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Old 19th May 2020, 10:11 pm   #1
Alan_G3XAQ
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Default Rewinding a toroidal mains transformer

Has anyone had experience, good or bad, of rewinding a small toroidal mains transformer? I need a 20VA transformer with 6.3V and 125-0-125V secondaries and I've been quoted over £100 for a bespoke unit.

I'm wondering how hard it would be to take an off the shelf £16 transformer, perhaps with 2x6V secondaries, and remove or ignore one of the secondaries and just add an HT winding? I have polyimide tape in the drawer to avoid the more obvious chances of electrocution. It feels like it should be easy to put on a test secondary to determine the turns per volt and I don't need terrific regulation because I have a series regulator for the rectified HT.

It looks on the face of it like I can make something for a fifth the cost of the bespoke item. How hard can it be?

Thanks, Alan G3XAQ
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Old 19th May 2020, 10:33 pm   #2
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Default Re: Rewinding a toroidal mains transformer

I have done the opposite, taken a torroid with a high voltage secondary and added a heater winding.
I bought an off the shelf transformer with twin 120V secondaries. I wound a few turns with normal pvc hook up wire, say 10 turns and measured the voltage I got. This gave the turns per volt.
I then wound my heater winding as two windings wound simultaneously (Bifilar) to give 3.15V (plus a bit to allow for loading).
I then connected the two new windings in series to give a centre tapped 6.3V heater winding. The centre tap can be connected to 0V.

Peter
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Old 19th May 2020, 10:55 pm   #3
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Default Re: Rewinding a toroidal mains transformer

I'm no toroidal expert but I think you'd be struggling to get a centre tapped 250V over-wind onto an off-the-shelf transformer. How would you get all that wire through the hole? Do you have a machine?
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Old 19th May 2020, 11:42 pm   #4
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Default Re: Rewinding a toroidal mains transformer

Quote:
Originally Posted by McMurdo View Post
I'm no toroidal expert but I think you'd be struggling to get a centre tapped 250V over-wind onto an off-the-shelf transformer. How would you get all that wire through the hole? Do you have a machine?
Agree. I think you need a split wheel which is passed through the hole and then wound with enough wire for the winding, The wheel is then rotated to pass the wire many times through the hole.

My preference would be to use two transformers, one to produce 6.3V and another connected 'backwards' to produce the high voltage supply.

Another way would be to make a DC supply and then use a cheapo boost convertor to get the HV supply.
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Old 20th May 2020, 12:24 am   #5
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Default Re: Rewinding a toroidal mains transformer

You can get C cores which form a split squared off toroidal core. Windings are then done in a similar way to interleaved lamination transformers.
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Old 20th May 2020, 3:03 am   #6
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Default Re: Rewinding a toroidal mains transformer

It would be possible to do!!. BUT it will take ages. a 20VA toroid will be in the ball park of 15 turns per volt. 250 volts times 15 = 3750 turns. It will have to be thin wire to enable the centre hole to accommodate all that copper. This will all be done by feeding a shuttle through the centre of the core, with the necessary amount of wire already wound onto the shuttle. As per your PM, I would try "canning" the existing transformer in a thin steel case.

Joe
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Old 20th May 2020, 6:18 am   #7
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Default Re: Rewinding a toroidal mains transformer

I've done this several times but with bigger tfmrs. At one time I could get cheap 500VA + toroids off ebay for something like £30. Procedure, take off secondaries, usually something like 35 0 35v 6A, save wire, good for heater windings. Get some scrap wire, wind on 10 turns, power up, record voltage, EG 1.57v. Take off wire, measure length, say 6ft, so we have 1.5v per 6 ft. Divide intended wiring by this, EG HT 150v/1.5v = 100, so we need 6ft x 100 = 600ft say. Find a bit of space free from bushes, trees, kids, dogs etc, used my local park, peg one end, measure off 600ft, then add a good bit, say 50ft.

Make a bobbin, lets say bit of ply, well smoothed, 1ft long 2 1/2" wide, remember, it has to go through the center easily, as you put windings on the hole will get smaller. Fold your length of wire in half, so 325v doubled, wind this on your bobbin, careful not to get it too tight, we don't want no kinks, though we'll get em. Take loaded bobbin home, toroid on bench, wind the winding,taking care to get wndg well spaced, tight and free of kinks, doesn't take long at all, a few hours. Count wndg's if you can, after a good few, measure voltage. With wndg doubled though, be sure to cut both ends, so we'll have 2 x 75v connected in series. Less turns the better.

