Replacing centre-tap transformer with standard transformer

[Hewitson]

Hi guys,

I have here a cheap chinese set of 5.1 PC speakers I'm trying to repair for someone in which the transformer (14.5V centre tap) has gone open circuit. It is quite a large transformer and I am unable to get anything like it locally, and ordering from overseas would likely cost more than the unit is worth.

What would be the best way to replace this with a standard transformer?

Cheers

[alm]

Assuming the secondary windings are in series, you can replace it with two transformers (primaries in parallel, secondaries in series, watch phase) with about half of the total output voltage and power each. If they're in parallel (unlikely), you can replace it with a single winding.

[Simon]

is it actually using the transformer to produce a dual supply ? A long shot I know but I've often seen dual secondary transformers used for a single supply and only use 2 rectifier diodes, but this is characteristic of small supplies, if your dealing with a proper power amp it probably has a proper dual supply.

Another option is get a single output transformer of twice the amperage and use a voltage doubler rectifier to double the voltage an have a common reference, you will need to replace the rectifier and smoothing circuitry as well

[Zero999]

Is that 14.5-0-14.5V or 7.25-0-7.25V?

If it's 14.5V just use a 15V transformer, if it's 7.5V you might be all right with 6V but it may need to be 8V

[JohnS_AZ]

This is a total guess, but I'd bet it's a bipolar power supply. Probably something like a 30V center-tapped transformer and a power supply circuit to provide +12V/Gnd/-12V. That's really common in higher power audio amps. The TDA2050 amp I'm building is set up for +22V/Gnd/-22V.

If that's the case, two 15V transformers would work; primaries in parallel, secondaries in series with the center point being ground.

I've read dozens of times that you have to keep the primaries in phase, but I have never figured out why, or how to actually tell. Since you're ultimately making DC, what's the difference??

[alm]

I've read dozens of times that you have to keep the primaries in phase, but I have never figured out why, or how to actually tell. Since you're ultimately making DC, what's the difference??

Not sure what primaries in phase means, a transformer doesn't have an inherent phase, but the phase of the secondaries depends on the phase of the primaries.

You're right that phase of secondary windings does not matter as much if they're in series compared to in parallel. If each has a separate bridge, it doesn't matter, since a bridge rectifier produces a 120Hz wave either way (so 180deg phase shift doesn't matter). If they're connected to the same bridge (with center tap directly connected to common), the ripple in increased because each side only sees a 60Hz rectified signal (both diodes connected to the same rail will be conducting at the same time).

[JohnS_AZ]

To make the question clearer (excuse the rough sketch), is there any real difference between the following two configurations?

[JohnS_AZ]

Oops, scratch that. This drawing is actually configured as a center-tapped secondary.

[cybergibbons]

Probably not what you want to hear - a friend had a Creative something or other uber-5.1 speaker set, and the transformer suffered an untimely death due to lack of ventilation. I think that the tweeters and mids were powered by a class A amp, and the sub a class D, so the power requirements weren't huge.

It was a centre tapped transformer, and no direct replacements could be found. Two transformers takes up a lot of space in comparison. We kind of gave up fixing them.

But then one day in Currys/Argos/PC-World or one of the other stores, I noticed that their extra-cheap, pretty crap 2.1 speaker system for £14.99 had a transformer and DIN connector that suggested it would have a center tapped transformer in it. I bought it, took it home, and found the supply was almost a drop-in replacement, for less than the cost of 1 of the 2 required transformers from Farnell.

[alm]

To make the question clearer (excuse the rough sketch), is there any real difference between the following two configurations?

Not that I know of, but it does in the following schematic (switch the connections to pins 5 and 6 of the transformer):

[Hewitson]

Is that 14.5-0-14.5V or 7.25-0-7.25V?

If it's 14.5V just use a 15V transformer, if it's 7.5V you might be all right with 6V but it may need to be 8V

Sorry, it's 14.5-0-14.5.

is it actually using the transformer to produce a dual supply ?

