Transformer parameters

I am planning to simulate a 120 V AC voltage source that is reduced to a 6.3 V RMS by means of a step down transformer. This voltage will be converted to DC using a Diode. I will use a Capacitor as a filter to smooth out the variations after the diode. I need some help to edit the Transformer parameters. I do not know what to put on the blanks corresponding to N, L_PRI (H), R_PRI (Ohms), R_SEC (Ohms). I thought I have to use the parameters from the equation: V2 = (N2/N1) V1 where V2 would be my 6.3 Volts, V1 my 120 Volts, and N1, N2 the turns in the primary and secondary respectively. Please advise.

by alfredoclemente21
January 07, 2016


by signality
January 07, 2016

Questions. 1) I want to reduce my voltage from 169 V AC (Peak) to 8.9 V AC (Peak). That is a ratio of about 19:1. Does that mean a) The value of my Step Down Ratio is 19:1 ? b) The value of the Turns Ratio Transformer is (19/19):(1/19) or about 1:0.05 ? and c) N= 19

2) How do I calculate the value that is going to be use as the Primary Inductance in Henry units?

3) After the values of the Turn Ratio Transformer and the Primary Inductance are put into the simulation, is the value of the Secondary Inductance calculated automatically?

4) The frequency of my voltage source is 60 Hz, how do I calculate the value in Ohms for the Reactance of the Primary Inductance? What is the reason for using the IEC symbol instead of the one that corresponds to a regular resistor?

5) On the CL Transformer Symbol 02 simulation, how the values of 0.01 Ohms for the parameters R_SEC and R_PRI were obtained?

6) I am planning to close the transformer secondary loop with a Diode, a Capacitor and a 100 Ohms resistance. What will be the best location for the transformer nodes? Do I need additional nodes for the other components?

by alfredoclemente21
January 10, 2016

The curly style is the US symbol for an inductor.

The solid rectangle is the IEC symbol for an inductor.

Neither represent a resistor symbol.

I was making reference to the component rectangular in shape that appears between the Voltage Function Generator and the Primary Transformer on all three CL simulations. The units for this component read clearly OHMS and is labeled R1. If I open the "Edit Parameters" page for this rectangular component I will get several options to choose under DISP and they are: US, IEC, Variable US, and Variable IEC. all of them showing the symbol and the label of OHMS for their units.

By the way, how was the value of 1 Ohm calculated for this component on the CL simulations ?

by alfredoclemente21
January 11, 2016

R1 is nothing to do with the transformer parameters.

It is there to illustrate that because the reactance of the primary forms a potential divider with R1, as the frequency drops the voltage across the primary drops.

It also illustrates the effect of the primary resistance because that limits the maximum attenuation between it and R1at very low frequencies.

Without R1 you don't see V(pri) drop as the frequency reduces (although you would see the primary current drop).

That's what the reference to the transformer being not ideal is about: it does not have an infinite primary inductance and it does not have zero primary (and secondary) resistance.

The value of 1 Ohm was pretty arbitrary.

If you are interested in reading more about magnetics:

Fundamentals of Magnetics book:

up to


and google for Magnetics Design Handbook.

And if you think that transformers are tricky to understand, just wait till you find out what really goes on inside a capacitor...


by signality
January 12, 2016

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