It's a good idea to test the original 10 turns and final wndg with a load, so if our 150v is at 0.1A, we'd need a big 1k5 resistor, it's also a good idea to overload it in incremants to see how the tfmr regulates. This is usually doone till V falls by 10%. Listen to the tfmr and check for excessive heat. That done, load your wndg with the actual circuit if you can, for instance heater windings on toroids are tricky to get right. You have more volts per turn, so you can get something like 5.9v and 6.7v, but not 6.3v. Also if you were to measure a valves heater and get 20r say, then used a 20r resistor as load , then added or take away a few turns till you hit about 6.3v, you'll find that when you power up the amp, heater volts will be off, so use the actual load if you can. Lastly remember with high current wndg's like a heater, the wiring from the tfmr to the load will drop voltage, bear this in mind.

One last thing, make sure you have a good insulation layer between pri and sec, obviously, you can also include a copper screen band. See - https://www.youtube.com/watch?v=lyZ7nM6Fo94 and https://www.youtube.com/watch?v=6e87pi47CsI If you can do a megger check.

Have a go, it's well worth it and easy to do, hope that helps, Andy
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Last edited by Diabolical Artificer; 20th May 2020 at 6:23 am.
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Old 20th May 2020, 9:24 am   #8
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Default Re: Rewinding a toroidal mains transformer

I have had a look in the workshop and found "one I prepared earlier".

Its based on a 50VA torroid with 2 X 55V secondaries, the secondaries are connected in series to give 110V. I then use a voltage doubler as Armstrong did in their valve amps (see attached) this gives just over 300V

The transformer is Farnell No 1675063.
The tested turns per volt is 8.13.

I wound 2 secondaries of 3.15V simultaneously ie Bifilar, which is 26 turns.

The 3.15V secondaries are also connected in series to give 6.3V and the centre tap can be connected to chassis.

I attach a photo of the finished transformer, it isn't pretty, I really must replace the PVC insulating tape with something more suitable.

Peter
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Old 20th May 2020, 11:55 am   #9
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Default Re: Rewinding a toroidal mains transformer

All this feedback is really helpful. Although the EI transformer I am replacing has a 125-0-125V secondary I can just as easily use a 0-125V winding and a bridge rectifier. So half as many turns to tediously wind on. I'll get a 2x6V 30VA transformer from RS, which is the smallest they sell. If I can I'll remove one of the 6V windings to make more space. I might add a few turns to the 6V winding to get to 6.3V but I think that will be spitting hairs.

More anon,

Alan
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Old 20th May 2020, 12:41 pm   #10
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Default Re: Rewinding a toroidal mains transformer

I'm actually looking at rewinding a 9 + 9V toroidal transformer, 30VA from RS. I actually thought I would be able to wind this at work, out-of-hours, only to find we have nothing suitable - either too large or too small.

Removing secondaries (both were 139 turns, 0.75mm wire). Off-load voltage measured at 0.775V per turn, implying the (single, 230V primary) is 3,097 turns.

Resistance of primary measured 85.7Ω thus at the full 30VA, we are looking at about 4% voltage loss in primary. Similar would apply to secondary, if using similar current density.

For heater windings, 6.3V is OK by hand using a wire shuttle of a piece of wood with 'V' notches at the end. But HT would be rather time-consuming, so I am in process of trying to make a winding machine. Relatively early days as yet, but if I succeed in 1 turn per second, I'll be well pleased.

So, Alan, if you have any bright ideas, I'll be really interested!
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Old 20th May 2020, 12:59 pm   #11
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Default Re: Rewinding a toroidal mains transformer

Kalee20,

For the dim, please can you walk through the arithmetic that goes from 9V secondary, 139 turns, 0.775V/turn, to get to a primary containing 3097 turns?

Thanks, Alan
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Old 20th May 2020, 2:10 pm   #12
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Default Re: Rewinding a toroidal mains transformer

Yes! And good you have queried, for starters, this should be 0.0775V/turn (significant typo!)

I removed the 9V windings counting turns. Both were 139 turns (which is good, they are intended to be connected in parallel if desired, so any difference would give rise to circulating currents).

I wound 20 turns on, energised at 230V (trimmed with Variac), measured 1.55V. So that's 0.0775 volts per turn, off load.

With this figure, 230V primary will have 230/0.0775 = 2,967 turns (not 3,097, another error).

