# LT Spice Transformer Circuit Questions

I'm a TA trying to convert a physical transformer circuit design that uses a transformer kit to an LT Spice simulation for some students who need practice but cannot attend my physical lab due to health concerns.

The physical lab uses a Function Generator set to 20 Vpp and 60 Hz with a 2 to 1 step down transformer. The idea is to use oscilloscope probes to measure voltages at points to determine their transformer kit's parameters. The students have to use Vb to determine the primary side current I1.

I tried implementing this in LT Spice following guides posted in the documentation for LT Spice and was getting the same voltage at point A as I was getting at point B, which I know shouldn't be the case due to the voltage drop across the DC resistance and windings of the primary. I'm getting 20 Volts for the measured voltages at points A and B. I'd really appreciate any advice on how to properly implement this in LT Spice.

• How does inductance vary with respect to the number of turns of wire?
– jonk
Sep 23, 2020 at 22:53
• Also, what is the output impedance of your function generator? I'm assuming you are saying that the resistance of the transformer as $100\:\mu\Omega$ (since that's what you wrote up there) and I'm pretty sure that's not the signal generator's output impedance. But maybe it is?
– jonk
Sep 23, 2020 at 23:00
• Since you haven't responded, I was asking my first question because you mentioned 2:1 as the voltage ratio for your transformer. But this means a difference by a factor of 4 in inductance, literally by the definition of the inductance constant and assuming a shared core from using your Spice card. So you've already started off on the wrong foot. The rest isn't important until you get that much right. They are just details. If you are going to ever get anything like a realistic simulation from your own efforts, you will need to know the basics.
– jonk
Sep 24, 2020 at 3:11
• Please let me know if you need more information. I'm trying to better learn this for my students so I can better help them. Any help you offer with this will really go a long way for my class. I'm a Masters student and trying to get better at my field through experience. Sep 24, 2020 at 3:49
• @JetPirate When you write the voltages, write them as Vrms, or Vp, or Vpp. You mentioned 20 Vpp, so you'll have to set up your LTspice source to 10 V (you're setting up the amplitude). You also mentioned "a few mV across R", was that with an open secondary, or loaded? These things need to be mentioned, the devil's in the details. Once you start ennumerating them in writing, it's very possible you'll be able to make your own mental connections. Sep 24, 2020 at 7:16