Friday, September 6, 2019

Transformer voltage and impedance transformation

The turns ratio will determine the voltage transformation in a simple direct way. A 10:1 ratio will reduce 120 volts to 12 volts. How does the turns ratio effect the impedance transformation? If we load a 12 filament transformer to draw 1 Amp what is the primary current? What would the impedance transformation be? These are some of the questions that maybe answered in this post. I will try to use the spice program to help see the transformer action.

The first thing to understand is the primary has inductive reactance which is determined by the frequency. We must be concerned with the inductance as well as the turns ratio. Here I have a transformer with 1:1 turns ratio and 1mh inductance. The audio generator has 600 ohms series resistance. The primary reactance is so low the generator output sees a short circuit. I have 10mv output because that is all the transformer receives the rest is dropped in the generator.
I placed the black test lead at VL1 and measured the generator internal drop and the primary signal. Almost all is lost in the generator.

The same test after raising the signal to 10Mhz. The generator is still dropping 250mv. The primary receives the rest. Note that the phase shift prevents us simply adding the voltages.

We see the same voltage on the secondary as the primary. Note the load is 2k. With the 600ohm source feeding a 2kohm load we see voltage transformation based on the turns ratio. A 1:1 turns ratio gives a 1:1 voltage transformation.
The generator is back to 1khz and the coils are 2H. Now Vin = Vout.
Setting the inductance to a 10:1 ratio and dropping the load to 8 ohms is not pretty.
The generator is dropping the signal again?
I lowered the secondary to 100mh and the primary is not overloaded.
I added the current trace. You can see the out current is 10 times the input current and the voltage is reduced by a factor of 10. The inductance ratio is 100:1.

Anywho, If you followed all that The turns ratio is important but the primary reactance is the most important thing.
It would be a good time to open LTspice and do some sims;).

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