VA specs for CT transformer

[atferrari]

Transformer with following specs:

Prim 220V Sec 36V CT
Power 50 VA

After implementing a diode bridge and a 2 x 5000uF filter, I measured first:

- 25,4V - 0 mA
-23,7V - 270 mA
-22,4V - 620 mA
-21,3V - 1000 mA

and later:

+25,4V - 0 mA
+23,7V - 250 mA
+22,6V - 620 mA
+21,3V - 1000 mA

Ridiculous as it sounds a big box with power resistors is missing from my bench thus the separate test of each branch.

I tried to come up to 1390 mA but after burning several small value resistors I decided to stop.

I always get confused about the VA spec for CT transformers, thus my question: provided the specs are true, how much current (maximum possible) should I obtain from each output at the same time?

1390 mA or just 695mA?

 

[MrAl]

Hi,

It's actually simple. If you have say two outputs, both 10v, and the VA rating is 20, and provided each output is spec'd for 1 amp output current, then you can draw 1 amp from each. You cant draw 2 amps from one and 0 amps from the other unless one of those is also rated for 1 amp output.
If they are both rated for 2 amps and still VA=20 then you can draw 2 amps from 1 and 0 amps from the other, or 1 amp from both.

So add up all the VA's but dont exceed any SINGLE winding current rating.

If you cant find the spec's then look at the wire size. You can go up to say 300 circular mils per amp usually, but you might also want to check for heating.

 

[atferrari]

Regarding the specs of mine

Hola MrAl,

I cannot get this straight. My inability to understand at work, again. Could you please refer to the specs of my transformer?

a) Should I say, 695 mA to my question below?

how much current (maximum possible) should I obtain from each output at the same time?

1390 mA or just 695mA?

b) How consistent with the specs is / it is not what I measured? Rarely I see the output voltage drop mentioned much less considered in any calculation, but it is real and noticeable (easily!).

c) It seems that 1390 mA is the right value adding both outputs; if so, provided I load, say, the + side, with 1000 mA, should I be able to load the - side with 390 mA as well? Would I be still within specs?

d) How to estimate actual voltages at the outputs? Better measure them right?

Had a bad afternoon yesterday trying to get 160 mA under 5V from an LM7805. It seemed unable to cope with it. I finally found that were the transformers (all small ones) that colapsed earlier. Learnt that when the regulator, connected to my power supply, provided those 160 mA gallantly.

 

[MrAl]

Hi again,

A CT winding is made from two separate windings that are connected end to end. That is, one 'finish' is connected to the other 'start' and that makes up the center tap.
So if you have a transformer that is rated 50VA and is CT then you can get 50VA from both windings, or 25VA from each winding. You dont need to connect rectifiers to find this out, but if you do connect rectifiers and want to measure the DC voltages (which just complicates things) then you have to show the circuit, full wave bridge (four diodes) or just full wave (two diodes) and cap connections.
It's easier just to measure the AC voltages and currents.