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simple questions regardin isolation transformer and variac combo.

Discussion in 'High Voltage' started by fezder, Jul 30, 2013.

  1. fezder

    fezder Well-Known Member

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    Hey, i decided to ask you pro's about his manner, as i couldnt verify is my knowledge right....but here goes:
    I have this isolation transformer, which i wrote in blog little bit, but there are however couple things i need to ask before i start designing the case and so on. The issues are its safety lead, which is supposedly in center between primary and secondary, to prevent harm to user IF transformer gets impaled or otherwise damaged so mains is no longer isolated in addition to noise reduction, is this correct? And now there's this thing, i know that in order for safety-earth to work as intended, it needs to be connected in ''input'' mains, which is at the wall. But, what about the other side, secondary, which is used in test equipment? Is it needed in secondary side as there is isolation between earth and secondary live?. Also this transformer which i have has 2xmains in secondary, according to teacher they can be paralled as they are in same phase.

    And as i mentioned in title, i have also question/s regarding variac. I have already pre-assembled two-variacs in parallel to increase current handling. What becomes of fuse/thermal overload-fuse, they should be connected in winding, right? but what about noise filter, as it seems to need that too, which i also have, does that go to right at the input of variac, or even before I-transformer?...Of course i'll add fuses in-line before I-transformer.
    voltage and amperage meters go of course the output of the variac in series and parallel, as normal. I may sound that i have no clue whatsoever i'm doing, but hopefully you dont' ''throw me out of the class'' or something. Main purpose is to make this thing so it can be used ''safely'' as test gear.....as mains is never safe.

    thanks for any assistance people :). I do have schematic if you want to check it out, but hopefully you understand what i'm after with this ''bomb''....
     
  2. JimB

    JimB Super Moderator Most Helpful Member

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    Yes.


    Correct

    The secondary of the ISOLATION transformer must not be connected to earth. Otherwise it is no longer an isolation transformer.
    However, it is good practice to make the earth connection available for use if required by the equipment which is connected to the secondary side of the isolation transformer.

    If the two secondaries are isolated from each other, then yes you can connect them in parallel.
    BUT, you must make sure that the phasing is correct, otherwise it will go BANG when you connect the supply.

    This sounds like a really bad idea!
    Are the variacs on a common shaft so that they turn together?
    Are the variacs identical and track perfectly so that the output voltage of each variac is the same at all positions of the wiper?

    I would connect the fuse or circuit breaker on the input side of the variac/transformer unit where it connects to the mains supply.


    Why do you need a noise filter?
    What are you powering with this unit?
    Consider why you need a filter, is it to stop mains borne noise from entering the equipment?
    Or is it to stop noise coming out of the equipment back into the mains?
    Probably best to put it at the output of the variac/transformer unit, but having said that, a noise filter will not enhance the operation of this unit in any way.
    If you need a filter, put it where it is needed... at the equipment under test.


    And this "bomb" will also give you a nasty nip if you are not carefull!

    JimB
     
  3. fezder

    fezder Well-Known Member

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    Yes, they are indeed isolated from each other, completely their own winding, no connection on anywhere. Now this is new, how come phasing can be incorrect if they are fed from single phase mains? Or, is it because the polarity switches between positive/negative, like mains: positive period secondary:negative, so i need to watch out NOT to connect negative/positive periods together in secondary? Or am i totally wrong in this?....and how to check phase? with ac non-contact pen? just asking to make sure :)

    Yeah, they have common shaft, and they have identical specs and wiring. Output voltage i haven't tested yet, didn't thought it would be issue as they are identical.

    and as of that noise filter, i said it wrong, sorry i meant like these what are found in CRT monitors, in fact this was connected to variac, see attachment. some sort of over-voltage protectors/suppressors?

    And thanks for replies!
     

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  4. dave

    Dave New Member

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  5. JimB

    JimB Super Moderator Most Helpful Member

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    The secondary windings on a transformer have a start and an end.
    If you connect the two "start" connections together and measure the voltage between the two "ends", you will see 0v.
    The windings are connected in phase and you can connect them in parallel - no problems.

