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The calculations in this question are based on my previous quesion: How many turns do I need for the primary coil of a transformer?

Marko Buršič said : "B_max shall be between 1.3T and 1.6T, but not more than 1.6T."

I calculate the minimum number of turns so that the core is not saturated:

B=1.6 and f=50Hz

Area = 0.009 * 0.015 = 1.35 * 10^-4 sq m

N = 220 / (4.44 * 50 * 1.6 * 1.35 * 10^-4)

N = 4587 turns.

I think the number of turns is too much, Am I missing something ?

According to Andy aka equations:

L = N^2 * AL = 210 H

XL = 2 * PI * f * L = 66 K ohm

I = 3.33 mA. (primary coil)

If I need 12 volts on the secondary coil:

I = 60 mA (This is too little for the secondary coil, I'm missing something here too)

Here's my core dimensions (cm): enter image description here

Thank you for all your assistance, Andy aka and Marko Buršič, I really appreciate your help. And special thanks to Andy aka who answered a lot of my question and taught me much useful information :)

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Michael George
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    These (1), (2) might be of some help. – K. Rmth Dec 18 '15 at 09:40
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    I don't follow how you calculated cross sectional area - what is the 0.015 number? Also a link to the core would probably clear this up. – Andy aka Dec 18 '15 at 10:09
  • @Andyaka A website says: "The 'effective area' of a core represents the cross sectional area of one of its limbs". A = 0.9 * 1.5 but this Area would be in square cm. So, I converted each length to m. The new lengths are: (0.009 and 0.015) so that the area would be in square meter (SI units). I don't have a link to the core because someone give it to me, I did not buy it myself. – Michael George Dec 18 '15 at 12:14
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    How did you calculate inductance then? – Andy aka Dec 18 '15 at 13:16
  • @Andyaka Before I try different values of inductance. I wanted to know the minimum number of turns. so, that B does NOT exceed 1.6 as Marko Buršič said.

    To calculate the min number of turns, I used the equation: B = V / (4.44 * f * N * A). The minimum number required to keep B less than 1.6 is 4587 turns!! I don't know but I think this is too many turns.

     – Michael George Dec 18 '15 at 18:35
  • @Andyaka I calculated the inductance of that number of turns. using the equation L = N^2 * AL. I take AL = 10 μH/turn2 as you told me. The minimum inductance is 210 H to keep B less than 1.6 – Michael George Dec 18 '15 at 18:40
  • @Andyaka The primary coil inductance is 210 H as I calculated before, and it has an impedance at 50 Hz of 66 K-ohm. My mains are 220 volts. So, the current in the primary coil is 3.33 mA.

    If I need the voltage of my secondary coil to be 12V. If I used the equation: "input power = output power" The Max. current of the secondary coil will be 60 mA only.

     – Michael George Dec 18 '15 at 18:49
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    It sounds like too many turns but the core is really rather small for 220V. Also, with an "unknown" core how can you be sure what the material is and this means you cannot go ahead and trust any value of AL. – Andy aka Dec 18 '15 at 19:17

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I don't know much about transformer calculation, but I would think that 12mA will be your primary current when there is no load on the secondary, and that seems fine. Connecting the load to the secondary winding will cause the current flow and effectively reduce inductance seen from primary side. (I think).

Dario Dentes
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