Looking for LTspice model for 5W Hammond 125B push-pull output transformer - Page 1
Forum section: electronics / projects

author: Greybeard
author_rank: Frequent Contributor
timestamp: August 04, 2025, 01:18:38 pm
content: Hi,I'm looking for an LTspice model for a 5W Hammond 125B push-pull output transformer with information on winding DC resistances, parasitic capacitances, etc.A Google search didn't help much:Some of the specifications found are incomplete or inconsistent: primary inductance (one or two half-windings? 8.97 H or 3.70 H), missing parasitic capacitances, missing DC resistors, missing coupling factor.Can anyone help me?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 05, 2025, 06:00:54 pm
content: Measure the transformer winding inductances and resistances and edit the centre tap model.Or just build a model from inductors.K will be very close to unity in a quality audio transformer, typically >0.98Add the parasitics as external discrete components.

author: jonpaul
author_rank: Super Contributor
timestamp: August 05, 2025, 08:12:46 pm
content: What is the objective of your simulation?The Hammond specs seem very complete with all inf9o needed for intended application.j

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2025, 08:13:33 pm
content: The leakage inductance is an important parasitic parameter that limits the high-frequency performance.You can measure it by attaching your LCR meter to the primary and shorting the secondary winding.Spice usually deals with it by converting it to the coupling co-efficientk< 1, or you can setk= 1 and add the leakage inductance externally.Of course, as in all iron-core coils, the inductances will vary with the flux density in the core; the time derivative of the total flux gives the voltage induced by the linked flux, and the flux density must be less than saturation for good performance.

author: Greybeard
author_rank: Frequent Contributor
timestamp: August 06, 2025, 11:33:35 am
content: Quote from: jonpaul on August 05, 2025, 08:12:46 pmI don't have the transformer yet.I want to simulate and redesing a guitar amp before I order the components.Quote from: jonpaul on August 05, 2025, 08:12:46 pmI wrote before:QuoteDoes the core need to be demagnetized before measuring the inductances?

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 06, 2025, 12:07:03 pm
content: Modelling parasitic capacitance in transformers is quite complex. Even with just two windings, there are at least three capacitances to solve for, and doing so requires quite a bit of measurement and math. Usually this is done in tandem with measuring the inductances (as it's impossible to measure one without being affected by the other). As the number of windings goes up, the number of capacitance terms goes up (with the number of windings squared I think). Many people have done their PhD's on this topic.It's extremely rare for vendors to provide detailed info on parasitic capacitance. At most, they might provide info on the overall interwinding capacitance, or the self-resonant frequency of one of the windings, but that's only a bit of the full picture.And that's generally fine, because the vast majority of applications don't care about the fine details of the parasitics.The datasheet(s) seems to provide the necessary info on how to choose a specific transformer and the tap points depending on your terminating impedances. Not sure about the discrepancy in primary inductance, could be that they measured at different frequencies.Quote from: Greybeard on August 06, 2025, 11:33:35 amProbably not, as the core should never be magnetized to begin with...