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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    \$\begingroup\$ These (1), (2) might be of some help. \$\endgroup\$
    – K. Rmth
    Commented Dec 18, 2015 at 9:40
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    \$\begingroup\$ 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. \$\endgroup\$
    – Andy aka
    Commented Dec 18, 2015 at 10:09
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Dec 18, 2015 at 12:14
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    \$\begingroup\$ How did you calculate inductance then? \$\endgroup\$
    – Andy aka
    Commented Dec 18, 2015 at 13:16
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    \$\begingroup\$ 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. \$\endgroup\$
    – Andy aka
    Commented Dec 18, 2015 at 19:17

1 Answer 1


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).


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