increasing the voltage of a transformer

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Hi I have a transformer that I want to increase the output voltage by extending the secondary windings
the transformer has taps at 20.4-16.5-0-16.5-20.4 vac
so if I wind back and cut off the copper wire back to the 16.5vac tap on both secondary windings counting the number of turns as I go
the number of turns = 4v ( 20.4-16.5= 3.9)
if I then attach a length of the same gauge copper wire to the tap at 16.5 and triple the amount of turns per side will this give me about 28vac x2 ?
also the transformer is 50mm x 100mm any guess at the VA rating ?

An externally hosted image should be here but it was not working when we last tested it.
 
I guess glen does not know the turns-per-volt rating for this particular transformer and tries to find it out that way.

No need to go that far, just wind, say, 10 or 20 turns of *any* wire, even plastic hookup wire around that core, measure voltage and then divide t/V (turns/Volts).
Then it´s easy to calculate needed turns to reach desired voltage.
 
done

job done just need to get some black cloth self adhesive tape to wrap it in both winding are at 28.8vac


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amps

An externally hosted image should be here but it was not working when we last tested it.
just rigged up a load to see how many amps I can draw from it and very surprised at the results
not sure if my meter bottomed out at 10 amp as this is as high it will go!
any guess at what va I am getting ?

An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
 
Looking at the case with both windings in series.
You measured 10.16Aac around the circuit.
The load is 8r0||8r0 = 4r0.
The voltage across the 4ohm load is 10.16*4 = 40.64Vac. Did you measure this?
The output during the test is 10.16Aac * 40.64Vac = 413VA
The regulation is ({28.8+28.8} - 40.64) / 40.64 = 41.7% This should be somewhere between 5% and 9% @ maximum rated output.

Your transformer is massively overloaded.

And you are pushing 413VA through 200W of load ! What temperature is inside those resistors? Enormous?
 
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Find two more 8r0 100W resistors.
Wire them in series parallel for an effective 400W 8ohms load.
Now apply the 8ohms as a load to the two series secondaries.
measure the AC current and the AC voltage.
Calculate the VA = Vacoutput * Iacoutput.
Calculate the regulation = (Vout open circuit-Voutloaded) / Voutloaded.

A better way to measure output current.
Wire 16 1r0 1% 600mW resistors in parallel. Reff=1/16th ohm = 0r0625
Allow a gap between each resistor.

If the output current is 7Aac, then the loading on the 0r0625 is Iac² * Reff = 7*7*0.0625 = 3.0625W
The loading on each 1r0 is 191mW. These will warm up but should not get hot if well ventilated and should stay fairly close to their rated resistance.

Now measure the voltage drop across the 0r0625 during the test. Expect ~440mVac
Iac = 440mVac/0r0625 = 7.04Aac
 
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Your transformer is massively overloaded.

while it may appear so, transformers can actually be overloaded,
but thing to watch out for is the core temperature rise.....
and since sine waves are involved, as long as the temp rise is within reason
and the duration of overload is only for a short time, then this can happen...
resistors are easy loads, they do not distort the current waveforms
the way rectifier circuits do...

for audio applications, i doubt that your traffo will ever be overloaded,
clipping will probably set in first and you will have to back down on volume...
 
Sorry if this too elementary but hey, that's my level! I wonder about the original outside plastic wrap. Now that it's covered with more winding and outer tape, is there chance of melt or fire if the transformer is run hot?

Thanks

fuses should take care of this...do not worry too much, listening to music draws little current than what was tested using a resistor load....

toroids uses mylar films of about 10mm width all throughout, separators for primary and secondary coils,
they are not even dipped in varnish....dismantling is very easy....at least from my experience..

and the good news is, the dismantled copper can be reused....
 

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