Amveco toroidal transformer.....R.G.?

[Bill Bergman]

If you read this R.G., I notice you had a post a while back on these transformers. Do you know if they make a model that would put out any where between 120 and 150VAC @ 80 to 90ma?

[puretube]

maybe you should note if you want it in a "back-to-back" topology...
i.e.: drive it from a 12V source
(that`s what I thought!).

10VA means: 10W; not to be mistaken for: 10V~   (AC);

:?

[Bill Bergman]

I suppose what I'm looking for  could be driven by any where from 7 to 24VAC as long as the output would be from 120 to 150 VAC @ 80 to 90 ma if possible.

Sorry for my lack of understanding the physics. :oops:
Ton, I didn't want to bug you any more so I asked here on the forum.

[Bill Bergman]

Ok,
please bare with me, I actually talked to one of the design engineers at Amveco.  I was ablle to confuse him farely well but I think he understood what I was trying to do. I would be able to get about 120VAC at 80ma with the 10VA, 7vac - 70040 model running parallel in reverse secondary to primary.
So then, I thought where could find a 7v transformer that would drive the toroidal and power the filiments on the tubes......doh!  the same transformer run in normal primary/secondary parallel configuration.

So you say why the hell buy two 70040's, why not just buy a Hammond 269AX......because two of the toroidals are still smaller and a little cheaper than an 269AX.

[Bill Bergman]

Good news and not so good. Good - it works!!  Not so good - it's bigger than I thougt. I bought 2. One to power the other while also suppling the tube filiments. It's quite a bit bigger than the one Aron sells(the smaller black one), but should supply more current for more tubes. I hooked it up parallel so 7vac in gives 116vac out and when rectified sould give around 150vdc which should be good for the sub mini tubes. here's a pict...

Puretube you were right....made in India.

[Bill Bergman]

If the picture isn't veiwable, look here.... http://www.freewebs.com/elroy2/

[Regan]

Bill,
How about trying this,
since the transformer has dual primaries too, does it have separate center windings on the primary? if so then why not power your heaters off of the secondary and then power your b+ off of the other primary winding.
Sometimes you can do this.
Then you would only need the one transformer and save lotsa room:)
Regan

[puretube]

A very cool idea - however only in 120V mains countries...  :cry:

Yes, they have separate windings!

CAUTION: those xfmrs have excellent prim/sec isolation (>4kV),
but I don`t know about the isolation between the 2 hivolt-windings...

maybe Bill should ask again in the company? (or look into the datasheet).

Maybe it needs a little re-calculation, too... .

(P.S.: that`s why I prefer posted messages over private ones:
so everybody would know what`s been said/wrote/linked already...  :wink: )

[Bill Bergman]

Puretube,
Is it because I'm using it in parallel instead of serial that I need to worry about the high voltage isolationn between the outputs ? I used the ps program from Ducan to estimate the output I'd get from the 116vac that I measured. The simulator showed about 150 vdc.
Also is there any way to test the ma output?

Regan,
there is not a centertap as I understand it, just has 2 pins per for each of the 2 inputs. Doesn't a center tap require a third pin.

[Bill Bergman]

Oh, I forgot to ask. I had planed to also take 7vac from the first transformer to also power the heaters. Is there a way to reduce the voltage to ~6.3 vac wih out rectifing it to dc and using a voltage regulator so I can keep it 6.3ac?

[puretube]

What regan meant, was:
only use one xfmr...
It has 2 independant, isolated "primary" windings (hivolt),
plus a (lovolt) "secondary" winding (or probably also 2 of those,
which can be parallelled.

So 1 hivolt (120V) winding can be used as ("primary") input,
whereas the 2nd hivolt (120V) winding can be used as the hivolt "secondary" output, to produce the plate-voltages,
while the lovolt-winding(-s) (in your case 7V) can be used for heating...

This scenario is all inside a single xfmr, wound around 1 core...

Like mentioned before, however, those 7V windings are intentionally very well isolated against the mains (= hivolt-)windings,
while those 2 hivolt (="primary") windings amongst each other may be not be that very well isolated, because in normal life,
they only have to withstand a voltage-difference of max 2 x 120V = 240V...
(and are wound around the core very close, physically,
while the 7V winding(-s) have extra layers of isolation between them
and the 120V windings)

[The Tone God]

I've been using what I call the "split primary" trick for awhile now. One added note. If you use only one primary you can only provide half the amperage on the secondary(ies) so you will need a transformer with twice the secondary amperage of your needs.

Andrew

[Bill Bergman]

Thanks!

I need 1.05amps total  for the heaters and each side of the transformer is .714amps if used separately. Therefore I'll have to use both side of the transformer in parallel to get 1.428 amps to power the tube heaters and to power the second transformer for the B+ to the tubes. Right?

[R.G.]

OK, mostly good advice above.

You're taking a safety chance using two parallel primaries from a toroid transformer with one as a primary and the other as an output. That's because as pure noted, the primaries are only isolated from each other by only the insulation of the magnet wire, which is good only for maybe 300-400V. Primary to secondary isolation in all consumer products is at least 1500V and often 4000Vac. A power line transient could easily puncture the magnet wire film and give you a primary-secondary short and shock hazards.

