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Old 14th October 2012, 10:49 AM   #1
M Gregg is offline M Gregg  United Kingdom
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Default Toroidal (adding a winding)

Just a quick question,

Whats the best way to wind another winding onto a toroidal power transformer?

Small bobbin and keep joining the winding or a long shaft?

I could just trial and error, however some must have done it..

Is it better to spread the winding across the whole core or limit it to about 2/3 of the way around the core?
Thinking about the tail outs from existing windings..


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Last edited by M Gregg; 14th October 2012 at 10:53 AM.
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Old 14th October 2012, 11:20 AM   #2
Yvesm is offline Yvesm  France
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Old 14th October 2012, 11:26 AM   #3
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Try to spread the windings evenly around the core.

The best way to wind it is with a bobbin but it's easy enough to just loop it through by hand.
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Old 14th October 2012, 11:46 AM   #4
AndrewT is offline AndrewT  Scotland
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if it's low VA then you need lots and lots of turns for required output voltage. That requires a lot of wire that is very long.
Best to wind that onto a long flat bobbin that fits through the eye of the toroid.

If it's high VA then you need a lot fewer turns but need thicker wire. That is again a hassle.
Use that long flat bobbin.

If you only need a low current winding from a high VA transformer then you can handwind,

But in all cases you have to calculate the length of wire required for the number of turns you have worked out.
Wind on a test winding by hand. Just try 10 turns. Power up from the mains, with the bulb tester and then measure the AC voltage available at the open circuit test winding.

The long flat bobbin can be a piece of plywood, 300mm long, by 5mm thick, by the "eye" diameter less 5mm. It has to be able to pass through after you wind on the second last turn. Round the ends of the ply so that you curve the wire around the ends and not create "kinks".
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Old 14th October 2012, 11:56 AM   #5
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It all depends on what purpose the winding is for. If its a supplementary winding for heaters then its relatively simple to hand wind onto the core using appropriately sized wire. Do 10turns and test what you get and then scale to what you need - testing at the end to fine tune. Bind up with insulating tape at the end.

I have done this a number of times to build a heater transformer.

Higher voltages are unlikely to be practical since feeding the required wire through the hole is unlikely to be practical.

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Old 14th October 2012, 12:12 PM   #6
balerit is offline balerit  South Africa
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Sometimes a gem of a story comes up and this is one of them. Thanks for posting.
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Old 14th October 2012, 01:31 PM   #7
M Gregg is offline M Gregg  United Kingdom
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Regards the windings,

I am thinking of putting two 60V bias windings on the heater Toroid which is 120VA. So they are low current windings.

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Old 14th October 2012, 02:06 PM   #8
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I take a long length of wire and fold it in half. Then put it on the opposite side for the other wires coming out of the toroid and tape the middle of the wire to the transformer and start winding one side at a time. This will get you in the ballpark. Take 5' or so and wind about 10 turns. Power it up and measure what you get. Take the desired voltage and divide into your voltage. Say 60V/10v if you got 10V out of 10 turns. You get 6 after division. Multiply that by 10 and you know you need 60 turns. Take a measuring device and see how long your windings are. Then estimate how tight your turns need to be spaced to stay even around the core. I've done this a bunch for filaments and bias.
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Old 14th October 2012, 02:16 PM   #9
AndrewT is offline AndrewT  Scotland
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a 120VA is more likely to be 3t/v to 6t/v

120Vac is going to need between 180T and 360T.
That is a lot of wire and a lot of winding.

Buy a small biasing transformer.
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Old 14th October 2012, 02:19 PM   #10
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Seems that 60V is going to take a few hundred turns from my experience. I personally wouldn't attempt it.
There is also the small issue of toroidals passing large amounts of hi-freq line hash.

Far better for you to find some small EI transformers to press into service - these can usually be hidden under the case without taking up to much real estate.

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