Hi Martin! Hello Everybody!

Since I'm building my own variable bench power supply, I gladly read and listen to anything related to this topic. Today I found a company in my country, which produces toroidal transformers based on the customers wishes. In their brochure they list methods of determining secondary voltage and current ratings. I would like to know more about the formulas presented here (theory as well as practice) because choosing the right transformer for a bench power supply (or any other project for that matter) is important. In my case, for example, I found that I chose a too large transformer concerning the voltage.

Please find below some .jpg files with the different formulas. As I first looked at them, it wasn't obvious, but the formulas on the left are without capacitors and the ones on the right are with capacitors (except for the last one, which is a bit special). If the more experienced could give a short explanation to each of them, that would be a good start.

A few of my questions are:

- Why does it matter to show the formulas with and without capacitors? Isn't everybody using filtering capacitors?

- In the case of full wave bridges, I learned that I have to divide the desired DC voltage with 1.41 and add about 1.2 - 1.4 Volts to cater for losses on the diodes. For example: 30 V DC / 1.41 + 1.4 V = 22. 67, so I could go for a 22 V AC transformer, but these formulas give different results.

- Current wise, I thought that if I plan the power supply to put out 4 A then all I need is to choose a transformer that can handle 4 A + 25% = 5 A; or in the case of a 22 V AC transformer it should be about 110 VA.

From time to time I become sooo confused!