Help figuring out how to wind new secondaries on toroidal power transformer. - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 21, 2024, 08:04:08 pm
content: I was given this pretty hefty toroidal transformer a few years back and I could never figure out the windings, even after tearing it apart there were nonsense things like two inputs leading to the side of one of the coils. Anyways I decided I wanted to rewind the thing, its a nice core and I would like to use it to build a dual rail 100v supply. This is what I have after taking off what I believe was the secondaries.There are two coils on there measuring 1.59Ω and 1.53Ω using a four wire tester, they both measure ~8mh on one of those eBay lcr/transistor testers. So I am assuming these were the primaries set up for 240v in series or 120v in parallel. I connected one coil to a function gen and the other coil to a scope, then vice versa, I verified the transformer currently behaves like a 1:1 transformer as excepted. Doing a bit of research I believe the transformer is about 250VA based on measurement of similar transformers.What I would like to do is wrap two new secondary coils that can output 102 volts at 1 amp a piece. What I don't know is what gauge wire to use (seems like the wire ampacity chart doesn't really apply to transformers, and the core is a bigger factor). Also I'm not sure how to figure out the number of turns I need to step down from 120 to 102v. Usually you would calculate transformer windings based on a ratio, but i have no idea how many wraps the current primaries are? I have seen people rewrap secondaries on E-I core microwave transformers, i am sure they had no idea how many wraps the original primaries had on those transformers. So how do you go about calculating secondary wraps and gauge in this situation?

author: wasedadoc
author_rank: Super Contributor
timestamp: July 21, 2024, 08:26:34 pm
content: Do a test wrap of known number of turns and measure the voltage ratio. Adjust number of turns accordingly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2024, 09:18:10 pm
content: 24AWG will be close enough for 1A.There isn't a rating rule within components: as long as the component temperature and insulation system are consistent, and efficiency and regulation are as expected for the application, it's fine. You could bury a thermistor here (suitably insulated) if you need the confidence of measurement.How much voltage does one turn create, and at what primary voltage exactly? (Also, you know how to count 1 on a toroid, right? It's passes through the center, because one pass through, no matter how straight or long it is, always closes in one loop around the core.) Can you measure primary current (preferably with a clamp probe and scope)?You'll want a double layer of PET tape over that. 3M #56 is an example. If you saved what you removed, it would be better not to trust it; it's been stretched, crinkled and folded from winding before.Can you see the core itself anywhere, get some measurements of dimensions? Count primary turns?Tim

author: Zero999
author_rank: Super Contributor
timestamp: July 21, 2024, 09:43:49 pm
content: Quote from: rwgast_lowlevellogicdesin on July 21, 2024, 08:04:08 pmThat reading is well off. 8mH would be an impedance of just 3 Ohm at 60Hz, which would be a huge magnetising current of 40A at 120V.What was the test frequency?The inductance should be several orders of magnitude higher.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 22, 2024, 01:21:12 am
content: Maybe it was 8H, im not sure I dont put a lot of stock in the inductance measurements produced by those cheap testers, I dont think they test at any frequency they probably just solve for L in a tank circuit.I can see the core a little bit, it is a huge green toroid with ~22awg primary coils. Its about 102mm diameter, 60mm tall, center hole is 41mm diameter, it weighs 6.3lbs or so. I wrapped 10 turns earlier and put 52v 60hz AC in to it and got 1.99 out. After some math I would need about 220 turns to 102v out at 120v in. That comes out to 130ft of whatever gauge wire. From what I understand if I want two secondary coils they need to be wrapped at the same side by side. So figuring what to wrap two 130ft wires on that will be able to slip through a 40mm hole isn't that easy.If a connect one of the primary's to 120vac and load the second one at different currents will I see waveform distortion when I max the current out? If so would I then take that number and double it when the primaries are connected in parallel?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2024, 02:08:58 am
content: That sounds more like 200ish VAper winding, that's a pretty beefy core.Let's see, Ae = 1830 mm^2, le = 224 mm, if Bmax = 1.2T, it should be 0.585 V/t. Triple what you tested it at. Could it actually be like, 380, 400V--?Or it's over-wound for exceptional low core loss, of course this increases copper loss, or reduces VA rating for given [reduced total] loss.Distortion is independent of load. You want to drive the -- say you take the 10t test winding, and put that on a 6.3V (or even 12.6V!) transformer plus a 1-ohm power resistor in series to limit current, and read the waveform on the winding. Plug it into a variac and give it a spin, see where the voltage starts clamping down at / waveform goes flat in spots (at onset of saturation, it looks like crossover distortion).As for inductance, say mu_r = 10k, A_L = mu Ae/le, about 100uH/t^2, ballpark 200 turns should be 4.1H. So 8H is very reasonable, but mind it may read as low as some 100s mH at small signal levels because steel is nonlinear like that (mu_i can be < 1000).Tim