Electronics > Repair

Transformer polarity review please

(1/3) > >>

cybermaus:
Hi all.

Some help please.

For some equipment (CNC machine, now in hobby use) I have a small burned out 6VA PCB transformer.
Long story short, I cannot find it, but I found someone willing to custom made one, so I need the exact specs.
Which are all clear to me, except the polarity. (and yes, in this case polarity is important)

The vendor told me it is usual for him, and without instructions he will simply, wind all coils clockwise from low pin number to high pin number.

I need some help interpreting what I measured please.


My measurement setup is such:

- On a friends different still working machine, not allowed to disassemble, I measured hot.
- Scope behind an isolation transformer
- Scope in AC trigger mode. That means it will always trigger on the same point in the 50Hz phase
- First I put GND on pin 1 (low pin) and probe on pin 5 (high pin) for the 400V high side (picture 1 left)
- Next I put GND on pin 6 (low pin) and probe on pin 7 (high pin) for the 1st 18V (picture 1 right)
- In this machine I can not reach pins 8,9,10 to measure the other coils.

(Note: The Rigol *overwrote* my USB stick images on the second measure setup, but lucky I also made a (tiny) hand sketch)

Please confirm, but I think this shows the 18V is wound in the *opposite* direction from 400
And thus opposite to what vendor tells me is usual.



Next I also measured the 3 secondary coils on the broken & burned transformer (primary coil being busted)
Using the same 6-7 18V coil as comparative baseline

- Scope behind an isolation transformer
- Scope in AC trigger mode. However, different house, different location, different phase
- I put 12V AC on pin 6 and 7 (with a 220R current limit resistor)
- First I put GND on pin 6 (low pin) and probe on pin 7 (high pin) (picture 2) to reset baseline
- Next I put GND on pin 7 (now low pin) and probe on pin 8 (high pin) (picture 3)
- Next I put GND on pin 9 (low pin) and probe on pin 10(high pin) (picture 4)

Please confirm, but I think this shows the 16V is also in the opposite direction from the 18V
And thus the 16V is normal direction related to 400V
And the overall conclusion is that *only* both 18V coils are opposite, and they are indeed opposite to convention.


Please also tell me, following convention, where would I put the dots in the attached transformer pinout schematic


Many thanks
Maurits

bob91343:
Maurits I could follow your description better if you would post a diagram.  Why is the phase important?

cybermaus:
I originally had explained all of that, but the posting was getting to long and I feared tldr; reactions.
Thank you for asking, giving me the opportunity to tell anyway:

The phase looks to be used to determine the Phase Quadrant for SCR/Thyristor driving. Schematic attached.
(This is the best quality I have, sorry. Right-click open image in new tab, then Ctrl-shift-plus to enlarge)

bob91343:
On that diagram I only see one transformer.  Its outputs are rectified and filtered, so polarity makes no difference.

BradC:

--- Quote from: bob91343 on January 22, 2022, 11:57:00 pm ---On that diagram I only see one transformer.  Its outputs are rectified and filtered, so polarity makes no difference.

--- End quote ---
Have a closer look at the top of the 18-0-18 winding.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version