Stepdown Transformer Turns Vs. Ampere

[electronics_shamim]

In step down transformer is it that Ampere changes in accordance with Turns.

I mean to say that in the Table I found:  38 swg wire has 0.0365 amp @ 200 amp per square cm. and 3507 turns per sq.cm.

If i wind 4000 turns shall I arrive at 0.042 Ampere that means current increases with increase in Turns.

Raw Formula = (0.0365/3503) x 4000 = 0.0416

Please help am I right. This is required for small step down transformers from 300mA to 1 Amp. 

[HarryA]

if:
Vp = primary voltage, Ip =primary current, Np = number of primary windings.
Vs = secondary voltage, Is = secondary current, Ns = number of secondary winding.

then: Vp = Vs(Ns/Np)  and Ip = Is(Ns/Np)

example: To transform 120 vac to 12 vac,  
Vs = 120 vac (100/1000) = 12 vac Ns/Np = 0.1, say 100/1000

Using Ip = Is(Ns/Np)  I am not clear which way you are going here.
    0.3 amperes = 1 ampere (Ns/Np)  Ns/Np = 0.3  say 180/600
or:      1 ampere  = 0.3(Np/Ns)     Np/Ns = 3.33..    or say 600/180

For power: Pp = Ps  at 100% efficiency.
Ps = n * Pp where n is the efficiency factor. n between  65 and 99 percent

For 50 watts at the secondary and 75% efficiency the primary power would be:
Pp watts= Ps/n = 50 vac/0.75 = 66.66... watts.

For Impedance: Zp = Zs(Np/Ns)^2

To connect an 8 ohm speaker to a 400 ohm amplifier:
Np/Ns = (Zp/Zs)^0.5 = (400/8)^0.5 =  7

I must confess I have no idea how many windings real transformers have; do not go by my numbers.

Somewhere I have a book that has information on designing transformers if you need more info I will try to find it.

 

 

 

 

[electronics_shamim]

Thanks Harry A. I was really waiting for it. It is great you have book on Designing transformers. I will need help later. But my question is different. To make it easier let me clarify, so that one can understand better. Say you have to design a stepdown transformer. Specifications as under:

Input voltage- 230v, Output volt is 14v @ 650 mA. Core Area is 1.90 cm (tongue width) and 2.00 (stack height)

Now what will be the Primary Turns, Primary Current, Primary wire Gauge.

What will be the Secondary turns and Secondary wire gauge.

What is the Turns per volt.

How to choose wire gauge (enamelled copper wire)

My calculations says:

TPV: 12.73, PTurn-2929, STurn-178

Pri.wire gauge-38/39 swg; Sec.wire gauge-25/26 swg

Am I right or wrong?

Please suggest.

 

 

[HarryA]

If you are not big on building your own transformer look at:

http://www.surplussales.com/homenew.html#Transformers

example; 95VA Toroidal Transformer 117/234V to 2 x 15V or 30V at 3.2 Amps

at 230 v that  would give you say: 14.7 volts; a fat resistor would remove the 0.7