Need a little transformer Theory help..


2008-01-31 9:01 pm
I'm new to diy audio but up to this point, felt I had a pretty good understanding of transformers. That is till I began looking for the proper transformers to power my kit. Here's what's troubling me.

1) Some transformers have more than two primary leads. Now, I thought you merely connected the hot and neutral from the wall to the primary side and the transformer transformed line voltage to the required voltage on the secondary side. Why are there more than two leads on the primary? What advantage does that give you? How do you use such a transformer?

2) What are center taps for and how do you use them? I've seen transformers listed as 220-0-220 but I'm not totally sure what that means. Is the zero the center tap and if so is that like a neutral or ground? If I wanted to connect such a transformer to a dual mono power supply would one 220 lead go to one and the other 220 lead to the second power supply? If you only want to use a single power supply do you just cap the other 220 lead.

I hope this is making sense. Thanks in advance, I feel I'm on the verge of a breakthrough but I've been visiting a lot of diy sites and have never come across exactly what I'm hoping to learn.


2008-01-29 9:26 pm
Transformer Theory

Ive attached a pdf to help illustrate.

First - the primary wires:

Many transformers can be wired for either 120v or 240Vinput to give you the same output.

In T1a of the attached you see a transformer wired with the "Hi" side of one primary winding tied to the "Hi" side of the other. The "Lo" sides are both tied also. this makes the windings in parallel and you get the rated output when you apply 120VAC to the input.

In T1b, it is the same transformer but with the windings in series, (center two tied together) and to get the rated output, you need to apply 240VAc to the input as shown.
(Note the difference in fuses!! Very important!!)

A trick you can use for a lower voltage out is to series the windings, and apply the voltage rated for the two windings in parallel to it, like in example T1A.

NEVER put more than the rated voltage onto a primary configuration or you are likely to let the "mysterious blue smoke" out of your transformer :D

If you tie the high of one to the low of the other, on both windings, you will have NO output, and a HOT transformer!

However, to apply LESS than the rated voltage, is OK!

Next, the center Taps:
In the upper left, you see a center tap used as a full wave rectifier. It uses two fewer diodes than a full wave bridge. And it applies the secondary of the transformer directly to ground. (don't really know what the advantage tying it to ground is, but that is what happens!)

Below that is the best use of a secondary center tap, in my opinion, it can be configured with a full wave bridge to give you a bipolar (plus and minu) DC supply. (Often needed for ICs and chipamps). The center tap provides the ground.

Hope this helps some!:cool:


  • xformer.pdf
    20.4 KB · Views: 95
The ones that I use a lot of, that have two pairs of primary leads, are spec'd to run from either 115VAC or 230VAC. They actually have dual primary windings. For 115VAC, you just use the two primary lead pairs in parallel. And for 230 VAC, they are wired in series. That way there is 115VAC across each primary, either way, and the secondary outputs are the same for either 115VAC or 230VAC.

A center-tapped secondary just gives you a "middle" voltage. Remember that "voltage" is (only) defined relative to another point. So it's up to the designer to decide what to do with the center tap (versus the other two). But, for example, it could be defined to be zero volts, for a dual-polarity power supply. (But someone could just as easily define one of the ends of the secondary to be zero volts, and the other two could then be two positive or negative voltages, with one twice the magnitude of the other.)

A transformer listed as 24-0-24 gives 48 VAC RMS (or about sqrt(2) times that, zero-to-peak, i.e. 67.9 v pk), between the two outer secondary leads, and 24VAC between each outer secondary lead and the center tap lead. For a dual mono setup, you'd probably want both positive and negative voltage rails for each mono block. So, unless they were single-supply designs, you'd need all three secondary wires for each one. (That should be able to work. But I think I'd just use two lower-power tansformers, for that, if I could.)

I usually would rather have two center-tap leads, i.e. dual secondary windings, since some types of dual supply designs will not work with just one center tapped secondary (e.g. using two positive voltage regulators to get a dual-polarity supply).

If the transformer has the dual primaries and also has dual secondaries, i.e. four leads for each (eight leads total), then you have the most options: The secondaries can be used in series, for twice the voltage with half the max current (compared to parallel). Or they can be used in parallel for half the voltage with twice the max current (compared to series). They can also be used independently, for two separate power supplies with either opposite or same polarities. And the primaries can be wired for either 115VAC or 230 VAC without affecting the secondaries' voltage or current ratings.

By the way, here's a link to a site that has a clear explanation of the calculations needed, for simple transformer-based power supplies:


2008-01-31 9:01 pm
Thanks to you all. I've read and re-read your responses a few times and every time I do, this subject gets a little clearer. I was worried that I might get a, "use the search fuction, newb!" kind of response and I must say I was pleasantly surprised.

Thanks again. :)
Dug (4/8ths),

If you need to go to Duvall, better do it before this summer, as they're going to tear up Main Street to install new water pipes and a traffic median. The telephone poles will be gone as well. Hopefully, people will still be able to get to Duvall Used Books without too much trouble.

Totally World-Class Traffic Problems coming to the Eastside:bawling:

Best Regards,
I'll throw a bit more into the mix; Two trafos with single scondaries can also be configured for your intended use. Vetco in Bellevue has (as of last week) 4, 20V, single secondary toroids, mfg'd by Toroid for 20 bucks ea... pick up a pair and I'm quite sure you'll be pleased. Besides, I have a whole pile of them and seeing the last 4 just sitting there is tempting me beyond my comfort level! Oh wait... on second thought:)

Tommy Q,
How the heck are you? Damp??? The weather here reminds me of the movie, "Groundhog day". I predict that three weeks from next tuesday will be 40 deg and raining. Masterful prediction or what ?



2008-01-31 9:01 pm
You know, I've been to vetco several time to buy components for guitar pedal building and never thought to look at transformers. Pedals typically just use wall warts and a 9v battery so the whole transformer thing is new to me.

I'll have to stop by this afternoon. We've got a large tax refund coming and my wife gave me a kitchen pass to go a little nuts. Oh, I'll be going nuts allright.

Thanks again all for the input, it has been a big help.
seventenths said:

Tommy Q,
How the heck are you? Damp??? The weather here reminds me of the movie, "Groundhog day". I predict that three weeks from next tuesday will be 40 deg and raining. Masterful prediction or what ?


Deer 35/50,

Given that my property tax is so #%&%*%# high, it should come as no surprise that our area it's been in the mid to upper 70's with sunny skies since early October. The County Council has allowed rain a couple times a week (only late at night, of course) so that everything is green and smells as fresh as a daisy. All my Rhodies are constantly blooming, month after month and the Roses...Well, can I say?

As for your prediction, in any area but mine, I'd say you're probably right on the money

Seriously though, I'm doing fine. More importantly, how are you doing? Have you and your son done any more speaker projects lately?

Best Regards,
Nice! I could use a little of that. We decided to fore go our usual winter/chase the sun vacation and I'm regretting it severely. No worries though as we're expecting the sun to show up by june or july:)

No, no speakers lately, but I've built several GClones and I'm working on a 6 channel unit to take advantage of that surround processor I won at the event. More full rangers to follow if I can make up my mind.

Cheer up a little, would you? ;)



2009-10-24 6:21 pm
I also have an audio power transformer with more than 2 primary wires. In fact, it has 6 primary wires and 6 secondary wires. Of the 6 primary wires, 2 of them are blue, 2 are red, 1 is purple and 1 is black. Does any know if there is a standard color code for these wires? How would these wires be connected to the AC?