transfo question

[bent]

Hello,

transfo turn 110v/220v to different voltage like 12vac , 24vac, ect...

does it exist a transfo that turn 24vac to 12vac ? so primary 24vac and secondary 12vac ?

thank's for reply !

bent

[R.G.]

Transformers don't have voltages. They have *maximum* voltages at a given frequency and ratios.

So to answer your question, **any** transformer that has a 2:1 stepdown ratio and at least a 24V rating at your power line frequency will do that. A 120Vac to 60Vac works. So does a 240:120 stepdown. Seems strange, but works. An audio transformer would do it, except you need a very high quality and expensive one to get down to 60Hz.

Another and probably simpler way is to use two transformers, one of them 120Vac:24Vac and the other 120:12Vac, and connect the two 120Vac windings together, using the 24Vac "secondary" as the input and the 12Vac secondary as the output.

Of course, none of these are what you really wanted, I'd guess. Sorry. 24Vac to 12Vac is simple, but not a standard item.

[bent]

Thank,s RG,

i was hoping a answer from you, cause you always give the explanation !!!!  

How do i calculated the Amp for the transfer....i know the w=va .. so probably the same, i take a 120/60 80va , it will be 24/12 80va so 6,66 Amp on 12v ?

Bent

[R.G.]

No, sorry. The power rating of a transformer is primarily determined by how hot it can get inside without damage, and by how much waste heat it generates in operation.

Since you are using an existing transformer rated at, for example, 80VA, it still can't generate any more heat. heating in transformers is dependent on two things: core loss and copper loss. Core loss is the heating the iron does inside because of the magnetic fields, and copper losses come from heating of the copper by current, according to the I-squared-times-R equation.

Core losses go down a lot when you reduce the primary voltage, so a 120:60Vac transformer used with 24Vac on the primary will have significantly lower core losses. However, the copper losses still are I-squared-times-R, so you can't pull much more current through it. Some, but not much. In transformer design, it's normal to assign equal power densities per unit volume to core losses and copper losses, and it turns out that these are roughly equal, no more than about 2:1 different when you add it all up. So if the core has half as much loss, that only lets you increase the copper losses by about 50%, and that increases the current by square root of three-halves. So the voltage on the 12V output is 1/5 of the rating, and current is about 1.23 times as much, so the power rating goes down to 0.245 of the 120Vac rating. Call it 1/4th.

This is an estimate full of assumptions that may or may not hold, but you can test it by seeing how hot the transformer gets in use. Hotter than about 130F/54.5C on the surface when it's fully heated up is beginning to be questionable.