Note then that the "9V" secondaries will show 139 x 0.0775V = 10.77V off-load. On-load, due to winding resistance, voltage will drop. At higher temperature, voltage will drop more due to resistance increase.
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Old 20th May 2020, 4:46 pm   #13
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Default Re: Rewinding a toroidal mains transformer

Now it makes sense! Thanks. As you did with your project I think I might go for a voltage doubler circuit with half the number of turns: about 965 in my case.
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Old 20th May 2020, 5:53 pm   #14
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Default Re: Rewinding a toroidal mains transformer

you must all be very patient! The last coil I wound myself was in 26swg and on a plain flanged bobbin. It needed 280 turns, which took me 10 minutes on my mini lathe. By turn no.50 I was already bored to the point of narcolepsy.
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Old 20th May 2020, 9:44 pm   #15
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Default Re: Rewinding a toroidal mains transformer

Memory lane time. Back in 1960, I had an ex WD 750 SV Harley Davidson. (mistake really). It had a double ended HT ignition coil, and it was dodgy. Loose terminals on primary side meant it failed. It was oil filled and the top unscrewed so I decided to try to fix it. After dinner, I hid in the "constant temperature room" used for viscosity testing. I unwound a few thousand turns from BOTH sides of the secondary, finally getting to the primary. I remade the bad end, and set about rewinding the thing. I had the job finished before home time. Connected it back on the bike, and road home. Seeing all those coils of turns along the bench and back a few times was a daunting sight.
So, you coil winders, it is all possible. Patience and in Andy's case a 600ft garden lawn.
Les.
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Old 21st May 2020, 5:30 am   #16
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Default Re: Rewinding a toroidal mains transformer

"you must all be very patient! The last coil I wound myself was in 26swg and on a plain flanged bobbin. It needed 280 turns, which took me 10 minutes on my mini lathe. By turn no.50 I was already bored to the point of narcolepsy. " Winding with a drill, lathe or other motor is boring, but winding a toroid by hand isn't so bad, put some music on and after about 10 turns you get into a rhythm and sw the brain off, it's like meditation.

Winding an EI by hand is different, it's just round and round, boring and easy to lose count, with toroids there's a bit more to it so it's less mind numbing. Miguel in Cuba winds his own tfmr and numerous coils for relay's by hand, or he did till he motorised the job, he's the king of hand coil winders. If your skint or whatever needs must when the devil drives.

That's some story Les, that's motorbike fixing par excellence, brave man, it's very Zen and the Art of Motorcycle Maintenance.

Andy.
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Old 21st May 2020, 9:31 am   #17
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Default Re: Rewinding a toroidal mains transformer

Most toroids seem to have a moulded in centre with a captive nut, not easy.

Why not get two 6.3V transformers and run one backwards to get the 125-0-125 from the primary?
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Old 21st May 2020, 2:03 pm   #18
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Default Re: Rewinding a toroidal mains transformer

Quote:
Originally Posted by woodchips View Post
Most toroids seem to have a moulded in centre with a captive nut, not easy.
The Farnell I reported on earlier in this thread has an open centre.
The Vigortronix ones from Rapid are also open centre.


Peter
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Old 22nd May 2020, 9:53 pm   #19
Alan_G3XAQ
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Default Re: Rewinding a toroidal mains transformer

I've added an HT winding to an RS 671-8937 30VA 2x6V toroidal mains transformer

https://docs.rs-online.com/8baf/0900766b8157a9ae.pdf

I've aimed for a 120V DC output via a full wave voltage doubler and wound the wire bifilar fashion to minimise the number of turns. A circuit simulator suggests I need 49.5V RMS off load with 25 ohms series resistance, although that ignores iron losses (do they change between offload and full load? Steinmetz implies not, so perhaps I really can ignore them).

A test winding gave 0.083V/turn, which is a little bit different to Kalee20's figure for his "identical" 30VA transformer. I've no idea why. That's 596 turns for 49.5V, or 298 bifilar. Ten turns round the transformer is 89cm so I measured 59.6*89 = 53m of enamelled wire (.25mm dia or thereabouts) plus 10% for errors, halved it, and wound it onto a crude 50cm shuttle.

The actual winding wasn't too mind numbing. I did it in three stints with a cuppa between sessions. I confess I quickly gave up on trying to make a neat two-layer winding and instead just scramble wound it.

I wound all the wire on the shuttle onto the core without trying to count turns. On test the open circuit voltage is about 15% high. A voltage doubler is far from a nice resistive load so rather than try to be clever I will just remove turns with the transformer in circuit until the DC output is 120V. My enamelled wire isn't the nice tough polyurethane insulated variety so once I have adjusted the turns I'll apply a coat or two of varnish.

OK, it's not pretty but £16 as compared to over £100 for a commercial unit looks good to me.

73, Alan
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Old 22nd May 2020, 10:19 pm   #20
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Default Re: Rewinding a toroidal mains transformer

Well done, glad it worked out OK.

Peter
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