Electronics > Beginners
Transformer Winding
codingwithethanol:
I'm trying to wind my own transformer for a homemade PSU, and I'd like to ask a few questions first to avoid costly mistakes. Firstly, regarding turn ratio, lets say I wanted a 2:1 step down. How many turns should actually be made and why? You could do anything from actually only doing 2 and 1 turn to 200 and 100 turns and beyond. How do you calculate the "sweet spot" so to say? Secondly, would plugging a transformer with no load into mains cause a short? And lastly, would winding multiple layers be ok? Thanks in advance to anyone with info.
TheHolyHorse:
If you Google "winding your own transformer" you'll find all the formulas you need. :-+
codingwithethanol:
I did and all I read is theory and no advice on practical design. I'd just like to make sure that I dont melt wire or trip my circuit breaker, which is why I made this post on a board frequented by enthusiasts and professionals.
forrestc:
--- Quote from: codingwithethanol on July 15, 2019, 11:24:42 am ---I'm trying to wind my own transformer for a homemade PSU, and I'd like to ask a few questions first to avoid costly mistakes. Firstly, regarding turn ratio, lets say I wanted a 2:1 step down. How many turns should actually be made and why? You could do anything from actually only doing 2 and 1 turn to 200 and 100 turns and beyond. How do you calculate the "sweet spot" so to say? Secondly, would plugging a transformer with no load into mains cause a short? And lastly, would winding multiple layers be ok? Thanks in advance to anyone with info.
--- End quote ---
I'm really not trying to dampen your enthusiasm, but the concept of someone winding an AC transformer as a hobby project for something which is going to be connected to the AC power line seems to be not all that safe. As in 'could start a fire' or 'could cause you to be electrocuted' not that safe.
There are lots of things done with commercial, UL listed, transformers to keep the end users safe, such as inherent current limiting, proper insulation of the primary from the secondary, and so on. If you get any of these wrong, the outcome is potentially deadly.
I would strongly encourage you to find a commercial transformer for your power supply project. If at some point you want to experiment with winding your own transformers, then I'd probably start with transformers which aren't plugged into the AC line directly.
MagicSmoker:
--- Quote from: codingwithethanol on July 15, 2019, 11:24:42 am ---I'm trying to wind my own transformer for a homemade PSU, and I'd like to ask a few questions first to avoid costly mistakes. Firstly, regarding turn ratio, lets say I wanted a 2:1 step down. How many turns should actually be made and why?
--- End quote ---
This is akin to asking, "how long is a piece of string?" The number of turns is inversely proportional to frequency, magnetic flux density and core area, and directly proportional to peak voltage. For typical mains frequency transformers the maximum (or saturation) flux density is somewhere around 0.8 to 1.1 Tesla (8000 to 11000 Gauss), while ferrite for high frequency transformers saturates around 0.4T.
The equation I like to use regardless of transformer type (unambiguously valid without weird conversion factors) is:
N = (Vin * Ton) / (Bpk * Ae)
Where Vin is peak voltage (square or sine); Ton is the time that voltage is applied in microseconds; Bpk is the allowed flux density excursion for said voltage and time, in Tesla; Ae is minimum or effective core area (e.g. - the center pole in an E core, the cross-sectional area of a toroid, etc.) in square millimeters.
Ton is either 8333 or 10000us for 60Hz/50hz mains, respectively, while Vin is around 170V for US mains or 310-350V for most of the rest of the world. Typical silicon steel used at mains frequencies can tolerate up to 1.1T, but it is good practice to wind a few more turns to bring that down to 0.8T.
--- Quote from: codingwithethanol on July 15, 2019, 11:24:42 am ---Secondly, would plugging a transformer with no load into mains cause a short? And lastly, would winding multiple layers be ok?
--- End quote ---
If there are sufficient primary turns such that the transformer doesn't saturate then, no, the transformer will draw very little current from the mains when the secondary is unloaded. The better the transformer the lower this no-load (or "magnetizing") current will be. Otherwise, primary current is directly proportional to secondary current (technically only limited by the heating of the windings!).
Ideally, the number of layers should be minimized to reduce losses from proximity effect and leakage inductance, but these effects mostly apply to high frequency transformers; you need not worry about them too much at mains frequencies.
All that said, it might be less work to start with an existing "split bobbin" transformer (in which primary and secondary are physically separated) with the right power and primary voltage rating, then unwind the secondary and count the number of turns vs. the voltage rating. Wind the new secondary using the same turns per volt ratio, as that will automatically incorporate the core area and flux density parameters.
Ideally you should use enamel coated solid "magnet" wire for the winding(s) because less winding area will be wasted on insulation (and it will invariably have a higher temperature rating than "hookup" wire with a thermoplastic insulation), but you can use common stranded hookup wire in a pinch and/or for prototyping.
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