Author Topic: Choosing the right transformer - formulas, theory and practice  (Read 2993 times)

birrbert

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Choosing the right transformer - formulas, theory and practice
« on: September 20, 2013, 07:00:29 AM »
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! :)









"Dubito ergo cogito, cogito ergo sum." Descartes

birrbert

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Re: Choosing the right transformer - formulas, theory and practice
« Reply #1 on: September 30, 2013, 03:28:20 AM »
I found out a couple of things. Please allow me to share this with you, because only now I understood it, thanks to a friend, and maybe there are others out there who need this approach.

With or without capacitors?
- Since the secondary side of the transformer I'm talking about has AC output, that means that the signal is sinusoidal. Taking into consideration the properties of sinusoidal AC, the output voltage is a mean value, let's say 24 Volts, but actually the voltage alternates between +33.84 and -33.84 Volts peak-to-peak. To be more precise, the 24 Volts is the one that's called RMS (Root mean square) value and the peak value is the result of 24 * 1.41 = 33.84. It's the same situation like with 220 V RMS in my home where the peak voltage is 220 * 1.41 = +310 Volts and -310 Volts respectively.
- Capacitors come into the picture because while they filter they don't "work" with the RMS voltage; instead they "try" to maintain the peak Voltage. This was the part of the story that was missing for me. For a full understanding it would be best to show it on an oscilloscope, but unfortunately I don't have one.

Regarding the formulas that I posted earlier, I'm still not clarified, but hopefully in time wisdom will come. :)
"Dubito ergo cogito, cogito ergo sum." Descartes