Big Toroids for amps ..variac rewind

[GS300]

Hi guys
Quasy said i should post this as it will be for my new amp

rewinding an old Variac core with a secondary......
i have done before and it works a treat about 1500VA
what a brute...
it feeds an old version 1 / ACTRK amp 700w into 2 ohms

i found an old variac at a metal recycle depot... the man behind the counter said $25 and i stuffed the money in his hand and i had some of the biggest cores i had seen.....you guys should keep your eyes out as i have collected a few since my first purchase

they have a great primary that is overwound ... that is too many windings for a primary usually about 10 - 15% so they could be set overvoltage.... but nice and safe . so leave that side alone.... they give a low no load idle current.

Unlike the first time i tried to wind a primary on one of these things .....it blew the power fuse at Quasy's house, blam instant dark...

keep you posted
Marcus

( there aint no replacement for displacement )

 

[GS300]

Hi Guys

The next step is to insulate the windings where they are shaved usually on the top for the wiper with polyester transformer paint. i picked mine up at a winding wire distributor but i think you could get some at a motor rewind factory ...as you really only need a little bit ..... i over did it and bought 4 litres so i could dip rewound E and C cores to stop them humming.... my hand winding not as tight as factory stuff.
After that is dry maybe a week is to wind with polyester tape
There are two types ... one yellow sticky tape style ...Well the one here in OZ is yellow, or one not sticky that is just clear..
the yellow one is fairly thin so two layers go on... maybe one layer of clear tape
usually not wider than 20 mm as too wide wrinkles as you try to wind over the primary ... and pull it tight as you go round.


see you next time

( there aint no replacement for displacement )

 

[unclejed613]

looks nice, i'll have to remember that..... since you can count the turns on the primary easily, it would be very simple math to figure out what turns ratio is needed for various secondary voltages. the secondary could also be wound with whatever wire you choose, like 12ga teflon for instance (pricy, but will never short windings from overheating). one word of advice, your transformer tape winding should be able to isolate the primary from the secondary to at least 1500Vrms if you plan on a)selling the amp, or b) you want it to survive line surges and near lightning strikes as well or better than commercial transformers. leakage current isolation is also a safety issue, and most standards state a minimum of 1500V.

 

[GS300]

Thats a good point so i checked the web site for the tape i use and all ok.........
http://www.lodestonepacific.com/distrib/pdfs/tape/1p802.pdf

3KV breakdown ........ phew

when winding the tape do a 50% overlap on the outside to get your double layer
oh yes take the plastic tape core out of the tape roll so you can feed the whole roll of polyester through the core....

Yes count your primaries as this one has 330 turns divided by 240v in Australia thats a turns ratio of 1.375 turns per volt
or .73 volts per turn

I need 75v secondaries DC.....
75 divided by 1.414 = 55v ac
55 divided by .73 = 75 turns

On finishing your secondary winding leave a meter of extra wire in case you have to put a few more windings on due to transformer losses. !!

but more on that later .... more to do yet

oh yes the new photo is one of my previous amps with one of these cores
the other is winding the tape

GS300

 

[AndrewT]

do a 55% overlap around the outside of the core.
This guarantees two layers of insulation.
I double wind a second time again with 55% overlap, guaranteeing 4layers. Just in case one gets punctured, or worse.

 

[GS300]

yes good idea
I was going to tape 2 more layers around the edges where the wire contacts hard to get my 4 layers

gs300

 

[GS300]

hi guys
sorry for the gap in update ...... family issues at home .. you know

anyway
my transformer has 4 tappings so i will wire out all 4 so i can adjust the output voltage a little by changing the the input turrns ratio....
I am using 105 degree C wire if you have some silicon covered all the better but i would not recomend 75 degree cable as transformers can get HOT.
My windings will be separate when comming through the polyester cover so i am only using 2 colours ... but if you bundle them try and have multiple colours.... i suppose you could always measure the voltages but colours makes idntification a bit easier

gs300

 

[unclejed613]

if you can afford it, Teflon insulated wire is the best, very high temp rating (about 200C if i remember correctly), and pinch resistant. it's a little difficult to work with because of stiffness, but well worth it, since you never have to worry about the insulation melting from overheating or chafing from magnetic slap (both of which are major causes of shorted turns). the only difficulty here is you still have a varnish insulated primary, which is susceptible to both failures.

also, i don't know if this has been mentioned yet, but you want to avoid using a mounting method that includes both the top and bottom covers with a bolt through the middle. this will cause the equivalent of a shorted turn....