Author Topic: An unexpected autotransformer  (Read 434 times)

Offline klugesmith

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An unexpected autotransformer
« on: September 28, 2024, 03:58:22 AM »
For a lab project I got several RF impedance-scaling transformers by Mini-Circuits.

First one is TC9-1-75+ 
It's got a ferrite binocular core like the one in my post about "4 mm cube microwave transformer". There are similar cores in consumer tv-antenna accessories like 300:75 ohm baluns (familiar to we boomers).
The datasheet https://www.minicircuits.com/pdfs/TC9-1-75+.pdf
says it's step-down, 9:1 impedance ratio (3:1 voltage ratio), 0.3 to 475 MHz, suggested application "matching laser diode".

I overlooked the word "autotransformer" until I'd studied a specimen under a microscope, before soldering it in.
Here shown as received in cut-tape form, with the cover tape peeled back.   Note unconventional direction of pin numbering; most devices (starting with vacuum tubes) have pins numbered clockwise when viewed from bottom.

Overlaid on photo is figure from datasheet (Config. D), with implied 3:1 turns ratio,
and a stock autotransformer schematic that matches the actual construction.
From pin 3 to pin 1 are 2 turns of both red and green magnet wire (bifilar).
Continuing from pin 1, in same clockwise direction, are 4 turns of yellow magnet wire; the end wraps outside left edge of core to pin 6 terminal.

Are any readers here familiar with materials and datasheets for cores like this?  Don't know if I'll have time to measure the ampere-turns or volt-seconds for core saturation.

The part served my purpose, which was adding MHz-range tones onto a 1-volt ASIC power net with more than 100 uF of MLCC's. Another transformer served better.
« Last Edit: September 28, 2024, 04:14:42 AM by klugesmith »

Offline haversin

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Re: An unexpected autotransformer
« Reply #1 on: October 01, 2024, 06:04:57 PM »
 The phase dots on the left figure don't match the autotransformer on the right figure.  A positive voltage applied to pin 6 would give a positive voltage on pin 1 according to the left figure. But on the right figure a positive voltage applied to pin 6 would give negative voltage on pin 1.

Offline klugesmith

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Re: An unexpected autotransformer
« Reply #2 on: October 02, 2024, 06:27:22 PM »
Why would positive on 6 give a negative on 1, assuming 3 is common? 
If you're confident, or not sure, I could check with an oscilloscope.
I've looked at that transformer only with a spectrum analyzer and a TDR scope (which verified the size & direction of impedance transformation).

Offline haversin

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Re: An unexpected autotransformer
« Reply #3 on: October 02, 2024, 10:49:03 PM »
Oops! your right I thought 3 was common on the right figure.

Offline klugesmith

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Re: An unexpected autotransformer
« Reply #4 on: October 03, 2024, 12:15:29 AM »
In the photo and both diagrams, pin 3 is common to primary non-dot and secondary non-dot.
I was initially surprised to see pin 3 as one end of the whole winding.
Pin 1 is the "off center tap", one end of 2-turn winding (red&green) and one end of 4-turn winding (yellow).   It's electrically identified as secondary dot.  Can we agree that dotting is implicit in conventional schematic symbol for autotransformer?

Offline haversin

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Re: An unexpected autotransformer
« Reply #5 on: October 03, 2024, 12:45:51 AM »
what I meant to say was I thought that the middle pin 1 on the right figure was common so that pin 3 would be go negative if pin 6 goes positive.  Yes I think the winding is assumed to be in the in same direction through out an autotransformer so the dots for it are implicit.

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Re: An unexpected autotransformer
« Reply #5 on: October 03, 2024, 12:45:51 AM »

 


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