It certainly is.

This is a total guess, but I'd bet it's a bipolar power supply. Probably something like a 30V center-tapped transformer and a power supply circuit to provide +12V/Gnd/-12V. That's really common in higher power audio amps. The TDA2050 amp I'm building is set up for +22V/Gnd/-22V.

That's correct.

If that's the case, two 15V transformers would work; primaries in parallel, secondaries in series with the center point being ground.

I'm not sure I understand how this would work.. Would you mind explaining it in a bit more detail or drawing a simple diagram?

Thanks for the help so far, wasn't expecting such a response so quickly! Think I might have to start making regular visits to the forum..

[Zero999]

Sorry, it's 14.5-0-14.5.

Is that the AC voltage measured across the transformer's terminals when no load is connected?

If so, a 12V transformer will probably work. The secondary voltage will typically be 20% higher than the rated value when no load is connected.

If you can't find a 12V centre tapped transformer, use a 12V transformer with twin secondaries and connect them in series.

[Hewitson]

That's the voltage silkscreened onto the PCB, I have no way of measuring the output as the original transformer has had it.

Getting centre-tap transformers isn't a problem, its getting one with a high enough current rating. 600mA is the best Jaycar can offer, I need ~3A.

[DJPhil]

Getting centre-tap transformers isn't a problem, its getting one with a high enough current rating. 600mA is the best Jaycar can offer, I need ~3A.

This is music amplifier territory, and the big suppliers charge dearly for large power transformers. You might find something reasonable from one of the salvage houses like Fair Radio. You can usually cut the cost by a factor of four this way.

[Zero999]

Jaycar don't seem to be very good. The look a bit like Maplin in the UK or RadioShack in the US.

I found a twin 12V 300VA transformer but it's probably out of your price range.
http://www.jaycar.com.au/productView.asp?ID=MT2130&CATID=19&form=CAT&SUBCATID=539

Farnel and RS Components have branches in Australia, perhaps you should look there or try your luck with ebay?

[JohnS_AZ]

Something that frustrates me just a little bit here on the forum (and I really don't want to start a firestorm like Simon did some time back )

I wish folks would put some sort of clue on their profile as to where they are located. Not an address or anything that would seriously identify them, just the country would be enough. Although state for the US would be cool, but that's because I keep an eye out for like-minded folks in Arizona and the Southwest.

In this particular case I'd be happy to search around a little to see if my favorite suppliers have a deal on the transformer that the OP needs but is he in the UK? Austrailia? The US? (and now Mars, I guess <grin> ) It obviously makes a big difference.

Sure, "just ask". Well I'm sitting here right now and have a few minutes to kill. If I ask, then wait for a reply, I may not have the time or inclination to go searching then.

It's just my humble opinion.

[Zero999]

You;re right, I'll fill my location in. I guessed Australia because he mentioned Jaycar.

[scrat]

Oops, scratch that. This drawing is actually configured as a center-tapped secondary.

About the "in phase" primaries of the transformer: if the schematic reflects the real magnetic cores, in the second one the two fluxes at the primary will act one aginst the other, resulting in null flux, so null voltage on the secondary. You can see it by setting one wire as positive and following the current flow direction.

[Hewitson]

Point taken John

Hero999: Yeah, thats about 3x what the speakers are worth, heh. And you're right, Jaycar arent very good. Far worse than Maplin, actually.

[Zero999]

Oh, I just realised they sell a 160VA transformer with a twin 12V secondary which is cheaper but still out of your price range.
http://www.jaycar.com.au/productView.asp?ID=MT2112&CATID=19&form=CAT&SUBCATID=539

My local supper sells a  suitable transformer which is more affordable but the cost of shipping will be unacceptable.
http://www.rapidonline.com/sku/Electrical-Power/Transformers/Toroidal/Toroidal-transformer/82719/88-3805