The principle Guide for ascertaining the Va of a transformer is the core size, you can have any gauge you want, but it makes no difference if the core is not sized for it.
The chart in post #20 will get you close enough.

 

Note that you need to derate the transformer AC output current about 50% for the DC output current of a full-wave rectifier-capacitor supply, due to the high peak transformer currents, that such a supply generates.

 

No, it will go to 2A each.

VA means just that, volts x Amps. At 56V you can use only 120 / 56 or a little over 2A

Bob

you mean for each 28-0 it will only supply 2A? and to extract 4A i should used parallel? will my transformer able to parallel connect the output?

 

You cannot parallel the output if it is a continuous winding, i.e. 24-0-24.
You can use it as I showed, with 0v as the common and each 24v out[put with a rectifier.

https://www.hammfg.com/electronics/transformers/rectifier

 

so hooking up 28-0 will only produce 2A?

 

you mean for each 28-0 it will only supply 2A? and to extract 4A i should used parallel? will my transformer able to parallel connect the output?

You have to check the phase of the secondary winding(s). They may be wound "out of phase" or "in phase" . The phase needs to be checked with a scope.

 

i dont have a scope, any other way to do it?

 

Simple, if only three terminations, then one pair must be common. if they were in phase, there would be no (or close to zero) voltage measured on the 'outer' ends. with a voltage seen of 48v across the outer ends, Then each 24v must be 180° WRT each other.
24v-0-24v.
Also if each WERE in phase, with each other then you would be able to parallel them. Otherwise NOT.

 

mate, thanks, but to close this thread maybe, please help me how much amps i can get 28-0. sorry for im still 1st year college learning to build linear ps

 

Theoretically, you can draw 4A from a single 28V winding, but it may get hot.

Use both coils to get your rectified DC like below. Now each coil is only active half the time, so it is equivalent to putting the could in parallel.

 

can i use 4 diodes instead of two on bridge rectifier? and capacitor instead of resistor(RL)?

 

can i use 4 diodes instead of two on bridge rectifier? and capacitor instead of resistor(RL)?

You can add a smoothing capacitor.
Where were you thinking of putting the other two diodes? a split-phase rectifier circuit only needs the two.

 

i mean like this

 

i mean like t

NOw you have shorted out your transformer. There was 56V between the two terminals that you have just shorted.
See @BobTPH ‘s diagram. There is Vm+Vm across those two,points.

 

ah see, ok thanks for the clarification, if i fllow bob diagram, the secondary current output will be doubled? 8A?

 

The output voltage will be the AC voltage x √2 = 40V
The current available is the VA divided by the voltage = 120VA/40V = 3A

 

how did you get 40V? sorry im not familiar yet with the formula.

update: ok its the 56V rms = 40V

 

The peak voltage of a sine wave is the RMS voltage times sqrt(2). The RMS voltage is 28, so 28 x 1.41 = 38.5. When you rectify and use a caoacitor, it charges to the peak voltage.

Bob

 

ok got it thank you for the explanation. atleast now i have a bunch of ideas not only on transformers but also the way how should i use it. i will be back in this thread if my mind wants some clarification again. i really appreciate all of you for being responsive and briliant. i cant wait to build my very first diy bench linear dc ps.

 

I’ll just add here that these voltage are not exact.
There will be a variation in your incoming mains voltage (it’s +10%-6% in Britain, +6%-10% in the rest of Europe, don’t know what it is in USA)
Then there’s regulation which is the variation between on-load and off-load voltage of the transformer, which is 11% for a 120VA transformer. You will get 11% more voltage than specified when there is no load.
So your 40V could actually by 21% higher (at 48.4V) so don’t go buying a 40V rated capacitor, a 50V rated capacitor is only just enough! Better use a 63V capacitor.