Winding tfmr's. Heater windings.

[Diabolical Artificer]

I'm winding another tfmr for an amp, again a toroid. I've stripped off the two 50v/5A secondaries and am winding a HT 280/310v @ 1A with several other wndg's for bias/CCS, etc. I'll need a center tapped 6.3v for elevated htr's for cathode followers, but this winding is proving problematic.

17 turns gives me 6.3v ish with a 4R7 load, this to power 3 off 6SN7's = 1.8A. With a tap on the ninth turn I get 3.6v and 3.2v, going back one turn, IE to the 8th, gives me the same voltages. no worries, I'll just use two 220r resistors to give me a virtual tap. I've had similar trouble with htr windings, it's tricky to get these low voltage high current wndg's to sit exactly at 6.3v with differing loads and as shown also tricky to get the tap exactly midway

To my question, how did they do it in the old days, is the fact that I'm using a toroid the cause of the problem? A E/I core must use more turns as the toroid core has a higher flux density, and thus easier to get the tap midway. Each turn on my toroid in 400mV, on a E/I core probably less.

I've wound the htr's wndg's first, right next to the primary for better regulation, and used thicker gauge wire than needed, not sure what more I can do.

Andy.

[Terry]

Andy, why can't you put tap on the eight and a half turn?

Anyway, as 3.2 + 3.8 = 6.8V, wouldn't 16 turns overall be better?

[boater sam]

 I would suspect that the old timers did not worry too much about getting the center tap bang on. I recently tested a few fairly new tapped mains transformers and they had center taps near but not exactly middle.

The same applied to the HT windings. One side will always be different under load as the length and hence the resistance of half the winding will be different due to the bobbin diameter increasing as the winding proceeds.

[pwdrive]

What will the tap be connected to?

Lawrence.

[Refugee]

To me it would appear that the tap is physically in the hole through through the middle making the lead out wire become part of a turn.
I think you are stuck without using a laminated transformer where the half turn can have the lead out run around the outside of half of the core.

[pwdrive]

What will the tap be connected to?

Lawrence.

I was meaning in circuit.

Lawrence.

[Diabolical Artificer]

"why can't you put tap on the eight and a half turn?" It don't work like that Terry, voltage is less or more only by complete turn, at least on toroid's. I spent ages mucking about with bits of turns to try and get wndgs bang on at first and would have thought that voltage changes along the length of a turn, it doesn't.

6.8v unloaded I should have said.

I've measured a few htr's wndgs on EI cores, they're pretty equal, on the whole. They have to be for hum cancellation no?

Tap was to be connected 100v or thereabouts to elevated htrs on cathode follower Lawrence, so maybe exact balance not essential, but . I was more interested in the question for future reference and curiosity . In this case I'll just use two 220r resistors to give a virtual tap.

Of coarse it's possible to get a tapped wndg exactly equal in resistance and voltage by using a twisted/bifilar wire, but I can't see tfmr makers doing this due to cost. I'll dig some tfmr's out and measure them this morning and report back.

Andy.

[Kalee20]

To me it would appear that the tap is physically in the hole through through the middle making the lead out wire become part of a turn.
I think you are stuck without using a laminated transformer where the half turn can have the lead out run around the outside of half of the core.

Refugee is right - you can't have a half-turn with a toroidal transformer (at least, not unless you drill a hole in the core).

With a laminated transformer, you can - if you have the start and finish coming out one side of the lamination stack, you can have the CT coming out the other side.

However, it is not to be recommended unless there is practically zilch current drawn from the half-turn terminal, because you get flux imbalance between the two outside limbs of the transformer, and this manifests itself as high leakage inductance. You can add flux-balancing windings which fix this, but it's all getting complicated now.

Best way with a transformer-from-scratch, is design the low voltage secondaries first, round them to a suitable integer, then fine-tune the voltage by selecting appropriate primary turns.

[Diabolical Artificer]

"Best way with a transformer-from-scratch, is design the low voltage secondaries first, round them to a suitable integer, then fine-tune the voltage by selecting appropriate primary turns." Good idea Paul.

Talking of winding toroid primary's I haven't had a go at this yet, having always used prewound tfmr's, just replacing the secs, but guess the basic gist is to calculate the toroid core area - pretty simple so far. I saw one video where they wound the primary and checked every so often with a lightbulb in series I think with said pri until the bulb stopped glowing. For the life of me I can't figure out what they're doing here.

The tricky thing with using unknown cores is that there is no data, no BH curves or type of material used for the core etc.

Andy.

[pamphonica]

Just a small suggestion, as half turns are definitely not available on toroids.
Could you add a small winding to put in series with the primary? That would slightly lower the output voltage, and thereby allow you to use exactly 9+9 turns for your heaters. You could adjust this simple additive winding a lot more easily than the centre-tapped low-turns secondary.
I've used this often in the past to modify output voltages on HT transformers where they are a bit too high or low. Add (or subtract) an unused heater winding to the primary and it can work nicely.
Jeremy