Wiring a Twisted Pair or Twisted Trinity in a Transformer Secondary

[jonovid]

if make a junk box power supply for an old 120W Toshiba Satellite laptop.
I have an old  used 700 watt microwave oven transformer
and hope to replace the 1kv secondary with my own 20 volts 6 amp secondary
by using wiring a twisted pair or twisted trinity of
19  AWG copper wire Gauge   1mm in wire diameter  x 2 so i can get a 6 amp secondary
on the 700 watt transformer  that has a 240 volt primary

my question is  wiring a twisted pair or twisted trinity in a  transformer secondary
a good idea? as I need current of 6 amps or more at 20 volts for an old laptop power supply.
an old 120W Toshiba Satellite laptop.

I do not have much in the way of 10 or 8 AWG gauge enameled copper wire
so must make do with twisted pair of 2 x 19  AWG 
I have never seen wiring a twisted pair or twisted trinity in a 50Hz power transformer
but do see the odd twisted trinity in CRT TV pcb choke transformers. were high current is used.
but this is a  high hz then 50Hz.
I have lots of 1mm enameled copper wire in my junk box
if my calculations are correct I will need double this for a 6 amp transformer secondary.
my question is will it work? are enameled wire strands bundled or twisted together in parallel a problem.
if its the transformer secondary.

[fourtytwo42]

There's no problem with using multiple wires in parallel providing they all have exactly the same number of turns and the total cross sectional area is sufficient to prevent excessive copper loss, a nominal value of 5A/mm2 is normally good enough.
If there is a big disparity in length (perhaps two windings in separate layers) current distribution will follow relative winding resistances.

[T3sl4co1l]

Yup, fine. Just takes up more space and gets hotter in the middle because of all the insulation.

Can also connect them in series, if you feel like saving turns.  (This has a high-frequency downside of increasing capacitance between turns, but that doesn't matter here.)

Multiple strands or builds are often seen at high frequency, to reduce losses; at mains frequency, there's simply no difference.


Mind that the primary uses a few turns less than it should, so the core runs in saturation and gets very hot under continuous duty.  You may wish to add some turns of your own.

Also FYI, knock out the magnetic shunts, they make the transformer "worse"; which is good for the intended application but not helpful here (you'll be using a FWB and cap to make DC I assume?).

Tim

[jonovid]

thank you! for your suggestions. 

Mind that the primary uses a few turns less than it should, so the core runs in saturation and gets very hot under continuous duty.  You may wish to add some turns of your own.

Also FYI, knock out the magnetic shunts, they make the transformer "worse"; which is good for the intended application but not helpful here (you'll be using a FWB and cap to make DC I assume?).

yes I may need to look at the loading and the core temperature 

looks like I be doing some rope making with my wire
.   

 electronics basket weaving 

[Red Squirrel]

This is a totally offtopic and unhelpful comment but Twisted Trinity would make a pretty awesome death metal album name.   

[T3sl4co1l]

Or Christian rock band, but for that matter, Dee Snyder is Christian so there you have it?

Tim

[gbaddeley]

As already mentioned, MOT are not generally designed for continuous use. Run it for 10 mins without a load to see how hot it gets. They can be noisy too, running close to saturation.

[NiHaoMike]

One trick that can be used is to phase angle cut the input so it's effectively a lower voltage. That was used in many Carver audio amplifiers back in the day. The downside is that the circuit to do it can be somewhat complex.

Most likely it would be cheaper to just buy a power supply. If you instead make it a variable output high current bench supply, you might have a chance of making one cheaper than what you can buy.