Hi folks. It's been a while since I've been on this forum, but I've gotten back into playing with electricity lately, and to do some of the stuff I want to do, I'll need some transformers.
For my very first transformer build, I found an autotransformer from a broken UPS. Being a noob, I cut the wire out rather than dismantling the core first and just unwinding it. I still have the wire, but it's now in 20 foot sections. At least I still have the intact spool and core.
Now, when I started this, I knew nothing of transformers or the math behind them, so I did the best I could. To start with, I figured Watts, Volts, and Amps on both sides of the transformer. Watts being the same on both sides. I never did figure out exactly how to estimate the maximum power a core could handle. I found an equation, core area (sq cm) squared = Watts. I still don't think that's quite right, but it's all I could find, so I used it. I came up with about 122W.
With 122W, at 122V, that's 1A on the primary side. I used 22 AWG wire for that side as it can handle 1.2A.
With 122W, at 9.7V, that's 12.7A on the secondary side. That's way more than I want, so I chose 18 AWG wire, as it can handle 3.2A. More than enough to power what I need.
While figuring all this out, I managed to get the core sheets apart so I could rewind the spool.
The number of windings on the primary side was not straightforward to me at all. I finally found an equation for 50/60Hz transformers: 42 / core area (sq cm) = Turns per Volt. This seemed promising as 42 is the answer to life, the universe, and everything. I learned later that this was a very simplified version with several assumptions in it. But my understanding is that you have to have a minimum number of turns per Volt, or the magnetic field will become saturated. This would cause current to start flowing excessively between the Live and Neutral wires, and perhaps trip the circuit breaker. I think that's how it works, anyway.
So, with the equation, I had 42 / 11.1 sq cm = 3.78 turns per Volt. At 122V, the primary would need 462 turns. I did my best to estimate the length of wire I'd need, and came up with 244 feet, so I bought 250 feet. Turns out, I was still about 2 feet short, so I need to come up with a better way to estimate wire length. But it all worked out, and I ended up with a nice neat primary coil. (Which I later wrapped in tape before adding the secondary coil.)
For the secondary winding, I guess I could have used the ratio of Volts, but I already had turns per Volt so I used that. For 9.7V I would need 37 turns. I didn't take inefficiency into account (because I don't know how), and so I ended up with 8.7V once I got the transformer put back together. But it was easy enough to add 4 more turns, and I got 9.6V. Close enough!
Also, I got to play with my new pocket oscilloscope. I've never had one before, and am still learning how it works, but I did get a nice sine wave, plus several numerical values that my multimeter can't do.
After confirming the transformer would actually work, I decided to make some improvements. I got some nuts, bolts, and washers at the hardware store, and a few L brackets to tighten up the core, and give it a way to be mountable. Squeezing the sheets together with the bolts reduced the buzz to the point where I can barely hear it. I also found a really nice power cable with a ground to replace the old lamp cable I was using before.
I had a lot of secondary wire left over, so I used the rest of it to make extra taps in order to have different Voltages to choose from. I wrapped the outside with tape, and squeezed a couple pieces of cardboard between the coil and the outside parts of the core. Just to make sure the wires will never touch the metal.
In addition to the 9.7V I wanted, I also tried to get 12, and 24V. I ended up with 12.8V and 24.7V, which is fine. By using different combinations of taps, I also got 3.1V, 15.1V, and 12.1V. I don't know what I'll use these for, if anything, but there they are.
These are all open Voltages, so they drop a little under load, but not by much.
Since building this transformer, I've continued to search, read, and watch videos. I know more now than I did, but not as much as I'd like to. I found a more involved equation for turns per Volt (T/V = 1/(4.44 x 0.0001 x B x CA x f) which tells me I probably had more windings than I needed. I've also heard a rumor that the core doesn't actually have a limit for Watts. It's all dependent on the windings. I don't know if that's true or not. And there's still more websites I haven't read yet. I'll apply all this to my next transformer.