Toroidal Question

[matta]

Hey Guys,

I am looking at replacing my current 'chunk of metal' transformer with a
Toroidal. I need a 15-0-15 30VA for the Greens I am building.

I can get one that has a pair of 15's at 1A each (15VA per wind)

Is it safe to assume I can use just one pair and if I don't use the other I will get 2A,
thereby having 30VA (15V x 2A), or is it 1A per winding?

Also on the secondary you have 2 pairs (one orange the other purple) of 15
and one blue, one brown, 0 (Ground). How the heck do you know which ground
is for which pair or is it irrelevant, i.e they both go to the same ground point?

Thanks in advance.

Cheers

Matt

 

[gyraf]

It's 1A per winding. To get 2A, use the windings in parallel.

 

[matta]

Thanks Gyraf,

Oh I see! In my mind I thought if I had them in parallel then it would up the 15 to 30V, but it remains a constant 15V, but with 2A worth of juice?
And I do the same for the 0.

So basically I would have the one orange and one purple together as a pair and the blue and brown 0's together as a pair. And that in turn will give me 15-0-15 2A, which is 30VA?

Thanks

Matt

 

[Guest]

Mark Burnleys diagram is a great explanation for this

 

[matta]

Hey Uk,

Thanks for the diagram, it is most helpful.

If I read it right the toroid I am looking at getting prob has it's
primaries in series (240V) and the secondarys are split, each wind having
2 x15V and a 0 center tap at 1A each, giving each wind 15VA

In my mind I guess I was confusing series and parallel on the secondarys...

Then is understanding correct above I need to take one 15V
side from both coils and join them together to get 2A out of the toroid.

As mentioned there is an orange pair of 15V and a brown 0 on one wind and a
pair of purple 15V and a blue 0 on the other (though the brown and blue
could be the other way around).

So in my mind I would take one orange wire and one blue wire from each coil
and join them together to get 15V 2A?

Thanks

Matt

 

[PRR]

> I can use just one pair and if I don't use the other I will get 2A

One 1A-rated winding delivering 2A will sag and overheat.

If you are not using the other winding, then you are not overloading the primaries or the core, you just have one hot secondary winding and one cold secondary winding. So in fact you could draw a little more than 1A with safety. How much more depends how well the heat is distributed around the transformer, hard to say.

Also note that 1A AC is good for not much more than 0.5A DC.

However: I thought a "green" ate +/-15V DC and about 0.03A per channel. So I'd think you want the secondaries in series, center tap to common, to make +/-21V DC before regulation. And that you could power a dozen or more Greens from a 30VA transformer. (Someone who knows the Green should check my assumptions.)

 

[CJ]

Be careful. Those torroids will give you hemroids if you mount them wrong.

 

[matta]

Hi Guys,

Thanks for all your input. I think I might have series and parallel confused again.... All I really need to know is is I combine the 15 of one winding with the other, so one orange, one blue will that give me 15V 2A and 30VA, maybe 30VA is overkill. I really am only using it for 2 Greens, but it might be nice to have the extra juice if I add another 2. Any is what I am suggesting in series or parallel?

Cheers

Matt

 

[alk509]

[quote author="matta"]I think I might have series and parallel confused again....[/quote]

Check out Mark's diagram. It has all the info you need!

[quote author="matta"]I combine the 15 of one winding with the other, so one orange, one blue will that give me 15V 2V and 30VA[/quote]

I'm sure you mean 15V, 2A. Yes, two 15V 1A windings in parallel will give you 15V 2A (a bit less, actually, but close enough)

[quote author="matta"]one orange, one blue[/quote]

Different transformers have different color leads... "orange and blue" doesn't really say anything.

[quote author="matta"]maybe 30VA is overkill.[/quote]

It is, but who cares?

[quote author="matta"]I really am only using it for 2 Greens[/quote]

Errr.... No, you're not. The green needs a +/-15V bipolar supply!

Peace,
Al.

 

[matta]

Hey Al,

Thanks for the input. Something is not clicking unless we are saying the same thing over and over and I am not getting it...

Currently I have a bulky CT type transformer

The primary has 2 wires which are fed 220V AC, the secondary has 3 wires out, 2 15V AC and one center tapped Common (0) this in turn feed my PSU which outputs the following regulated DC voltages +15, -15, 0 and 48V DC.

I am now wanting to use a toroidal instead of the CT. I think I can read Mark's image just fine and am just getting my head around it all.

On the toroidal you also have the 2 primary in, on the secondary there ate 6 wires. 2 pairs are the same colour and the other 2 are different color. My logic is that there are 2 sets of outputs, one on each wind.

