toroid psu, weird results...

[Stixx]

Well,

in my attempt to build my third headphone amplifier (aikido this time) I am planning to use two toroids for B+ and the heaters. They are both 80VA units since the heaters ( two 6CG7, two 6H30) are drawing about 2.8A , the other toroid for the B+ is well overdimensioned.

I' ve breadboarded the psu and immediately wanted to check some voltages...and here things are getting ...err...strange.

Both toroids have two 115V primaries with the secondaries being 2x12V for the heaters (regulated supply with 'dropper resistor' and LM1084) and 2x55V for HT.

To start with the HT I put the primaries in parallel and the secondaries in series, connected the whole thing to our 230V line voltage hoping to arrive at 220V after my filter network (byv99, CRCRC). As load I used around 4k WW resistors...I really don't know whether putting 230V on two paralleled 115V windings is a good idea or even a definite no-no, but I am getting around 172V AC before the filter network and basically nothing after the rectifier diodes...?

To make the situation really strange i measured the voltages of the other toroid (primaries in series, sec's in parallel)without load, hoping to see 15V+... Well, I'm having around 190V here!!

Q. Am I missing something important??
Q. Am I......................dumb??

Clueless so far,
Oliver

PS. MY DMM seems to be okay since I checked voltages on my MJ this weekend...

Edit: corrected typos

[Miles Prower]

I really don't know whether putting 230V on two paralleled 115V windings is a good idea or even a definite no-no, but I am getting around 172V AC before the filter network and basically nothing after the rectifier diodes...?

A really bad idea! Those xfmrs are designed to operate at a specific volts/turn ratio. By putting 230Vac on 115Vac primaries, you've doubled that volts/turn ratio and guaranteed core saturation. It'd be a miracle if you haven't poofed that xfmr.

[nhuwar]

You need to series connect the primarys not parallel them for doubled pri voltage.

Nick

[rcavictim]

Originally posted by Miles Prower
... It'd be a miracle if you haven't poofed that xfmr.

Miles,

Really, such language! Isn`t poof on the restricted words list here? Sounds like it shud be, since there seems to be a law against common and very useful soft, not even swear words here that appear in dictionaries as legitimate parts of our contemporary language lexicon.

I can complain, therefore I am.

[Shoog]

As Miles says you can't get you wanted 220V that way. If you apply a load the waveform will be badly clipped. You will have to try and work out a voltage doubler power supply.

Shoog

[thoriated]

You definitely don't want to exceed the primary voltage rating on a transformer. If you haven't already let the magic smoke out, correct that first.

[Stixx]

Thank you everybody for putting my head straight....
after some further reading on the subject I also realize that my plan was overly ambitious.
Luckily no harm was done and no xformers pxxxxxxd...

I'll ry a voltage doubler instead or have a custom toroid made.

[billr]

Originally posted by Miles Prower


A really bad idea! Those xfmrs are designed to operate at a specific volts/turn ratio. By putting 230Vac on 115Vac primaries, you've doubled that volts/turn ratio and guaranteed core saturation. It'd be a miracle if you haven't poofed that xfmr.

Hi. I was interested in this comment above. The important thing here is not the voltage per se. it is the VA rating of the transformer that is important, so if the voltage is doubled and the current take is halved, then no saturation problems should occur. what you then have to be careful about is the insulation, both between windings, and layers.


the following transformer equation highlights the point:

VA = 4.44 x B x F x I x ff x Sf x L x H x T x S
1,000,000

Where :-
VA = volt amps, or watts rating of the completed transformer,
4.44 is a constant for all equations,
B is the magnetic field strength in Tesla,
F is the frequency of the mains,
I is the current density in the copper, and we will allow 3.0 amps per square mm,
ff is the fill factor, ie, the fraction of copper area in the core window,
Sf is the stacking factor of the laminations, since there ia a coating of insulation, so about 0.95,
L is the length of the core winding window,
H is the height of the core winding window,
T is the core tongue width,
S is the core stack height,
1,000,000 is a constant for all equations.

[ak_47_boy]

Toroids are fussy creatures.

[billr]

Originally posted by ak_47_boy
Toroids are fussy creatures.

that's as maybe, they are senstive to dc bias, and saturation, but in the example that the original poster provided it would seem that he had a wiring problem.

I suspect that he connected the two primaries up anti-phase so that they cancelled each other out.

My basic premise that voltage across a primary does nothing to the saturation still holds, it is the current through a coil that determines the flux density, and as shown in the transformer equation

bill