Modifying toroid transformer's secondary. - Page 1
Forum section: electronics / projects

author: FullBridgeRectumfire
author_rank: Newbie
timestamp: April 12, 2017, 09:58:57 am
content: Hi all!Recently I've started my first serious project of a home-lab power supply(0-24V, 0-3A). Of course, I need to keep it as cost-effective as possible, so I'm looking right now at any chances to shave off the cost of most expensive parts like heatsinks, enclosure, and obviously transformer. Now, here is the idea.As custom-made transformers are beyond economical for a single application, and scrap yards are full of this bloody switch mode PSUs I'm forced to buy a new one wich not really meets my requirements. I thought that I'll go for 200VA transformer with a dual 12V secondary to connect it in series and get 24VAC center-tapped. Using just this transformer without any mod would give me huge power losses on output transistors, so to pre-regulate that I wanted to wind additional 12VAC center tap secondary and switch between theme with a relay. Optionally I will add some digital control and display in the future so next 6VAC low current winding would be also required.http://www.tme.eu/gb/details/tst200w_2x12v/toroidal-transformers/indel/tst200004/Now as you see, I will have to play around with winding additional outputs what is a quite new thing for me. My questions are:-Which material would be best for insulation between windings? It must be easily available and cheap.-Same question about insulating foil to wrap the transformer after modification.-How to wind it properly, so my mod won't destroy transformer's performance?-Does direction of winding matter? If so, how to determine which one is the right one?If you have a link to any tutorials or articles about the practical side of modding/making a transformer I would really appreciate sharing.

author: james_s
author_rank: Super Contributor
timestamp: April 13, 2017, 12:05:41 am
content: I built my own 650VA toroidal transformer for a bench power supply a number of years ago. I got a core from Alphacore and looked up their spec for the optional primary they offered then wound one myself, worked out the ratios and wound the secondary windings. I used polyester tape over the bare core and then several layers over the primary, then varnish dip, cure and several layers of polyester tape over the finished transformer, it came out really professional looking and works great.Toroidal cores are a pain to wind by hand though! I had to calculate the wire length and then transfer it off the spool to a long slender piece of wood. For the tape I unwound it off the roll onto an old D battery and used that as a spool that would fit through the middle of the core.

author: mariush
author_rank: Super Contributor
timestamp: April 13, 2017, 12:42:17 am
content: TME has some nice pcb transformers that you could use, for example9v AC 5A for 7.6 uk pounds :http://www.tme.eu/gb/details/tszz35_9v/pcb-transformers/indel/tszz45003m5/You can buy three of these and use a couple of relays to switch between 9v AC , 18v AC and 27v AC which (withouttaking into account losses in the bridge rectifier) would give you peak dc voltage of ~ 12.5v , ~ 25v and ~ 38v before the losses in the bridge rectifier.Seems like quite a lot above the 24v DC you want, but this would allow you to use smaller capacitors which would also be cheaper.For example let's say for voltages below 5v, you'd usethe first transformer only in which case you'd have a peak dc voltage of about 11v after losses in the rectifier. To output 5v, you'd probably need at least around 6v dc all the time, so you'd need to shove enough capacitance to keep the voltage above that threshold .. at 3A, that would be about C = 3A / [2 x 50 Hz x (11v-6v) ] = 3 / 500 = 0.006 farads or 6000uF - so you could use 2 x 3300uF 50v rated capacitors.. a 3300uf 50v cap is less than 1$ ,anything higher through hole is around 3$ ... snap-ins would be cheaper, something like 1.6$ for 10000uf (http://www.tme.eu/gb/details/he1h109m30050ha/105c-snap-in-electrolytic-capacitors/samwha/- good enough for a linear psu) but it's also quite large.With two transformers in series, you'd have about 23v peak dc, so you could set voltages up to maybe 18v DC using same capacitance, and with all three transformers in series you'd have a peak of around 36v, which would allow you to configure the output up to around 30vA fourth transformer with maybe 2 x 12v-15v ac secondary windings would give you the +/- 12v and +/-5v you'd need for opamps and whatever ... 12v would be needed for powering the 2 relays as wellthis 4 uk pounds 18v transformer would do:http://www.tme.eu/gb/details/tszz15_2x12v/pcb-transformers/indel/tszz18006m/ps. you could also mix and match transformers ex buy 1x10-15v AC 5A transformer for the base range and 2 x 9v ACps. and you could go for even finer control by using 4 transformers, here's 6v AC 35VA transformers, perfect for up to around 3.5A dc :http://www.tme.eu/gb/details/tszz30_6v/pcb-transformers/indel/tszz35001m5/

author: danmcb
author_rank: Regular Contributor
timestamp: April 13, 2017, 07:29:01 pm
content: Just a question, but is this really worth it? I see second PSU's regularly where I live and when I set up my lab a few years back I bought a dual linear 0-30V 2A Farnaell one for around 120€ and a single Philips 0-30V 1A for next to nothing. Both are old bit really pretty good quality, and there is just no way I could have made something comparable for less.

author: ggchab
author_rank: Frequent Contributor
timestamp: April 14, 2017, 08:02:17 am
content: I have the same problem and instead of winding an additional output to the toroid transformer, I used something like this. Until now, I have no problem. But I am wondering if this will have a negative impact on the transformer?I used a small isolated DC/DC converter and 6N137 to isolate the serial port used for remote control.