    However, if you connect a "start" and an "end" together, the voltages of the two windings will add together.
    If you measure the voltage between the unconnected "start" and "end" you will have twice the voltage of a single winding.
    If you connect them together, there will be a flash of sparks, a loud hum, and the transformer will get very hot very quickly.


    Sorry, I have no idea what that thing is in your picture.

    JimB
     
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  6. fezder

    fezder Well-Known Member

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    Hmm, tvtech might know what that component is by official name :D.

    if i got it right, you mean that windings can be connected in SERIES by an accident? the voltages add up just as you said like in series
     
  7. JimB

    JimB Super Moderator Most Helpful Member

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    Just to clarify what I mean, please see the attachment.

    JimB
     

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  8. fezder

    fezder Well-Known Member

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    ah, we vere talking about the same thing, goodie! :). i drawed exactly the same scematic in fact. But, how come low-power transformers CAN be connected like that? at least we have couple of transformers wire up like that, theyr windings are however lov-voltage, 5, 7 and 12v from single transformer. They were used all windings wired in series and no smoke.....but, 230v is bit different than 12v :S...

    and, how come it can be determined which end is end and which start? or does is matter as long other wire is determined to be start/end and connect accordingly?
     
  9. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Hi Fez

    Looks like two PTC's on a board. Am I correct?
    Now what are you actually trying to do :confused: Explain all and I can maybe help.

    Bear in mind, PTC's are commonly used on CRT TV's to control the Degauss Coil/s. A Degauss Coil does not have the same characteristics as a Normal Mains transformer. Because it is not one. I am stopping here until I hear more specifics from you. Think clearly now.

    Regards,
    tvtech
     
    Last edited: Aug 3, 2013
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  10. fezder

    fezder Well-Known Member

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    hey tech, i suppose its allright to call you tech? but, seems you were right! i googled ptc and bingo, excactly same components along with those more common, ceramic case came up. Thanks! :)
    And you asked what i'm trying to do, perhaps i should have posted picture earlier just to clear things out! :D here goes, pretty much same thing as in picture, just isolated variac with analog A/V meters along with fuses for isolation transformer and variac slides :)

    http://www.iseincstore.com/ProductImages/variac/3pndvam1.jpg
     
  11. tvtech

    tvtech Well-Known Member Most Helpful Member

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    And now...where does the PTC fit into this puzzle ????

    Don't blow yourself up. This is funny......:rolleyes:

    Stay safe :D

    tvtech
     
    Last edited: Aug 3, 2013
  12. fezder

    fezder Well-Known Member

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    well, seems there is no place for PTC in this puzzle, i thought that they were spark gaps lol.....well, teacher said that :D and funny, i saw marking ptc right where i pulled these off from crt pcb....

    in fact, he said that these are used to reduce inrush current, which PTC does too :D
     
    Last edited: Aug 3, 2013
  13. tvtech

    tvtech Well-Known Member Most Helpful Member

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    You know Fezder...............this is what worries me A LOT.

    Teachers apparently "teaching" and they don't know what the "****" they are talking about. Here we have Members giving their time without any Fees involved and the advise given is normally spot on.

    You know what, I am not surprised anymore that so many people here..read "Indians" beg for help. Because their own teachers are clueless/useless.
    As a Free Forum, we are probably treasured by all the poor buggers that come to us for help.

    Not one has ever been refused. Sure, Members get tired of repeating stuff over and over again. But the advise given here is stuff given from the heart and hopefully 100% accurate.

    In a crazy kind of way, it has to be.......I know if I dream up and give crap advise there will be people on my back ( and rightfully so) here. It's like we all take pride in what Technical advise we give. I know I do.

    And this is a Free Forum. The whole World has access to some of the very best Technical advise available on this Planet....for free.

    I am so proud and humbled too at the same time to be part of this awesome Community.

    ETO thank you

    tvtech
     
  14. fezder

    fezder Well-Known Member

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    Yeah, i agree with that. Once there was situation at school when one student said to techer right in the class ''you don't have a clue what you are saying, right?'' and theacher went pretty pale-faced :D.

