Step 4: Wind the new transformer secondaries

Picture of Wind the new transformer secondaries
We wound 20 turns of 10-guage wire on each transformer. That's just about how much wire would fit into the available space. It took a little over 20 feet of wire each.

tip: draw tally marks on your table to keep track of the number of windings.

How does a transformer work?
The primary winding is an electromagnet connected to alternating current.
The humming magnetic field of the primary induces a current to flow in the secondary winding. If both windings have the same number of turns, the output voltage is the same as the input.
(minus a smidgin due to eddy currents, resistance, etc.)
If the secondary has more turns than the input, its output voltage is higher. That's the type of transformer you started out with.


Our primary has 100 turns and gets connected to 100 volts AC. We're winding 20 turns on the secondary, so we'll get about 20 volts out.

The available POWER STAYS THE SAME regardless of what the output VOLTAGE is.

If the primary is made take 1000 watts (100 volts * 10 amps) out of the wall, we'll be able to take 1000 watts out of the secondary. With 1/5 of the windings, we can draw 50 amps out of the secondary.

That's the cartoon version with play numbers anyway.
Over here in our shed full of reality we've got two of these beasts in series and plan to short the outputs through a welding rod like Jennifer Beals.

Let's just say we're going to pull a whole lot of amps, which is why we need to wind our secondary with such thick wire.

The copper conductor in ten-guage wire happens to be 1/10" (0.1") in diameter.

Here's a table of conductor diameter, guage, and current rating.

n1cod3mus3 years ago
after yesterdays ramblings I got bored of keep working out how many turns i would need to produce the amps and voltage so i wrote a calculator feel free to use it

I have updated this with a few wire gauges and their top amps they can take and got it to work out if it would fail e.g. end up melting and causing a fire
n1cod3mus3 years ago
i'm in the UK so if i took one of my transformers with 112 turns on the primary, the UK mains voltage is 230v and about 13amps so...

230v * 20 turns on 2nd = 4600

4600 / 112 turns on primary = 42.67v

13amps * 230v = 2990w (could round it up to 3000w)

is that correct? if so then surely i would only need a single transformer to do the welding? as you guys are using 2 to achieve output voltage of a simlar value

which would mean i would be getting over 100amps right?
i was just reading about how to calculate the amps on this site


looking at number 4 it says that if your reducing 120v to 12v your doing it by a factor of 10 and thus the input amps would be times that factor to give you the ouput.

to work out my amps i would assume i would need to work out the factor for my voltage so

input 230v / out put 41.67 = 5.51

input 13amps * 5.51 = 71.63amps out

am i doing this right? I think this should be the approximate amount of amps i would be putting out from 1 transformer
i know i'm answering my own questions here but i'm hoping that this will help others. and hey its near 2am here in the UK so i'm doing my best work.... more caffine.
ok so looking around the net most home welders produce upwards of 80amps and around 34v and up.

what this means the the US is that with your 50amps (as above) you will need more so doubling up your transformers gives you 100amps and your in the golden zone.

with my higher power and slightly higher amps with one tansformer I would only be able to get 71.63amps so its clear i also need to double up too.

so heres what i got, I have 2 transformers with the following

A) 112 turns on the primary

B) 132 turns on the primary

assuming I will be doing 20 turns on the secondary and the input voltage is 230v with 13amps, i work out its output and my factor for each transformer and then work out its amps

working out A)
230v * 20 turns on 2nd = 4600

4600 / 112 turns on primary = 41.67v output

input 230v / output 41.67v = 5.51 (the factor)

input 13amps * 5.51 (the factor) = 71.63 amps output

working out B)
230v * 20 turns on 2nd = 4600

4600 / 132 turns on primary = 34.8v output

input 230v / output 34.8v = 6.60 (the factor)

input 13amps * 6.60 (the factor) = 85.8 amps output


so once i combine the 2 transformers output

85.8 amps + 71.63 amps = 157.43 amps total output

41.67 volts + 34.8 volts = 76.47 volts total output

now the problem is what wire do i use, i found this site which has a handy table for the amps different wires can handle


so i need wire that can handle up to 85.8 amps for B) and 71.63 amps for A)

looking at the chart i found I would need to use 7 gauge wire in the UK which can handle up to 89amps as you guys in the US are generating around 50amps 10 gauge is fine.

