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Old 4th December 2010, 02:15 PM   #1
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Default LTspice U/L Transformer Symbol

Hello from a new forum member.

I'm working on a tube power amplifier and have been struggling to find output transformer SPICE models that are compatible with LTspice. I just came across a thread from 2005-2007 that seems to offer most of the information I am looking for. Member Robert McLean developed a spreadsheet to calculate the parameters for a number of transformers, including the Hammond 1650N that I am considering for my design. Here is a link to that forum conversation:

spice model for UL output transformer?

I have studied the spreadsheet and believe I understand it well enough to create a subcircuit model for the 1650N. However, I have not come across an LTspice symbol (.asy) file for the transformers defined in Mr. McLean's collection. Has anyone developed such a symbol file, or can someone offer some advice about how to create one myself?

Thanks in advance for any help.

Ray Waters
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Old 4th December 2010, 06:41 PM   #2
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I link inductors in series to form the UL tap, and assign a K directive for the whole mess. The tricky part is figuring out the right inductance for each component in the network. If you only have a single untapped primary coupled to a single untapped secondary, it's easy. The inductance ratio equals the impedance ratio. Once you start throwing in additional taps, you need to account for mutual inductance. I'm probably doing it wrong, but it seems to work better than nothing.

My example should be a 5.5K:8 with a UL tap at 40%.

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Old 4th December 2010, 07:44 PM   #3
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Thanks for the quick reply, Ty. This makes sense.

I suppose what I am trying to do here is to create a library of u/l transformer models that I can use with a single LTspice symbol file (similar to using the triode.asy symbol), including a library or subcircuit file with a SPICE directive, and naming the instance of the component to the appropriate device. Robert McClean's spreadsheet caught my eye because it contained the information I would need to build the desired subcircuit (or library) files. All that is missing is a symbol file consistent with those models.

Has anyone tried this?

Ray
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Old 4th December 2010, 10:02 PM   #4
taj is offline taj
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Here's a UL output transformer symbol I drew for LTSpice, but I'm not familiar with the program at all (other than to doodle symbols) so I have no idea how to connect it up to a model.

I don't imagine it's too difficult. Usually you just rename/renumber pins to suit those contained in the model.

Rename the file to have an .asy extender.

..Todd
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Old 4th December 2010, 10:26 PM   #5
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Below is a nifty transformer model library by Steve Bench. It's only single ended OPT's, but you can add to it if you want, maybe even adding Taj's UL symbol.

hope this helps

Drop down transformer models for LTSpice
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Old 5th December 2010, 12:41 AM   #6
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Quote:
Originally Posted by taj View Post
Here's a UL output transformer symbol I drew for LTSpice, but I'm not familiar with the program at all (other than to doodle symbols) so I have no idea how to connect it up to a model.

I don't imagine it's too difficult. Usually you just rename/renumber pins to suit those contained in the model.

Rename the file to have an .asy extender.

..Todd
Thanks, Todd. This will give me a place to start.

Ray
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Old 5th December 2010, 12:44 AM   #7
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Quote:
Originally Posted by pedroskova View Post
Below is a nifty transformer model library by Steve Bench. It's only single ended OPT's, but you can add to it if you want, maybe even adding Taj's UL symbol.

hope this helps

Drop down transformer models for LTSpice
Thanks. I'm hoping to be able to do something similar with a library/symbol combination for ultra linear transformers.

Ray
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