Transformer question

[djk]

"then open up the trafo and cut it up...it should be quite easy...been there done that..."

On a toroid, yes.

On a vacum-varnished EI type layer wound with fish paper?

Last one of those I re-wound had the secondary on the inside, primary on the ouside.

 

[li_gangyi]

So...I forgot what kinda trafo is he using...anyways...he could try opening up the trafo to see if the primary is on the outide or is the secondary on the outside...even if the primary is outside (outermost layer) he still can cut the secondary into 2 as most probably the windings end and are tied up near the "tags" where you solder your wires...unless the trafo is wire-ended...but I guess almost all EI trafo's are tagged...only toroidal trafos are wire ended...am I right??

 

[leo14m3]

BAD ANSWER !

Loss in secondary of transformer:
top half at 100% of rating I^2R=1
bottom half at 100% of rating I^2R=1
sum of top and bottom 1+1=2

Double current in top half only I^2R=4
bottom half not driven.
sum of top and bottom 4+0=4
(flaming hot!)
. [/B]

You forgot : if you drive half of the secondary, you will have half of the wire lenght ---> half of resistance. ( if the wire is large enough, like I said).

 

[li_gangyi]

It is just not possible...unless you double the winding..yes you might get a bit more than the stated 10A...but not to the extend of 20A...

 

[millwood]

( if the wire is large enough, like I said).

you are essentially assuming away the biggest and most likely problem in this particular set-up.

 

[djk]

"You forgot : if you drive half of the secondary, you will have half of the wire lenght ---> half of resistance. ( if the wire is large enough, like I said)."

You don't get it.

I don't like to call anyone stupid, but this is a grave saftey issue.

If you follow your idea you will cause a fire.

Try it and see.

 

[technics]

You forgot : if you drive half of the secondary, you will have half of the wire lenght ---> half of resistance. ( if the wire is large enough, like I said).

No, this is not right.
You have exactly the same resistance (not mentioning heating effects).
You always have the same length of wire - when using the two windings you use for each winding the length from the center tapping to the corresponding tapping (plus or minus) from the winding...
Got it?

 

[sss]

hello thylantyr
your original question was :

Most amplifiers use a power supply
with a center tapped transformer on
the secondary windings. For example,
let say the transformer is rated for
1KVA, about 100v output with 10 amperes
roughly.
If I use the center tap, I can get
(50v @ 10A = 500VA) * 2 = 1KVA

thats correct , u will get 2 500VA trafos

What if I don't use both halves of the
secondary, what if I use only the
50V @ 10A section and don't use
the other half. Can this section
output 50V @ 20A because the other
side is not used ?

its possible but not that way though

u cant use one half of the secondary and get 1000VA because its not capable of that
the solution is simple :
u should connect both halves of the secondary in parallel
that way the secondary will give u 20A (1000VA)
btw, be careful doing so , u should know how to connect transformers in parallel .

 

[leo14m3]

You don't get it.

I don't like to call anyone stupid, but this is a grave saftey issue.

If you follow your idea you will cause a fire.

Try it and see. [/B]

I will try and I will let you know.
At least, there will be the first time I cause a fire with a transformer. I'm not a coward, so I will accept my mistake.

No, this is not right.
You have exactly the same resistance (not mentioning heating effects).
You always have the same length of wire - when using the two windings you use for each winding the length from the center tapping to the corresponding tapping (plus or minus) from the winding...
Got it?

I don't agree with this. You tell me that my ohm-meter is not working well?

 

[technics]

I don't agree with this. You tell me that my ohm-meter is not working well?

OK. Just measure the windings like desbribed above...you will measure same resistance, because you will not change measuring points
Just read my posting...

Again: You can use one secondary winding with 10A. Two of them in parallel for 20A. That´s it.

 

[hieroglyphics]

I believe the reason you cannot achieve 20A from a single secondary is because you are only using 1/2 the number of turns available. There simply isn't enough current induced onto 1/2 the # of turns for the available flux. The same goes for only hooking up a single primary coil. You'll only get 10A from the secondaries in parallel because you're cutting the flux in half.

 

[technics]

I believe the reason you cannot achieve 20A from a single secondary is because you are only using 1/2 the number of turns available. There simply isn't enough current induced onto 1/2 the # of turns for the available flux. The same goes for only hooking up a single primary coil. You'll only get 10A from the secondaries in parallel because you're cutting the flux in half.

Don´t mix up with voltages and currents...turns result in ratio of transformer (input voltage to output voltage).
Again: Current is limited by diameter of wire...

 

[hieroglyphics]

I am aware of that. I should have been more descriptive. I was not speaking of the turns per coil (N2/N1) but the number of secondary coils (that have the same turns ratio and therefore same voltage). Lets say for example a transformer has two secondary coils as opposed to one. The amount of flux through the core will remain the same in both cases (generated by the primaries). The transformer with two secondaries has flux through two coils of wire and therefore paralleling the coils will yield equal voltage and twice the current. An equivalent transformer with a single secondary has the same flux through the core but only one coil to induce a current.

It is true that the current a wire can pass is limited by the diameter of the wire. BUT, the current a transformer is able to induce on a wire is limited by a combination of the turns ratio and the core flux.