VA specs for CT transformer

Thread Starter

atferrari

Joined Jan 6, 2004
3,453
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?
 

Thread Starter

atferrari

Joined Jan 6, 2004
3,453
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?
Since the voltage from CT to each end of the secondary is 1/2 of the end-end voltage, calculate each, then add.
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?
 

#12

Joined Nov 30, 2010
18,076
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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The Electrician

Joined Oct 9, 2007
2,737
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?
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.

See this thread: http://forum.allaboutcircuits.com/showthread.php?t=68839

Post #3 of that thread applies specifically to your center tapped secondary.

The question of whether you can draw different currents from the - output and the + output, and how large those currents can be will require a somewhat different technique than I described for the case where there are not different + and - output currents..
 
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