You will create a step-down transformer circuit with a primary inductance winding inductance of 3.1 H and a winding resistance of 0.2 Ω. The secondary winding has an inductance of 31 mH and a winding resistance of 0.2 Ω. The transformer provides a step-down ratio of 10 and is connected to a 100 Ω load resistance.

1. Create a new project called Linear Transformer and draw the circuit in Fig. 9.6. The inductors, resistors and K_Linear are all from the analog library. V1 is a VSIN source from the source library. Set the coupling coefficient to 0.65.

2. If you have a previous version of OrCAD which does not have the dot convention shown on the inductors, then display pin 1 of the inductors and orientate the inductors accordingly.

3. Double click on the K_Linear device to open the Property Editor and add the inductor reference designators as shown in Fig. 9.7. Highlight L1 and L2, then rmb > Display and select Value Only (Fig. 9.8).

Note

A common misunderstanding is that the coil reference designators must be entered as L1 to L6 in the Property Editor. If you have coils with reference designators L3, L4 and L5 in a circuit, then you enter the designators as shown in Fig. 9.9. It is recommended that you show the reference designators on the schematic for a K core device, especially if you annotate and change the coil numbering.

4. Set up a transient simulation profile with a Run to time of 50 ms (Fig. 9.10).

5. Place voltage markers on the primary and secondary nets and run the simulation. You should see the response as shown in Fig. 9.11 with a secondary voltage of 6.4 V.
If you have not placed markers on the circuit then when the PSpice window appears, select Trace > Add Trace and select from the left-hand side under Simulation Output Variables, V(primary) and V(secondary).

6. The relationship between the primary and secondary voltages and the primary and secondary inductances for air cored transformers is given by:

$\frac{{\mathrm{V}}_{\mathrm{s}}}{{\mathrm{V}}_{\mathrm{p}}}=\frac{\sqrt{{\mathrm{L}}_{\mathrm{s}}}}{{\mathrm{L}}_{\mathrm{p}}}$

So, if you increased k to 1 you would see a secondary voltage of approximately 10 V. In practice, air cored transformers have a low coupling coefficient.

Exercise 2

You will create a nonlinear, center-tapped transformer circuit.

1. Create a new project called Non Linear Transformer and draw the circuit in Fig. 9.12. Enter the inductor values as the number of turns as shown. The E13_6_6_3C81 core model is found in the magnetic library. Enter a value of 0.99 for the coupling coefficient.

2. As in Exercise 1, double click on the K core device and in the Property Editor, add and display the reference designators L1, L2 and L3.

3. Create a transient simulation profile with a run to time of 50 ms.

4. Place voltage markers on the nets, primary, secondary1 and secondary2 and run the simulation.

5. Fig. 9.13 shows the voltage waveforms for the nonlinear transformer. You should see 25.1 V on each secondary winding.

6. Investigate the other transformers in the analog and breakout libraries.