer will draw about 2050 watts from the 120 V outlet. Oct 24, 2022 at 15:45

I'll give you the layman's easy to understand explanation.

A step up transformer works by having two coils of wire, with one coil having more turns than the other.

When an AC voltage is applied to the coil with less turns it creates a changing magnetic field which is proportional to the number of turns and the current. This magnetic field is also around the second coil and induces an electromotive force (EMF) in it. Since the second coil has more turns the voltage across the second coil will be higher, if you have 100 turns in the first coil and 200 in the second the voltage will be doubled.

There's no free energy though, the trade off is that while the voltage is doubled the current is halved, so the power (voltage times current) is the same (ignoring losses for the sake of simplicity).

In your case you have a machine that uses 2000 W at 240 V, so the current will be 2000 W/ 240 V = 8.333 A. That is what you're getting out of the secondary of the transformer. What the primary is drawing from your mains power will be 2000 W / 120V = 16.666 A.

So there's no energy stored, it's just trading voltage for current. The power out of a transformer is very close to the power in, there's a bit of losses so it's not 100% but it's generally pretty close.

If you want to get into the technical details you can read this.

• +1, nice layman's explanation. One small correction, the changing magnetic field induces an EMF in the secondary, not a current. (Consider an open secondary. The voltage is there but there's no current.) The EMF may drive a current in the secondary and load when connected. Oct 24, 2022 at 16:13
• @JohnD Edited. Thanks. Oct 24, 2022 at 16:25