Winding a Monster Toroid, Hum & Voltage Sag

[wg_ski]

Originally posted by EWorkshop1708
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.

Two in parallel will get it back to 30A capacity. Wire is good for about half in trafo applications. For small trafos THWN is a bad choice, but it works well with big ones. Even pre-made 1KVA units typically have a little space between turns and the slight loss of winding efficiency caused by the insulation isn't the end of the world. Especially at the price. I use it to "add" a few turns to existing trafos all the time, and did a few whole new secondaries. You will get better results with magnet wire, but handling stuff that big is interesting to say the least. If you use magnet wire a winding jig is a necessity or you'll end up scraping insulation off and causing shorts. The jig is helpful even if you use THHN. One more thing - Don't use stranded THHN. Use solid. The turns will stay in place much better and you'll be able to wind tighter. Another reason for the winding jig.

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.

0.72 turns per volt is a bit on the low side for an 8" toroid! I rewound some 1.1kVA cores that were 6" dia and it came in at 0.8 turns per volt. And the light bulb stays dark with no load so the magnetizing current isn't excessive. Maybe you ought to re-think which primary. Don't go just by sound - measure the mag current. Bigger toroids will make more acoustic noise at the same flux density compared to smaller ones.

[jackinnj]

Originally posted by AndrewT
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.

There are a gazillion wire and winding calculators on the web -- they'll give you the current carrying capacity in free air and bundled, the turns per inch, and the number of metres or feet in pound or kG.

there are any number of folks selling incomplete spools of magnet wire on the web or ebay, or you can purchase from a distributor. I've used sizes ranging from #16 and 18 to #32.

[pacificblue]

http://en.wikipedia.org/wiki/Transformer_design

[EWorkshop1708]

Originally posted by wg_ski

0.72 turns per volt is a bit on the low side for an 8" toroid! I rewound some 1.1kVA cores that were 6" dia and it came in at 0.8 turns per volt. And the light bulb stays dark with no load so the magnetizing current isn't excessive. Maybe you ought to re-think which primary. Don't go just by sound - measure the mag current. Bigger toroids will make more acoustic noise at the same flux density compared to smaller ones.

Thanks for that helpful info! So I can push it harder. I'll measure the input current. My meter doesn't read high AC currents, so I'll use a lightbulb and measure the voltage drop across it to figure out the input current.

I just wanted less hum, but now you got me thinking, that I would probably not hear the hum from 4 or 5 feet away, and with speakers playing, I probably would not hear it at all anyway.

If I go to the next connection down on the primary, I get 7.9V for 10 turns. That's closer to what you state. I could even go to the primary tap with the least number of turns and most power, and get even more voltage than that. (I'll have to re-measure to see what I get) It would save me some more wire-wrapping effort to get the required voltage.

I'll do some more measurements and keep you folks updated.

[AndrewT]

don't use your DMM to measure current.

Put in a dummy load resistor in series with the mains supply.
Measure the voltage drop across the dummy resistor using the 600Vac scale. If it reads near zero then change to the 200Vac scale.
if it reads near zero change to the 20Vac scale.
If it reads near zero then change to the 2.00Vac scale.
If it reads near zero then change to the 200.0mVac scale.

If you are trying to measure a 10mAac current, use a 1r0 +-1% resistor as the dummy series load.
It will indicate 1mA for every mVac across it.

This is mains you will be measuring.

BE CAREFUL.

[EWorkshop1708]

Originally posted by EWorkshop1708
I'll measure the input current. My meter doesn't read high AC currents, so I'll use a lightbulb and measure the voltage drop across it to figure out the input current.

Thanks Andrew. I was going to do the voltage across the "resistor" measurement anyway, using a lightbulb as my resistor as stated in my previous post. I also have a heating element from a hot water heater I could use as a resistor instead of the lightbulb if I decide.

I've worked with mains for years, and I'm well aware of the dangers present. I've done commercial and residential electrical installation. Electricity is my friend, a good friend I respect very much. Just like you have to respect a good friend so they respect you in return, you have to respect the dangers of electricity to avoid shock or burns. I do not fear electric in any way, I respect it. I 100% appreciate your concern, as we don't want nobody hurt on the mains.

