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I have one transformer that I am using it for my PS, but I don’t know what is his VA rating, or the maximum current it can give at his secondary, so does someone knows how can I calculate the current at the secondary???
Do you have a multimeter or some thing?With out a meter you can not do anything.What is the rating of the PS you are using and what is it for? If you can find the voltages and the i/p current then you can find the o/p current using this relation:
This is an ideal equation but will give a decent answer since a transformer has high effiency.
yes, i have multimeter, but it can mesure only DC current, and i dont know the power of the transformer. The PS is 3-30V 2,7A and i am using it for trying other circuits.... I just mesure the wire diameter and they are: 0.3mm-primary, 1.2mm-sec...can this somehow help me?
Sometimes you can compare the size (weight) of one transformer against another of the same manufacture and this will let you know the VA rating. In other words the "watts" of the transformer equated to AC values. If you have a similar transformer and it is 15VA, then the transformer is a 15VA type.
If the output voltage is 15 volts (AC), the current will be 1 amp.
Similarly, you can measure the gauge of the secondary winding and if it is similar to another type that will deliver 500mA, the same applies. This is only a guide so that you don't think 500mA from one transformer will deliver 2 amp from another, with the same gauge wire.
If you only have a DC meter, put a bridge on the output and a 1,000u electrolytic and you will get a DC voltage appearing across the electro.
A 15v AC transformer will produce about 22v DC from the bridge.
Because the voltage is increased during rectification, the current capability must be decreased accordingly so that the final result is 15VA. The current will be about 700mA.
My old electricians reference books say that for every square inch of the center core area you have about 100 VA of capacity.This is just a rule of thumb referance for 50 to 500 VA transformers though.
however that is not an exactly linear value.
It degrades below 1 inch (75 VA/sq. inch) and goes up at over 4 inches (120 VA/sq.inch)
Still, it gives you a basic starting point to work from.
Increase the load/voltages very slowly and monitor the entire transformer's temperature. Stop when you get to the same temperature any other transformer runs at under it's rated load. The key is VERY slowly.