Transformer polarity review please - Page 1
Forum section: electronics / repair

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 22, 2022, 06:09:10 pm
content: 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 400And 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 18VAnd thus the 16V is normal direction related to 400VAnd 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 schematicMany thanksMaurits

author: bob91343
author_rank: Super Contributor
timestamp: January 22, 2022, 07:10:31 pm
content: Maurits I could follow your description better if you would post a diagram. Why is the phase important?

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 22, 2022, 07:15:56 pm
content: 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)

author: bob91343
author_rank: Super Contributor
timestamp: January 22, 2022, 11:57:00 pm
content: On that diagram I only see one transformer. Its outputs are rectified and filtered, so polarity makes no difference.

author: BradC
author_rank: Super Contributor
timestamp: January 23, 2022, 12:14:11 am
content: Quote from: bob91343 on January 22, 2022, 11:57:00 pmHave a closer look at the top of the 18-0-18 winding.

author: srb1954
author_rank: Super Contributor
timestamp: January 23, 2022, 12:31:24 am
content: Quote from: bob91343 on January 22, 2022, 11:57:00 pmAs far as the PSU functions go the polarity doesn't matter as long as the 18V-0V-18V is wound in the same direction i.e. one continuous winding.However the 18V winding is also used as timing for triggering the SCR for an external load. Since the SCR only conducts on one half of the cycle the trigger phasing might be important depending on what load is controlled by this SCR. You really need to consider the load connected to the SCR to determine if the phasing is important.If you get the phasing of the 18V winding wrong the easiest fix would be to just lift one end of resistor R1 and connect it to the opposite end of the transformer.

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 23, 2022, 04:48:31 am
content: Quote from: bob91343 on January 22, 2022, 11:57:00 pmAs @Bradc and @srb1954 mentioned, have look at this OpAmp and where it is being fed from, copied out for convenience.Quote from: srb1954 on January 23, 2022, 12:31:24 amAnd this is where I am unsure. It is powering the fixed field winding of a brushed DC motor. But why does it appear powered only during one half?It may be relevant, it may not. It may even be that if I get it wrong, it works well enough to mask that it is somehow working inefficiently and get hot or not have the umph it needs.Best to get it right. Though your resistor hint is also good, I was hithirto thinking of lifting the and bodge wiring the 400V pads, as they would not be visible but bodge wiring only the R1 may indeed be easier and just as invisible.My biggest worry is it is somehow important, I do get it wrong, but that is masked by working, but working badly.Hence my request for a 2nd opinion, not on whether the polarity is important, but on how to interpret the measurements with respect to polarity.

author: BrokenYugo
author_rank: Super Contributor
timestamp: January 23, 2022, 07:11:50 pm
content: Unless it liquefied the copper, why not just unwind the burnt one and find out?Stands to reason they wouldn't spec out an oddball transformer if a regular one will work the same with a single bodge wire.

author: cybermaus
author_rank: Frequent Contributor
timestamp: January 23, 2022, 08:16:00 pm
content: Because it is potted. And heat will not melt it but hardening/burn the resin.So I have little confidence in my ability to remove the potting in such a way I can still make sense of the connecting wires.Also, I did poke a needle into the coil, see if I could make contact with it and could analyse at least part of the coilAlas, no matter how many needles I poked, and even if those needles contacted each other, they never were connected to either of the input pins.Seems whatever short happened, it disconnected both input wires from most of the coil.And also, I suspect this is a small series purpose build transformer anyway. Such a tiny PCB tfansformer, but with 414V in is not usual, and 3 coils out is also not usual.If it is a purpose build, they can ask for any orientation they like.Anyway, the hesitance here to share hard conclusions based on my measurements make me feel a little better about my own doubt.If no-one steps forward with credible authority, well I have a) hope I measured well. b) 50/50 odds anyway and c) can fix it with a single bodge wire.

author: BrokenYugo
author_rank: Super Contributor
timestamp: January 24, 2022, 12:48:34 am
content: You might try soaking it in acetone, that may soften it enough to pick it away.

author: cybermaus
author_rank: Frequent Contributor
timestamp: February 10, 2022, 06:47:36 pm
content: Some closure on this (I know no-one is waiting for it, but I like to close topics, even if only for myself):With all my measuring and double thinking, in the end I got it wrong.The 18V-0-18V should not have been reversed in polarity, and I ended up rerouting R1 to the other pin as @srb1954 suggested.Some keywords for future searching: Bridgeport Interact KTK drive MTI-1 SCR board transformerSerial No 664201089C