After much thinking, I'm not worried about stressing this big toroid, whether or not I use as much wire on the secondary as intended. I'm not sure I have as much wire on my spool as I had hoped, but I'll see what I end up with. I'll even settle for "only" 1100VA if I had to, as I'm only going to use about 500W RMS of amplifier power, or add a few extra turns to allow for rail sag with heavy load, but I'll keep you posted before making any final decisions

I originally was going to do offline SMPS, but I need more experience with them before I go making a 500W or larger SMPS, so the mains toroid and a 50A bridge rectifier and large capacitors is MUCH easier, more reliable, and less complex than an offline SMPS. I have plenty of 12V SMPS experience, I just haven't dabbled with the offline mains 320V DC ones yet. I know how parts can be vaporized in a microsecond from the mains, so the toroid will be fine in this case.

I'm not trying to go too big on amplifier power, as I completely melted the voice coil out of my last subwoofer with the last big amp I built. Even if I get a LOT of amplifier power, I'll be going easy on the volume control to prolong the life of my speakers.

[pacificblue]

Originally posted by wg_ski
0.72 turns per volt is a bit on the low side for an 8" toroid! I rewound some 1.1kVA cores that were 6" dia and it came in at 0.8 turns per volt.

Nice guesswork.

U = N * Afe * 2 * PI * f * B
N = number of winding turns
Afe = cross section of iron core
f = mains frequency
B = magnetic flux density

I = Acu * J / N
Acu = wire cross section
J = current density

The more windings you need for the necessary voltage, the lower is the permitted current density for the wire. The value depends on the transformer's heat dissipation. For 1100 VA the value should probably be in the range of 1,5 A/mm². There must be tables for that, which you should look up.

The alternative method is to try it out and if it gets too hot, unwind it and wind it again with thicker wire. That could become tedious after several attempts.

Originally posted by EWorkshop1708
Thanks Andrew. I was going to do the voltage across the "resistor" measurement anyway, using a lightbulb as my resistor as stated in my previous post.

A light bulb changes its resistance with the temperature. Use an adequate constant resistor for it.

What is the use of measuring the input current anyhow?

[AndrewT]

measuring the primary current with the secondary open circuit will show if there is a sudden increase in primary current using any of the 4 primary taps.
If one or two taps have a much higher no load current then it would indicate that the mains voltage is too high for those taps. i.e. one needs more primary turns.

[EWorkshop1708]

Thanks folks, for all the generous support and help and answering all my questions.

It ends up, I only have ~90 ft of wire, and it would only get me ~20-0-20 if I went parallels. I'm going to have to settle for the slight loss in power, and go with single 10AWG for each side and winding only bi-fillar instead of quad.

Due to my being between jobs and looking for work, I have to use what wire I already have. I'm making a bunch of smaller amps anyway for extra side income. I'll be selling the big amp when it's finished as well. I have to spend as little as possible right now.

I'll wind a few extra turns on it to get me higher rails at about 28-0-28 VAC so if the bass hits really hard, the rails will still hold a good voltage. I only needed about 500-600VA for the amp project before I got this toroid transformer anyway, so the estimated 1100VA will be just fine.

I'm using such oversized filter capacitors that a bit of sag on the rails will be OK. I'm sure the oversized core will still help some, despite any copper losses. I'm 100% sure I'll have plenty of power, especially now that I'm choosing higher rails. Most commercial amps sag the rails a bit, so I won't mind if this amp does some rail sag.

The current does rise some on the lowest primary turns, but not very much to be concerned with. I left the toroid sitting powered for several hours, and it's BARELY warm to the touch! I LOVE that about toroids! Thanks, you were right, the hum is no big deal.

I'll keep you posted when I finish wrapping the monster toroid. When I build the amplifier, I'll post pictures and specs.

[wg_ski]

Originally posted by pacificblue

Nice guesswork.

U = N * Afe * 2 * PI * f * B
N = number of winding turns
Afe = cross section of iron core
f = mains frequency
B = magnetic flux density

Not exactly guesswork - I kept the original primary intact, so it determined the turns per volt. It was originally 100V/50Hz, which I knew I could push to 120V/60 Hz and keep B and the primary current under load the same. I needed three secondaries, with the inner bifilar pair good for full current, and the outer one wound with one gage smaller because the peaking supplies won't be loaded all the time. I'd hate to even inquire as to the price of something custom like that, and those originals were $30 a pop because of the screwy voltage and frequency (for our zip code, anyway).