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EWorkshop1708 18th February 2009 11:23 AM

Winding a Monster Toroid, Hum & Voltage Sag
 
I just SCORED a transformer for a powerful audio amplifier! I got a toroid transformer from a huge battery charger. The secondary coil was shorted on the toroid, and the soft-start was fried, so the owner stripped & scrapped the charger, and let me keep the toroid!!! I unwound the secondary and let him scrap the copper. 5x12AWG litz coil wire! :eek: Two of those strands burned and shorted together.

This Monster Toroid is 8 inches across with just the primary alone! Much Bigger than an Avel 1KVA. It will be bigger once I wind a secondary back on it! I'll be winding 2x10AWG for a 25-0-25V secondary.

Anyway, I want to use this for an amp, and I'm wrapping my own secondary, but my concerns are voltage sag, and which primary tap to use.

To explain the primary coil: The primary has 4 taps, because the battery charger was originally 24V@30A, 12V@60A, 12V@40A, and 6V@60A.

It simply changes the # of turns on the primary for the 120VAC depending on which tap. The transformer hums audibly on the 24V hookup, and slightly on the 12V 60A. The tap for 12V 40A is nice and quiet, so I want to use that one, and I figure I can wrap more wire on the secondary for more voltage if needed.

What I'm wondering, is it better to use less turns on the primary for more current, and have audible 60hz hum, or more turns to make it quiet? I'm concerned that more turns on the primary will cause more voltage sag under load in the secondary by not driving the transformer as hard.

Thanks,
Eworkshop1708

richie00boy 18th February 2009 11:48 AM

I can't understand what you have. Surely the primary is a fixed winding (maybe with two windings to achieve US/UK mains capability) and it's the secondary that is tapped to give you the different voltages and currents.

AndrewT 18th February 2009 12:52 PM

Try to measure the mains AC current in the primaries when trying all four options.
I think you'll find noise is proportional to AC current.

Go for the option that gives least primary current.

You'll find it far easier to wind smaller gauge wire. Use two, or three or even four wires in parallel, to reach the current output you require. Keep all the taps separate. Keep the two sets of 25Vac windings separate.

That gives you the maximum flexibility when you come to use the transformer.

If you want good regulation use 3A/sqmm, if you want excellent regulation use 2A/sqmm.
1.6mm diameter is ~2sqmm and can pass 4A to 6A per winding.
2mm diameter can pass 6A to 10Aac per winding.

Is 10gauge about 3mm diameter?
That's just 7sqmm and can pass a maximum of ~22A.
From 0-25,0-25Vac that's only 1100kVA. You're strangling the toroid using 3mm wire.

EWorkshop1708 18th February 2009 02:42 PM

2 Attachment(s)
Quote:

Originally posted by AndrewT
Try to measure the mains AC current in the primaries when trying all four options.
I think you'll find noise is proportional to AC current.
.....................
Is 10gauge about 3mm diameter?
That's just 7sqmm and can pass a maximum of ~22A.
From 0-25,0-25Vac that's only 1100kVA. You're strangling the toroid using 3mm wire.

Thanks AndrewT, that clears up a few things! :D

This toroid is wound for 120VAC operation, with just different taps to change the secondary output voltage by the ratio of turns.

Yes, the higher current taps (less primary turns) make more noise. Original 6V tap draws the least current, and makes least noise, but I didn't want to wrap nearly double the amount of turns to get the voltage I need, I'll so the next one up, Original 12V 40A tap. So what you mention, having more primary turns won't reduce the amps output at the secondary then?

Also, you mention 10 AWG wire good for 22A. So two 10AWG wires in parallel should be good for 44A right?

My plan to save wrapping effort, and half the actual amount of turns I have to wrap, is to actually wrap FOUR 10AWG wires together in parallel with enough turns for just 25V, and when it's done, I wire them in series-parallel to get the 50V (25-0-25) so each 25V side would have 2 parallel 10AWG wires, and where they connect would be the CT. I'll be aware of the proper phase of course.

Would that provide the required VA?

AndrewT 18th February 2009 03:30 PM

4 windings of 10gauge (if this is 3mm diameter) will give max of 550VA per winding, but regulation is likely to be quite high, particularly if you try to draw 2200VA at once.

Reducing this to 2A/sqmm gives about 350VA per winding and regulation will be better. Total ~ 1400VA.

The limit for this transformer will be set by the primary turns and the core size. I cannot give any help with this side of the redesign.

Have you tried to wind bi-fillar before?
Have you tried to wind 3mm diameter before?

3mm and quad fillar will be some challenge.

