a quick transformer question

[exclamationmark]

I was looking at some cheap PCB mount transformers with dual secondaries for a heater/logic supply but have come across a little problem... Whilst these transformers are ideal, they state on the datasheet that the secondaries can't be used independently (or shouldn't be) and I either have to connect the secondaries in parallel or series. Why exactly is this, and what will happen if I ignore this warning ?

[Enzo]

Is this a practical question or just academic?


If you intend to wire them that way anyway, it becomes academeic. Nothing wrong with that, but I want to know what to answer to.

Your average low voltage transformer is rated typically in VAs - volt-amps. The difference between VAs and Watts doesn't really matter much to some little project circuit we might build. So you will see a 12VA transformer with secondaries either series or parallel, with the lower voltage arrangement having twice the current rating, and vice versa. If you do not wire it up their way, then it doesn't meet their specs. If the thing has either 6v at 2A or 12v at 1A, you wire it appropriately. Either way is 12VA - 12 watts. If you use just one winding, you get 6v, but not the 2A.

In my opinion, and one of the engineers around here may tell me I am wrong, you can use the two secondaries independently as long as you understand the output capability will not match the catalog. I mean after all, it is just a transformer with two windings on the secondary. A bazillion transformers are made that way. Just that the power transformer in a Fender Twin Reverb or something is not rated with all its windings hooked together. SO it is not rated that way.

[exclamationmark]

It's actually a practical question. I'm ordering some parts online and thought I would take advantage of the free shipping I get on get big orders. Plus it's only like 6 bucks. Anyway, here is a data sheet (it's a pdf).

http://system.netsuite.com/core/medi...6ad8b&_xt=.pdf

Notice at very bottom numbered with a '4', it says that the windings are not intended to be used independently, and I can't really figure out why. One would assume you'd get 10VA out of each secondary (6.3v at 1.6A), but perhaps there's something I don't know... I'd like to have 2 secondaries for the logic + heater supply of my pre-amp, so I can elevate the heaters and keep my logic on a separate winding (I haven't done any real thinking on a what a cathode-heater short would do to my logic chips at cathode follower voltages, but I'm just assuming it wouldn't be good).

[Enzo]

Well, note 4 also includes the primaries, and trying to use them independently will derate the thing too.

Um, I don't think you will be elevating your heaters above the cathodes on CF stages are you? You could put the heaters on the CF at +150 or whatever, but then they would also be at +150 for the other tubes, whose cathodes would likely be at a more common volt or two. Typically the heater hum thing will occur in the low signal high gain first stage or two. I don;t worry about heater hum in the phase inverter and similar.

As I see it, you'd want to be conservative with your heater winding draw. I can't imagine your logic circuits would draw much current anyway. Are you using "real" logic, like 4000 series CMOS? Or do you just mean various transistor switching circuits? Either way, not much current draw other than potentially relay coils. But definitely keep the primaries teamed up.

[exclamationmark]

I'm only using 3 pre-amp tubes so I'd at most be drawing 900~ mA from one winding. The elevated voltage thing is mainly for cathode longetivity, as I'm going to be running quite high voltages off a separate transformer I have lying around (that has no heater winding). The logic would probably be drawing close to 1 Amp, since it's some pretty serious stuff (a couple microcontrollers, a DSP, LCD display, 7 Segment displays, etc).

[Steve Conner]

The only thing I can think of is that the windings might be wound bifilar, so the insulation between them would be pretty modest. Some of these little transformers are rated to withstand 3kV between primary and secondary, but they might only stand 300V between the two secondaries or the two primaries.

For example, using one of the primaries as a secondary would certainly violate the Class 2 safety certification and probably present a real safety issue. You might reasonably want to do that, to provide B+ for a tube preamp, but it's not a good idea.

I don't see a problem with using the two secondaries to make two independent low voltage supplies, as long as you don't float one of them at hundreds of volts relative to the other one.

[R.G.]

Yep, what Steve said!

[Austin]

Really? Bifilar on a power transformer? I thought that was only for opt. Learn something new everyday...

[J M Fahey]

Well, it's actually the other way, bifilar OPs sound horrible, best way to increase parasite capacitance.
Not a problem on PTs.

[exclamationmark]

I ditched the pcb mounted transformer and instead went with a cheapo $15 transformer. It has a separate 5v winding for non standard rectifiers, in addition to heater and B+ windings.

The thing that confuses me, is that it has a '115/230v' centre tapped primary (115-0-115) as opposed to dual primaries. I didn't actually know how to connect this thing...but after some experimenting with a variac, I'm assuming for 230v operation you just connect the two outer taps and ignore the CT. And to get it to run on 115v you would stick neutral on the CT and connect live to only one of the outer taps (or the other way round). I've seen some transformers like this for universal operation that leave one half of the primary just sitting there when used in 115v mode - but isn't this more expensive to manufacture than having dual primaries, as you have to make sure the winding has sufficient VA rating, even if half of it is only used?.

[J M Fahey]

It's standard on cheapo transformers, clock radios, cassette recorders, etc.
And when you want many voltage options, as in older Marshall, Vox, even "export" Fenders.
Allows the same transformer being usable in 100/110/115/120/220/230/240V with a simple rotary switch.

[Steve Conner]

In a well designed transformer of this kind, the 115V half of the primary should be made of thicker wire than the rest, so do make sure you are using the right half.