# VA specs for CT transformer

Discussion in 'General Electronics Chat' started by atferrari, Nov 24, 2012.

1. ### atferrari Thread Starter AAC Fanatic!

Jan 6, 2004
2,648
764
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?

2. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
Since the voltage from CT to each end of the secondary is 1/2 of the end-end voltage, calculate each, then add.

3. ### atferrari Thread Starter AAC Fanatic!

Jan 6, 2004
2,648
764
So, as per the above, 695 mA.

Now, what if I load the + output with 1000 mA; should I be able to get 390 from the - output?

Is it how it works?

4. ### #12 Expert

Nov 30, 2010
16,321
6,818
One of the things you must consider is that the rectified voltage times the current is the wattage you are getting out of the transformer. In addition, the configuration of the rectifiers affects the net current you can use. This datasheet from Hammond tells the factors to use with each type of rectifier to avoid over stressing the transformer.

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5. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,283
326
This transformer's rating means that you can connect a resistive load across the whole 36 VAC winding and draw 50/36 = 1.39 amps; this means 1.39 amps from each end of the winding at the same time. However you can't draw 1.39 amps DC after the rectifier and filter because the current in the windings will be larger than 1.39 amps RMS.