# AC transformer question (lots of pins, why?)

#### exscape

Joined Jan 19, 2012
28
I'm looking at designing and building my own DC power supply as a project, and got a bit stuck on finding an appropriate transformer. All of the ones in the correct voltage/current range don't look like the ones I know (that have fewer pins!).
Here's the pinout of one:
https://www1.elfa.se/data1/wwwroot/assets/large/5617907-11.jpg

Some of the specs:
"With 1 or 2 secondary winds that can be parallel or series connected.

Connections secondary: pins 9-13
Connections primary: pins 1-7
Secondary: 16 VA, 1 x 6...18 VAC / 2x 6...15 VAC
Secondary voltage: 15 VAC(1x)

Data sheet: https://www1.elfa.se/data1/wwwroot/assets/datasheets/EI54_eng_tds.pdf
"
(The exact model doesn't matter since I picked this transformer semi-randomly from a pool of similar ones. The same principle should apply to all of them.)

So... What do I make of this? My goal is first and foremost to step down 230 V to roughly 20 V, but I have a secondary goal which is center tapping, or something to that effect, so that I can choose between ~10 and ~20 volts, perhaps even more than 2 choices... The point of this being to limit the input voltage to the regulator (LM317) to avoid wasting those excess volts as heat (via (Vin - Vout) * I = heat in watts) when using low output voltages.

Would this be possible with the above style transformer?
If not, why are there more than 4 pins?

#### crutschow

Joined Mar 14, 2008
27,743
Transformers often have two or more secondary windings so can serial connect them for more voltage (and the connection between the two becomes the center tap) or connect them in parallel for more current. Sometimes they also have two or more primary windings (for either 120V or 240V operation for example).

An interesting way to easily get two output voltages is to connect a bridge rectifier to the outer terminals of a center-tapped transformer. The bridge output will be the high DC voltage and the center tap will have 1/2 that DC voltage (the bridge acts as a two-diode full-wave rectifier for the center tap). Of course you will need filter capacitors at each output to smooth the ripple.

You do know that the maximum DC output of a rectified and filtered sinewave is approximately 1.4 times the AC RMS voltage so pick your transformer output voltage accordingly.

#### exscape

Joined Jan 19, 2012
28
Transformers often have two or more secondary windings so can serial connect them for more voltage (and the connection between the two becomes the center tap) or connect them in parallel for more current. Sometimes they also have two or more primary windings (for either 120V or 240V operation for example).
Hmm, right, but how would I do that with these transformers? Is it even possible with the one linked? I'm a bit confused.

I'm looking for roughly 16 volts DC output (before the regulator), and it'd be great if I could also get ~8 and be able to switch between the two. Also, somewhere in the ballpark of 1.5 amps (though more like 1 in practice; the rest is mostly to be safe).

#### crutschow

Joined Mar 14, 2008
27,743
To get 8VDC and 16VDC you want want a 12VAC center tapped (6VAC to the tap) secondary. The Hahn BV EI 542 1152 should do what you want. You would connect pins 9 and 13 together and that would be your center tap. Pins 8 and 14 go to the bridge rectifier. The negative bridge connection is the common. Add a filter cap of the appropriate size from the bridge positive output to common, and a filter cap from the center tap to common. That should give a bridge DC voltage of about 16V, and about 8V from the center tap.

Edit: Alternately you could just switch the bridge between the center tap and pin 8 to change the bridge output voltage. That would save a filter cap, since you don't need both voltages at the same time. The disadvantage is that you have a extra diode drop for the 8V output which will lower its voltage by 0.7V or so as compared to using the center tap output for the DC as described above.

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