Yes I agree with your sketch, bring all the tappings to the outside.
Remember to attach flexible tails to each tapping so that you don't fatigue the solid core wire. And keep the junction between solid core and flexy tails well insulated and strongly attached.

wg_ski 18th February 2009 10:02 PM

Just how much power do you expect to get out of a +/-35V rail anyway? You'll probabaly only end up with about 250W at 2 ohms, which can be done easily with a little bitty 500VA toroid.

Winding 10 ga bifilar is enough of an s/b to make it worthwhile to go for 80V rails, at least.

EWorkshop1708 18th February 2009 10:08 PM

I forgot to mention the type of wire. It should be not TOO hard to bend or wrap, it's stranded black THWN house wiring, so it should bend easier than the solid wire. According to specs, and some online research, I found it's rated for 30 amps, so two in parallel should do.

The insulation is not very thick, because it's THWN, so it should wrap nicely on the toroid, hopefully not taking up too much space. I'll wrap it very tight to maximize the space available.

Thanks, you helped me figure out the primary side, so I'll focus on wrapping the secondary now. I tested 10 turns of wire, which produced 7.2VAC, so I now have it figured how many turns and feet of wire are needed for the new secondary.

This core is huge, and I'm not sure the planned amplifier will really stress this transformer very much. Toroids run cool, and that's why they are GREAT!

I was originally planning on only ~500W RMS combined from all channels, but since this toroid ended up being much bigger in size than the original plan, (the amp is way overbuilt anyway, it would handle it) I could eventually run half the speaker impedance, double the watts, and use about 1KW at the most.

AndrewT 18th February 2009 10:15 PM

Quote:

Originally posted by EWorkshop1708
I forgot to mention the type of wire. .....................
The insulation is not very thick, because it's THWN, so it should wrap nicely on the toroid, hopefully not taking up too much space. I'll wrap it very tight to maximize the space available.

you have not got a hope in hell.

Buy a few kg of 1.6mm diameter enameled copper wire. It is made specifically for this job. How much weight of copper came off the old secondaries? look at a 1kg reel of enameled copper wire and then think about extra volume the plastic insulation would take up.
Now multiple that by how many kg the chap sold for scrap.

EWorkshop1708 18th February 2009 10:20 PM

Quote:

Originally posted by wg_ski
Just how much power do you expect to get out of a +/-35V rail anyway? You'll probabaly only end up with about 250W at 2 ohms, which can be done easily with a little bitty 500VA toroid.

Winding 10 ga bifilar is enough of an s/b to make it worthwhile to go for 80V rails, at least.

I'm winding for low voltage, and high current for low impedance. All amps run from these rails together.

The amplifier system will have 4 main speakers, 4 ohms each channel running Single Ended (SE). Also a 4 ohm subwoofer would be driven from two heavy-duty amp channels in BTL.

I conservatively figured 60W RMS x 4 = 240W RMS total for all main speakers

Also 240W RMS for 4 ohm sub in BTL from 2 channels.

480W RMS total power.

MondyT 19th February 2009 09:04 AM

Hi Workshop

I would be very careful with high power toroidal primaries as they can pump heavy currents into short circuits! Be very careful also that you do not damage the primary major insulation as it is the only thing that is really isolating you from the mains and the possibility of electric shock.

Can I suggest you have a look at the warning I gave here...

http://www.diyaudio.com/forums/showt...54#post1749954

Size is not always everything when it comes to transformer power and regulation. It is directly related to regulation but there is a little more to it. You may be much better off and safer obtaining a proprietary toroid in the region of 1KVA or so with your required secondary voltage. This will have much lower leakage reactance than a larger toroid with a hand wound secondary, and therefore will provide a lower impedance and a more dynamic power source for your PSU.

The problem you have is winding the turns you require tightly enough on the toroid, which is nearly square in cross section, with the required copper sizes that you need for the current, you will practically have to hammer each turn down on after each right angled bend (4 of these per turn!). By the time you get to finish the winding, your transformer primary and its insulation will be pretty badly stressed! If you donít do this then your turns will be closer to circular in cross section and you will loose on-load voltage due to leakage inductance. Also, unless you cover most of the toroid area with secondary copper, you are going to have some pretty nasty stray fields radiating all over your amp, not good!

With wire current capacity, you should not use cable ratings as these are generally a guide for installation work with a temperature rise on the cable of around 30 to 40 deg.C and with the wire in straight runs. Wire used in transformers, is subject to strong magnetic fields and proximity to itís neighbouring current carrying turns as well as the temperatures generated by the transformers losses and therefore the current handling capacity is reduced.

Mind how you go!

Ray


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