If I wire the secondaries in series I will end up up with +- 30V (PRR stated 21V, which is prob a real world number) but only only 1A.

If I wire them in parallel I'll have 15V 2A.

Parallel seems like the way to go, but then will 15V into my PSU be enough?

I am going crazy, someone shoot me or put me out of misery! LOL!

Thanks

Matt

 

[peterc]

Matt

Allow 120mA for each Green pre, so .12A x 30v = 3.6 VA. Doesn't take losses into account, but it is close enough.

So for 2 channels a 10vA trafo will be fine. Avnet Kopp have a 10VA low profile Era transformer for about R60-00 that I've used before with success. It will also fit into a 1U box.

I dont think it is a toroid but it is encapsulated & not very noisy.

Peter

 

[MikoKensington]

OK here are my questions. Seems like a nice time to ask them. Very basic, I know.

I too am shopping for a toroidal transformer. Transformers are listed with Power VA. Can someone describe what that rating means? Just so I know.

What's an appropriate current to run 4 discrete opamp based preamps? Ala, Quad Eight, JLM. Bipolar 28V.

Also the transformers I'm looking at, when the Secondaries are in series the current is halved? It's the Amveco Mini line.

I'm trying to get my head around this stuff too. Thanks guys.

 

[PRR]

> If I wire them in parallel I'll have 15V 2A.

No, 15V at up to 2A before overheating.

In this case the Amps are a total non-issue. You don't even have 0.1 Amp of load.

> Parallel seems like the way to go, but then will 15V into my PSU be enough?

No. How could it be? You need +15VDC and -15VDC, total 30VDC. You need two 15VDC supplies so it makes sense to get them from two windings. They happen to be connected in the center, so you can use a center-tapped 30VAC winding. But that is really two 15VAC windings connected together. Hint. Hint.

> I am going crazy, someone shoot me or put me out of misery! LOL!

Your mind is stuck in some rut. Sleep on it a few nights.

 

[alk509]

[quote author="matta"]on the secondary there ate 6 wires.[/quote]

6 wires? You never said anything about no 6 wires! :wink:

You need to ohm them out and see which ones are connected to each other and in what order... There may or may not be two separate 15-0-15 CT windings there (which you could use).

[quote author="matta"]If I wire the secondaries in series I will end up up with +- 30V (PRR stated 21V, which is prob a real world number) but only only 1A.

If I wire them in parallel I'll have 15V 2A.[/quote]

No, no... wire two 15V windings in series and you get 30Vrms, or 42V rectified and filtered DC. Wire them in parallel and you get 15Vrms, or 21V rectified DC (Paul's number).

[quote author="matta"]Parallel seems like the way to go, but then will 15V into my PSU be enough?[/quote]

If you have two 30V CT windings, you just parallel them so that you get one big 2A 15-0-15 output. If there's two 15V CT windings, then you need to wire them in series and ignore the center taps.

Do you have specs for this transformer? make and model? Can you plug it in and see what voltages you get between the different secondary windings?

Peace,
Al.

 

[PRR]

> Transformers are listed with Power VA. Can someone describe what that rating means?

Volt*Amps. If the rating is 10 Volts and 5 Amps it is a 50VA transformer. VA really means the core-size (and cost): you can find 5V 10A, 25V 2A, and 100V 0.5A, all the same size and price because they can all be wound on the same 50VA core and primary.

> What's an appropriate current to run 4 discrete opamp based preamps? Ala, Quad Eight, JLM. Bipolar 28V.

It Depends.

Jensen 990 driving the rated 75 ohms at full blast could be over 120mA.

Most amps in most normal jobs, 10mA-20mA, but Read The Docs.

 

[MikoKensington]

Many thanks PRR! :thumb:

 

[PRR]

> Allow 120mA for each Green pre... 3.6 VA

I was going to say it is less. But after sliding all my abacus beads around, worst-case, I get 30V AC 0.2A AC per Kev's Green with Phantom. 6VA per channel. Matta's 30VA is marginal for 5 channels (ample for two). It will actually work fine until you have all Phantom-hungry mikes making clipped outputs in 600Ω. With happy audio and typical Phantom mikes, 2VA/ch is marginal so a 30VA core might feed a dozen channels for most users. I guess 3.6VA is a nice conservative target.

 

[matta]

Hey Guys,

Thanks again for all your input. It looks like I'll be able to get a
transformer from Farnell, does anyone have any recommendations? Part
number(s) etc? Has anyone else bought from them? I looked into the low
profile ERA ones Peter recommended, but they seem to be PCB mounts.
Any suggestions for either a small EI or Toriodal?

Thanks

Matt