adding windings to a toroid tranny - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: June 26, 2019, 02:08:18 pm
content: Hello!I have a 30VA toroid (230V primary, 2x9V@1.5A secondaries). I need to have two additional 12V windings, one for 100mA, another one ~10mA for auxilary supply, they will provide power to opamps, etc.Questions:1. Which type of wire should I use? Enamel? What diameter?2. How should I wind it? In one layer, or multiple layers?3. Can I use kapton tape for outer insulation?Additional info: it seems I need 160+160 turns for 14V output (~20% more voltage to include transformer regulation). Measured voltage is ~0.086V per turn when idle.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2019, 02:57:48 pm
content: The exact wire diameter is not important. It just has to be thin enough to make sure it does not take up too much space. Enamel wire is good because it has a thin insulation and thus takes up less space. Chances are one could use the same wire size, as very thin wire for 10 mA is difficult to handle and 160 turns are not that much. There is a rule of thumb for the current density - if in doubt a little thicker does not hurt much. With this size transformer the voltage drop under nominal load should be less than 20 %, especially if the wire used tends to be on the thick side.Wether the windings are in single layer, evenly spread or localized at one point makes a small difference in the form as additional series inductance / slightly less coupling. Due to the low power it does not matter here.As the voltage is relatively low, there is little need for extra insulation, especially if as normal the primary winding is on the inside.The outer tape is more like a mechanical protection.

author: Brumby
author_rank: Unknown
timestamp: June 27, 2019, 12:53:06 pm
content: I've added a winding to a toroidal transformer. Similar need - low voltage (15V), low current (100mA). I just used a light gauge hookup wire (which was overkill for the need) but that worked fine.The only important thing is to make sure that the total VA draw by the loads does not exceed the stated VA rating for the original transformer.

author: julian1
author_rank: Frequent Contributor
timestamp: August 19, 2021, 10:36:35 pm
content: Is it necessary to match the winding direction of the existing secondaries?From one point of view I think it should not matter, and I can just adopt whatever polarity a quick meter test shows.But perhaps the polarity of inter-winding capacitance matters/has some influence?Dots only indicate instantaneous polarity.When winding bare toroids, it's easy to see and just make all windings match the same sense/direction.The problem for adding extra windings to an existing toroid, is that I cannot see through outer insulator tape - to identify the direction of the secondaries.I tried placing a magnetic compass against the toroid, with a 0.5A load through the secondaries - but it doesn't reveal anything.Looking at videos of professional winding machines shows different directions/ and no consistent convention used,professionaltoroid body rotated counter clockwise. while winding over the top.toroid body is rotated clockwise.

author: bob91343
author_rank: Super Contributor
timestamp: August 20, 2021, 04:44:23 am
content: Don't worry about winding direction. The gauge needs to be heavy enough to handle the current and light enough to fit in the window. I assume we are working with mains frequencies, 50 or 60 Hz so capacitance isn't important. The rule for current is a few hundred circular mils per ampere. 300 is on the low side and 600 is conservative. So your 100 mA load needs at least 30 circular mils wire. Any insulation will do.

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 21, 2021, 02:40:32 am
content: If the two additional windings do not need to be isolated from the 9 volt windings, you can just add enough turns to boost the 9V to the required level. 30 gauge wire wrap wire (100 circular mils) would be an easy solution, wind and secure the ends, no need to cover the whole core.If you connect add-on windings in series with the wrong direction, the voltages will subtract, not add. It is also knowing as bucking (reduction of voltage) or boosting (increase in voltage). A test winding of a few turns connectable in either direction will show you which direction is required.

author: amyk
author_rank: Super Contributor
timestamp: August 21, 2021, 06:12:06 am
content: Quote from: julian1 on August 19, 2021, 10:36:35 pm1. I hope you used DC, because it certainly wouldn't reveal anything with AC. 2. Now you know why toroidal transformers are used - very little magnetic flux leaks outside.