# Crude sim transformers in LTspice

#### ebeowulf17

Joined Aug 12, 2014
3,274
If I want a very simple, rough approximation of how a transformer will work in LTspice simulation, it seems like a few inductance values and resistance values would be a good starting point, but if I don't physically have a transformer and the tools to measure it, I don't know where to start in estimating those values.

I know how to add the inductors, couple them, and set their values, but I have no idea what the values should be. Right now I want to simulate a circuit running off of this transformer: http://catalog.triadmagnetics.com/Asset/FS12-090-C2.pdf

It takes 115VAC and outputs 6.3VAC (depending on choice of serial/parallel connections.) This is a turns ratio of roughly 18.25:1. I know you square this to get an inductance ratio of around 333:1, but where should I set those values? If I make them to large I get a transformer capable of seemingly infinite power transfer, with inductive spikes to match! Too small and I can't get any power out of it. I haven't the faintest idea what a reasonable value would be.

1H:333H?
1kH:333kH?
1mH:333mH?

As you can see, I'm not looking for precise accuracy, more like getting within the right order of magnitude - very ballpark. When you make such a guess, what's it based on? VA rating of the transformer? Physical size/weight of the transformer? I'd love to make reasonable estimates like this in the future without coming here begging for help, so I'm more interested in your thought process (or links to external resources I've failed to find in my own searches,) than in just having the answer for this one specific transformer.

Thanks!

#### ebeowulf17

Joined Aug 12, 2014
3,274
hi ebeo,
Look here for a step by step guide.
http://electronicsbeliever.com/transformer-model-in-ltspice-step-step-guide/

I could post a LTS circuit I use for 230Vrms mains if it would help.
E
I've read through several such guides, and I'm able to get that all working fine - the problem is where to start with inductance values. In that guide, it just jumps to:
For a 400uhenry primary inductance, the secondary inductance would be...
There's no discussion of how one might guess what the primary inductance was in the first place. I don't have the transformer yet, and the datasheet doesn't list an inductance value.

I'm sure many users here have some intuition from experience as to whether a transformer with a 1.1VA rating is likely to have inductance in microhenries, or miili, or kilo, or mega... but I haven't a clue.

If you don't know the actual inductance value of either side, and just want to make a ballpark guess, where do you start?

#### ericgibbs

Joined Jan 29, 2010
9,795
hi,
I use a rough value of 1H or 10H for a primary voltage of 230Vrms 50Hz.
You must have an idea what size VA rated transformer you need for your actual project.?
E

#### ebeowulf17

Joined Aug 12, 2014
3,274
hi,
I use a rough value of 1H or 10H for a primary voltage of 230Vrms 50Hz.
You must have an idea what size VA rated transformer you need for your actual project.?
E
Yes, I think the 1.1VA device listed above is fine for this project - just didn't know how much inductance that would correspond to. I'm simming it based on the 115V mains input option.

#### ericgibbs

Joined Jan 29, 2010
9,795
Hi,
I did have a detailed document for LTspice transformers, still looking.
If I was using 115V 60Hz mains I would go for the 0.4H primary.
In the past I have tried a range of primary inductances for mains transformers and providing it was in the range 1H thru 10H it made little or no difference to the sim.

E

#### noweare

Joined Jun 30, 2017
112
You can use the permeablility of the material to calculate inductance based on the geometry of the core. They should give you that info.
L = u0*Um*A*N^2/path length You''ll have to look this formula up. N is turns, u0 is permeability of free space, Um is material permability reference to free space, A cross section area of core,
path length is magnetic path length (the path the flux takes around the core)

Also you probably want to makes sure you are not saturating the core material that comes down to a volts/turn value.
There is an equation for that also.