# How should we calculate L (inductance) value for a transformer

I am trying to design a transformer using LTSpice with the following specifications:

1. Vin = 230 V @ 50 Hz
2. Vout = 12 V @ 50 Hz
3. Power = 10 W

The thing I do not understand is how to calculate the inductance (L) value for these specifications. I know there are some formulas (N1 / N2 = V1 / V2), but how do we determine L1? How can we calculate it?

• For what purpose? Are you modeling a particular transformer? Show the datasheet. Do you know what magnetizing current or leakage it should have? Do you need a generic mostly-ideal transformer to support a circuit, nothing in particular? Commented Jul 16 at 18:58
• Thank you for the answer. I do not have a particular transformer to simulate so that ı do not have a datasheet. I just want to learn how can we calculate these values and to be honestly ı do not know what should be the magnetizing current or leakage inductance. But ı want to calculate a basic model transformer maybe with a leakage and magnetizing current Commented Jul 16 at 19:11

A starting point would be 20 % magnetizing current of your full rated load, in this case 2 VA. At 230 V, that’s 8.6 mA. Z = 2*Pi*L*f. Solve for L.

If the transformer would have been 2 kW, about 5 % would be reasonable to assume for general purpose.

• Thank you for this detailed information. Can you help me to solve these equations with step by step Winny. I could not understand some points. For example Z Commented Jul 16 at 21:00
• Z = U(rms)/I(rms). In this case 230/8.6m. Commented Jul 16 at 21:05
• Why we have not found L by the help of 10/230 = 43.7mA. Did you mean Lm with L? Commented Jul 16 at 21:11
• We did indirectly as Lm is related to the full load current. Yes, Lm. Commented Jul 17 at 6:13

The inductance is essentially a characteristic of the transformer core, and rises with the square of the number of turns. If you study the datasheet for a particular core you should find this value known as AL (expressed as Henries/turn^2) and you should also find the maximum magnetisation, expressed in Tesla. The latter can be used to determine the maximum throughput of the transformer.

• Can you help me to do a basic calculation? the values that you mentioned Commented Jul 16 at 20:21
• @TechnoLover you’ll need to start by specifying a core.
– Frog
Commented Jul 17 at 19:54
• Looking at this RM10: farnell.com/datasheets/3920295.pdf the untapped version using N95 compound has an AL value of 5500nH per turn^2, so for example 100 turns would give you 55mH.
– Frog
Commented Jul 17 at 20:13