How to calculate amps when adding secondary coils to toroidal xformer?  - Page 1
Forum section: electronics / beginners

author: jerry11
author_rank: Newbie
timestamp: January 04, 2016, 06:47:12 pm
content: Hello. I have an 250va 25v+25v toroidal transformer that I want to add secondary windings to. The secondary windings are to eliminate the need for an additional 12v center tap 3A transformer. I understand the voltage will be determined by number of turns, but what changes the amperage available? Is it wire guage? Also, since I need the center tap, do I make two windings in opposite direction, connecting the start of the windings for the center tap and the ends for the voltage (about 6v+6v for my application)?Thanks for the help.Jerry

author: w2aew
author_rank: Super Contributor
timestamp: January 04, 2016, 07:08:38 pm
content: Quote from: jerry11 on January 04, 2016, 06:47:12 pmCurrent capability will be limited by the wire gauge and insulation type (resistive loss, heat generation) and core material (flux density). If your primary is rated for 250va, then that will set the input VA limit to the transformer. You'd size your secondary windings appropriately, assuming that the sum total of the secondary output will need to be <250va. For the center tap, it's probably easiest to start winding, when you're halfway through, add a tap wire, and continue winding in the same direction.

author: Simon
author_rank: Unknown
timestamp: January 04, 2016, 08:40:09 pm
content: I think you want to wind the sendaries the same way so that they are 180 degrees out of phase,

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2016, 09:18:38 pm
content: The amperage is limited by heating of the transformer. So it the wire gauge that determines how much heat is reated by a given current. The VA rating is not a brickwall limit - it's just when the temperature reaches the rated maximum temperature of the insulation.As about half the loss is from the primary, don't expect much more power than the rated VAs without the extra windings.The magnetic saturation determines the volatge per turn that can be used - thus no influence on the current rating of the turn, except sometimes saturation magentization is lower at higher temperatures.

author: jerry11
author_rank: Newbie
timestamp: January 05, 2016, 12:05:58 am
content: Thanks for all the replies. Sounds like I would need to know some fine details of the wrapping wire to plug into a formula to figure the amps that would be available. I don't have any details on the wire, and don't have a way to measure that fine or accurately.So,Would your opinion be that 18 awg winding wire would get me at least the 3 amps? When I get a chance I will experiment with some wire I have on hand, as I don't currently have any winding wire.Regarding the winding direction, I guessed that I would need opposite directions to get 180 degrees out of phase. I will draw myself some pictures and try to wrap my head around that.ThanksJerry

author: Brumby
author_rank: Unknown
timestamp: January 05, 2016, 02:50:30 am
content: Quote from: jerry11 on January 05, 2016, 12:05:58 amDon't fret about winding directions. Just do this:Quote from: w2aew on January 04, 2016, 07:08:38 pm

author: retrolefty
author_rank: Super Contributor
timestamp: January 05, 2016, 03:13:43 am
content: You can draw 3 amps from a new added secondary, but only if your not going to also draw the maximum rated output current of the existing secondary winding(s). No free lunch here. If your project has dynamic current changes in normal operation then adding an additional winding is probably not a big deal. However trying to pull > 150VA rating of the transformer continuously is not a good idea no matter with the existing secondary or adding an additional winding.