transformer basics

[dvdwmth]

Could someone please explain some pratical basics of tranformers to me. All the sites I've been able to find explain the theory but not the practical stuff.

(Im pretty green at this so your assistance and patience is appreciated)

Specifically:

1. what is the significance of VA when choosing a transformer
2. I'm guessing that the second number is the secondary voltage.
If so, am I right that to calculate the voltage produced that you multiply by 1.414
3. if i wanted to build a power supply for a chip with an ideal voltage requirement of 25v, how would i determine an appropriate transformer.


If the answers to these question is more complex than is suitable to explaining here, could someone direct me to a site that explains the practicalities of choosing a transformer.

Thanks in advance

David

[fcel]

I always find the technical section of transfomer manufucturer very helpful. Example Plitron transfomer ... http://www.plitron.com/Pages/technote.htm

[RobPhill33]

1. VA is the complex power handling of the transformer. Although there is a major difference between complex power (VA) and real power (W) it may be easier for you to think of VA in terms of W. Example, if you need 3A at 30V then you need 90W or 90VA. It is rare that you can purchase the exact VA rating you desire so you should always choose the next highest value.

2. Yes, multiply the secondary voltage by 1.414 (converts RMS to peak, caps hold peak)

3. If you want 25VDC with dual secondaries and say 3A per channel, you would calculate ((((25V+1V)/1.414)*3A)*2channels)=110VA. You need to figure out what your current draw will be. Also, the extra 1V is for the voltage drop across the rectifier.

There is lots of information on this site about transformers and perhaps a more thorough search would find you what you're looking for.

[dvdwmth]

thanks for your help.