I recommend that you use either two toroidal transformers back to back or one flatpack semi-toroidal. The flatpack has the two "primaries" wound in separate sections of the two bobbins and so they're isolated by the bobbins, not only the magnet  wire film.

When you size the tranformer, you have to make note that all of the secondaries suck from the primary. If you pull 80ma out of the 120V "secondary", then 80ma plus the magnetizing current penalty will be going into the live primary. Add to that the current pulled from the low voltage secondary. If, for instance, you use 1A of 7V, that's 1A*7/120 = 0.058A on the primary, and now the real primary must  conduct 80+58+5ma = 143ma. That means that if you used a dual primary transformer with one primary hot and one "primary" used as a secondary that you'd need to have specified the original transformer for primaries that *both* carried 143 ma. Call that 150ma, and the original VA into the transformer had to be 120* (.150+.150)= 36VA. Less than that will likely overheat the AC-line driven primary.

If you use two identical tranformers, you get a similar result. If you use 120V at 80ma on the high voltage secondary driven by the 7Vac, then you need 0.08A*120/7 = 1.37A of 7V. If you use the same 1A of 7V to drive filaments, then the primary of the transformer which connects to the AC line needs (7/120)*2.37A = 138ma of 120Vac just for the secondaries, and perhaps the same 5ma of magnetizing current. That's ...um... gulp... the same 143ma.

No surprise really. The same amount of power is being used, and we did not subtract out anything for the various inefficiencies, so the answer *had* to come out the same. Mother Nature again, First Law of Thermodynamics: energy is conserved. We just didn't go deeper and include in the Second Law consequences of losses.

Actually I flim-flammed you - I didn't include the magnetizing current of the second transformer, which would add maybe another 5ma to the needs of the two-transformer case.

In the two-transformer case, you're going to find that you get significantly less voltage on the high voltage secondary than you think you will. That's because transformer makers deliberately make the open circuit secondary voltage of small power transformers higher than is ideal to make up for the resistive losses in the small wires and many turns that small transformers need. If you then drive the second transformer from the low voltage winding, all of the resistive losses in the first transformer are subtacted from the drive voltage that the second transformer gets, and the output high voltage is significantly low.

When I first did a back-to-back using two identical small 12Vac transformers, I only got 132Vdc out of the setup. Took me a while to figure it out.

To get 6.3Vac from7Vac, either use a resistor or two (Good Idea! cuts down on input surge when they're cold) or a pair or two of back to back diodes to cut the peak voltage a bit.

Final shot: to size a transformer, add up all the secondary voltage and current needs. If you are using AC like for heaters, the AC rms current is whatever it is. If you rectify to DC, then multiply the DC current by 1.8 to get an approximation of the AC RMS current (accounts for the high peak current pulses in a FWR/cap circuit).

Add up all the secondary loads, then transform these to primary current by multiplying by the turns ratio. Add them all up in the primary, allow a few percent for magnetizing current, and what you have is the necessary VA rating of the transformer.

[Bill Bergman]

Thanks R.G.!
I was planning on using two identical toriods. I feel so damn stupid. I've read your expanation several times and still can't decide if your saying it's possible or not.

Sorry if I'm wasting you guys time.

[R.G.]

Don't feel stupid. This is a particularly arcane bit of knowledge that you have to have run into before to understand. This is your first time. I took a three-semester-hour course in transformers and motors.

No, the 7040's will not work as you want. One 7040 under full load (as I read the Amveco specs) will produce enough output power to do what you want. With the 7Vac (8.3Vac open circuit) secondaries paralleled, it will put out 1.4A of 7Vac. You need 1A of that to make your 120Vac load, leaving only 400ma for heaters. That's probably not enough for more than one tube - of course, I don't know the heater requirements of your tubes or how many.

It would make - just about - the 120V output by itself,  but none left over for the heaters.

The 120Vac primaries are designed for 10VA of power in, about 10/120 = 83ma of current plus two times the no-load (magnetizing) current, about 95ma. That's OK for the output "primary" but the input primary needs to do over twice that. So you could do with a 7040 driven by a 7060 or a 7060. The 7060 will barely do it, the 7070 is comfortable.

I'm sorry, I was not being clear.

[Bill Bergman]

I'm sorry, I was not being clear.

Thanks R.G., actually you were clear and also it's more of a benifit to others when you go into detail. I'm still trying to learn.

All this really makes me appreciate even more, the technology and design that went into Zach's Nanohead. Even though I'm just trying to push one more tube than the Nano, my power supply is turning out to be the size of a cinder block. Geez, I just should have bought a conventional Hammond transformer.

[R.G.]

Once you make the step to high speed switching, the power becomes easy. There are many vendors that could make a switching power supply for 150Vdc/100ma and 6vdc/1A and put it in a case the size of a pack of cigarettes.

You just have to want to do it enough to get the three prototypes done to get it right and pay for 500 to 1000 units. Over that hump, it's a snap.

That's an economic, not a technical problem.

[Bill Bergman]

I unsuccesfully attempted to build a high speed switching power supply designed by Steve Bench a year ago or so. I had to sub a few parts. Maybe that was the problem.

[R.G.]

I unsuccesfully attempted to build a high speed switching power supply designed by Steve Bench a year ago or so. I had to sub a few parts. Maybe that was the problem.

Could well be. The details matter in high speed switching. It's probably best left to companies that already do it. But there are lots of those.