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I'm a hobbyist and I have 2 doubts regarding a power supply described into this schematic:

1) How do I differentiate a center-tapped power transformer from a non center-tapped one? My primary language is portuguese and I cant manage to translate it... Can I assume that "24V 2A" is a center-tapped transformer and "24+24 2A" it's not?

I need a +-20V power supply. Into the diagram, it recommends a 30V center-tapped or a 15+15V secondary in order to build one. But:

2) If I split a 30V center-tapped transformer, it will give me +-15V, not 20... Neither if I use a 15+15V, it will not amplify to 20V...

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  • \$\begingroup\$ Some times the third pin or cable, it is the "electrostatic shield screen", a float one layer winding or piece of copper sheet betwen primary and secondary. There is no connection eather with primary or secondary. \$\endgroup\$ – GR Tech Feb 10 '15 at 2:33
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A center-tapped transformer has three connections on the secondary, where a non-center-tapped transformer has only two. Transformer datasheets should indicate this by showing a schematic symbol of the transformer.

The output voltage of a transformer is always given as the RMS value. The peak voltage is about 1.4 times the RMS voltage, and after a bridge rectifier and filtering the DC voltage will be 1.4 volts less than the peak-to-peak voltage.

A 30 volt center-tapped transformer with a bridge rectifier will produce about 40 volts DC. If you use the center tap as the common, or ground, terminal, that gives you +/- 20 volts.

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The "24+24 2A" is probably a center-tapped transformer. If it has two separate 24 Volt windings (4 leads), 1 lead of each winding can be tied together to produce a 48 VAC RMS Voltage on the other 2 Untied leads. If the two leads that are tied together are incorrect you will have somewhere near Zero Volts AC on the untied ends. If that is the case then untie the 2 leads and substitute just one of the tied windings 2 leads for the other lead of the same winding and tie it to the unswitched winding lead. This is a little hard to describe I hope you can understand. From this you would have a 48 Volt AC center taped transformer (the 2 leads tied together is the center tap) with 24 volts AC measured from center tap to each of the 2 untied leads. Note: when the 2 lead are tied together their should be no current flow example buzzing, getting hot, sparks etc.... When not hooked to a circuit.

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X+X is probably center-tapped with X appearing between the tap and each end. So 15+15 would give you 30 end-to-end.

That said, the ratings are RMS, and the rectifier will give you peak minus one or two diode drops. This is how a circuit can be powered by slightly less AC than the DC that is derived from it.


Taken to somewhat of an extreme, there is the voltage doubler power supply. In this configuration, one end of the winding is used as a DC center tap while the other end alternately charges the positive and then the negative DC supply capacitor. Thus, a single 9V transformer can power a 12+12 DC circuit for a total of 24VDC.

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