problem with this is that 7 gauge is much thicker so will i get the turns in, well thats where someone on instructables has the answer, you have to work out the area you have to use check this out


this is a series of videos that explain how to prep the transforms and in one of them it shows how to work out the area and how to wind your cable to use the best space and if the wire your planning to use will fit.
just been working this out if i wanted to use 10 gauge wire i would have to have an additional 10 turns so 30 turns and not 20, if anyone wanted to know, again you have to make sure this will fit, if not you will have to increase the turns and thus reducing the amps and you can use thinner wire
the only other way you could get round using 10 gauge with 20 turns is to introduce cooling, otherwise the wire will just melt and fuse together and then your stuffed.

good job i salvaged the fans from those microwaves, of course if you used wire that is enamel coated then i suspect you could fit the 7 guage in, but i'm thinking it will be tricky if its covered in a standard insulation.

i'm rambling i should go to bed, its 2.20am
KCMude3 years ago
Hey Engineers, technicians, Craftsmen and many more in this forum, am happy to report that following instructions is easy. With 2.5mm diameter wire 18 turns on the secondary was able to get 18.5volts hence, I must be able to get 37volts if I manage to make the second transformer. I will give a full report when my project comes to an end.
from Kaychi.
zfolwick5 years ago
I was only able to get 18 turns of naked copper and it looks like a rat's nest. This is hard enough I'm going to look to purchase my other transformer somewhere... What's the other wire? 8 gauge?
0087adam5 years ago
I think you need to check your math a little bit, your equations are off. But other than that the concepts are the same and i applaud you for figuring this all out.
jamisan915 years ago
hey could I use 12 gauge
VanUnamed5 years ago
I've made it with italian 220V transformers, i winded 2mm cable 24 times to obtain 19.4V each transformer. 2,5mm is the diameter including the rubber casing

thanks alot for the idea
ampeyro5 years ago
so if mine is made for 220v(europe) and have near 100 turns too, i will ned to make the secondary with 10 turns?
tonhe5 years ago
If I am concerned about blowing the breaker (using 115v AC here in the US ~15amp) would using a 3rd transformer help? I assume it would lower the voltage further and raise the amps up, but as far as welding goes, I'm not familiar enough to know if that would help.
jonogt6 years ago
I saw in a prior comment and the reply to it that 10 ga. stranded wire is acceptable in place of solid wire. I got ahold of a good amount of 12 ga. solid wire for free, and i was wondering if this would be OK. Should I just add a couple more winds to bring the current down a bit? Or can I do the same number of winds and just weld in short spans to prevent overheating? I don't need this thing to have massive bite-your-head-off penetration... I'll mostly be making lighter welds. thanks for any input -Jon
eric m6 years ago
No explanation of phase or direction of winding versus primary.

skuitarman6 years ago
how many amps can you get out of it???
openwater6 years ago
I got about 20' of 10ga. 3-strand copper stranded wire leftover from an old job. Should I use it, or just go buy the solid wire?
maxpower496 years ago
is there still some shilding on the new wire
Coffee bean6 years ago
one word......tesla
jdpower7 years ago
Can I use 8ga. stranded insted of 10ga. solid ? Also what if I move on down the scale to a larger wire?
Wait, what are you winding the wire around? Is it around the primary windings? Or through the transformer somewhere?
yanggers7 years ago
Do I have it correct that the direction of this secondary wire winding is negligible as long as they fit into the space? clock wise vs. counter-clock wise, all-over-the-place vs. neat-like-a new-twine-spool ? Thanks for the awesomeness.