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Old 18th Nov 2018, 10:13 pm   #1
bikerhifinut
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Default Working out wire diameter for transformer secondaries.

After much internet searching, I can not find a simple resource for calculating or looking up the wire sizes needed for various voltage versus current combinations of transformer secondaries.
It seems to me that there isnt a simple answer, there never is of course, but is there a rule of thumb I can use?
For instance, I wish to make a simple over wind on a 600VA toroid that will give me 9 volts at 2A for a DC valve heater regulator.
You can assume that the primary is running at half or less than its rated load. turns ratio around 2.5 per volt from my own empirical measurments.
Why? Well I would like to have this and after rewinding the existing pair of 9v secondaries to give 6.3V ( they appear to be wound bifilar so its not easy to just unwind one section) I will need to have the choice of a 9V seconadary to rectify and regulate to 6.3V.
You may safely assume that the windings are on the surface of the toroid and thus all is accessible.
I had initailly assumed that as long as I chose a wire size that could handle 2A at 9V AC i would be fine. But it seems i am mistaken.

A.
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Old 18th Nov 2018, 10:19 pm   #2
Ed_Dinning
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Default Re: Working out wire diameter for transformer secondaries.

Hi Andy, bifilar wire is often used on heavy secondaries as it is easier to wind than a wire of twice its CSA.

Rule of thumb is 3A per mm^2 for wire area. plenty od wire table on web to give you diameter from area. Not that insulation added gives a larger diameter than bare wire.

Also note that copper has a large positive tempco so resistance figures are often given at 20c and 50c.

Ed
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Old 18th Nov 2018, 10:25 pm   #3
bikerhifinut
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Default Re: Working out wire diameter for transformer secondaries.

Quote:
Originally Posted by Ed_Dinning View Post
Hi Andy, bifilar wire is often used on heavy secondaries as it is easier to wind than a wire of twice its CSA.

Rule of thumb is 3A per mm^2 for wire area. plenty od wire table on web to give you diameter from area. Not that insulation added gives a larger diameter than bare wire.

Also note that copper has a large positive tempco so resistance figures are often given at 20c and 50c.

Ed
Thanks Ed,
I'd have Pm'ed you but I appreciate you are very busy for the forseeable.
that explains the bifilar winding on the toroid as its capable of delivering 16A!
I wanted to just take the windings off one secondary as that would still be plenty for my needs, and leave the other 9V alone for the other part of the circuit, but as it is feasible to overwind I thought that might work too.

Andy.
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Old 18th Nov 2018, 10:27 pm   #4
bikerhifinut
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Default Re: Working out wire diameter for transformer secondaries.

Quote:
Originally Posted by Ed_Dinning View Post
Hi Andy, bifilar wire is often used on heavy secondaries as it is easier to wind than a wire of twice its CSA.

Rule of thumb is 3A per mm^2 for wire area. plenty od wire table on web to give you diameter from area. Not that insulation added gives a larger diameter than bare wire.

Also note that copper has a large positive tempco so resistance figures are often given at 20c and 50c.

Ed
So a length of 1mmsqd pvc insulated wire stripped from a bit of twin and earth would work just fine for 2A?

A.
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Old 19th Nov 2018, 1:49 pm   #5
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Default Re: Working out wire diameter for transformer secondaries.

As long as the operating temp of the TX doesn't worry the PVC, it should be OK. PTFE or just enamel insulated wire might be better.
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Old 19th Nov 2018, 2:47 pm   #6
kalee20
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Default Re: Working out wire diameter for transformer secondaries.

Quote:
Originally Posted by bikerhifinut View Post
I had initailly assumed that as long as I chose a wire size that could handle 2A at 9V AC i would be fine. But it seems i am mistaken.
No you're not. But it depends on what you mean by, "Can handle 2A."

The piece of wire in a 13A fuse is quite happy carrying 13A, but you wouldn't want to use the same wire gauge for a 20m extension cable. Similarly, even the wire used in your off-the-shelf
extension cable that's happy carrying 13A when all unwound, may overheat bunched-up in a tight coil with limited airflow.

Ed's rule-of-thumb is pretty good. I have designed transformers using much less, and also rather more. One, for example, is running at nearly 7A/mm², but it is sitting in a howling gale from a fan.

Sometimes, heat generation is not an issue but you may reduce wire gauge deliberately to give extra resistance. An example is for feeding a rectifier valve with a capacitor-input filter, where the rectifier valve specifies a minimum limiting resistance to keep its peak current down. So you reduce the wire gauge of the HT secondary. This gives you a bonus as there's extra space to beef-up the wire for the primary and the heaters.

For your application, if you have wire that's happy carrying 2A, and there is space on your toroid, use it!
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Old 19th Nov 2018, 10:03 pm   #7
bikerhifinut
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Default Re: Working out wire diameter for transformer secondaries.

Thanks for the information and advice folks. All very useful to me.

I have just now succeeded in taking 5 or so turns off one of the 9V secondaries and adjusted the turns on load. getting a nice 6.4 to 6.5V across 3 x 6550 filaments and the voltage didnt sag when I put a 4th one in the circuit so It'll nicely cope with 4 x EL34 or even 4 X 6550/KT88 if I get a notion to try them in the power amp. (I have an idea to have a switch to alter the cathode resistors to achieve the correct bias in this situation.)
So I think I am sorted with a nice quiet toroid for my heater and HT needs including a 9V winding for the DC heaters on the preamp and phase splitter valves. There was no sign of the very thick wire on the secondary even gettiing slightly warm so I am happy all is sound.
Just need to reapply the mylar insulating tape around the transformer and then ponder how best to mount it.

Thanks for all your help. Much appreciated.

Andy.
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Old 21st Nov 2018, 4:24 am   #8
joebog1
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Default Re: Working out wire diameter for transformer secondaries.

You only had to ask Andy

If these are too small I will email a PDF to you.

N.B. !!! your toroid is rated @ 12 amps for both 9 volt windings in parallel.
As you are not using the HT capability, or the maximum heater current available, nor are you using the bias winding, the toroid will cruise along.

Cheers

Joe
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