    There are couple of teachers who i like the most, but other has pretty annoying way of teaching: he just tells the theory, but don't know how to make it so student will understand it too! and sometimes topic is even other than what was asked. Like once i asked ''how do i determine voltage, or something like that from scope?'' he replied about telling capacitors maximum instant current voltage when in AC polarity changes....and no surprise, not helping a bit, in that day that is.

    Now then, other teacher, he really knows what he knows, and what doesn't know. He says it clearly if he doesn't know. And he knows how to teach so others undersntand! Like in parallel resistors, transistors, fets, 'scope etc....he taught me at least 80% what i learnt in school. Other very valuable source of information is indeed ETO here, i'm so lucky and thankful for this amount of aknowledge you guys have.

    last time i was at job, yes my contract was cancelled due the fact materials ran out :(. boss said that he would have taken me full-time if things were different, i got no complaints in that week i was working it there. i learnt there in week more than half year in school, at least. We don't install those bigger fuses, don't know their names, but what have handle on them. and funny thing was, even thought i haven't done those things before, i was constantly running out of things to do :D. My supervisor said ''ok, now you can take a little break'' :D.

    little bit off topic perhaps. But, as for topic, i found perfect case for this project: microwave ovens inner case. I just install plywood/stiff metal to cover it more. and of course there is work for panel meter installments and so on. But those things simply cost too much locally, and with this material on-hand, there's no harm in little DIY no? :)
     
  15. tvtech

    tvtech Well-Known Member Most Helpful Member

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    I will always be at you side Fez. Time spent on you is time well spent......because you have the ability to listen and learn and appreciate all help here.

    Appreciation is probably the biggest thing though. Many of us here are regarded as Grumpy old Bastards.....but we know our stuff. And we guide PROPERLY. We never guess.

    And you Fez have the mindset to be an absolute Champion one day....because your grounding started here at ETO. Never forget it Buddy.

    tvtech has a gut feeling you will be a achiever of note. Study hard, stay a nice person and remember your Roots. And never forget ETO.

    Hows that for a Grumpy old tvtech :D

    Regards,
    uhm..tvtech
     
  16. fezder

    fezder Well-Known Member

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    experience has told to me that you old cranky ''SOABs'' (just kidding here..'' have the best knowledge. And i appreciate, a lot that you help with these matters....
    there came little set-back on this project, i may need to re-thing construction. I inspected that duo of variacs, two were paralled. I saw that coal-contact on wiper didn't make contact, and somehow ''floated''. After further inspection, i noticed that wipers ''arm'', made of plastic/bakellite, was broken. Now i'd need to tinker a lot to make safe parallel, but i think that maybe it's not worth risking the variac, and it is enought for now to have 4A variable. But i still intend to have transformers winding in parallel, less stress for transformer, at least i think so. And perhaps it's wise to add relay for power-on purposes.

    I need to buy little bit of hardware before it is done thought, can you tell me what you of these? just an opinions/suggestions :) sorry but one link is in finnish, hopefully translator could help you....

    http://www.vekoy.com/product_info.php?cPath=66_498_1640&products_id=21595
    ^fuse for variac wiper, i might add those for transformer secondary too. primary could also use fuse, in case of primary fault. 5A should be enough, protection but still doesn't affect secondary's load. I mean that it doesnt blow even with full load on variac's side.

    http://www.vekoy.com/product_info.php?cPath=78_656_845&products_id=9863
    http://www.vekoy.com/product_info.php?cPath=59_732_734&products_id=8825
    ^i'm thinking to make mains cord fixed,or with jack, not sure yet.

    http://www.vekoy.com/images/PM2-15VDC.gif
    ^panel meters are analog, pretty much MUST to me, i simply dont like that much LED/LCD meters...of course, they give better resolution, but if that's needed, there is DMM :)
     
  17. JimB

    JimB Super Moderator Most Helpful Member

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    Be carefull!
    Look at the pictures of those connectors, I think you want to use one that looks like P595, not P596 or P598.
    The Pins of P596 are turned by 90 degrees to make a non-standard connector.

    JimB
     
  18. fezder

    fezder Well-Known Member

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    woa, good point....honestly, i din't saw that ''little'' detail...thankfully i dont have those non-standard, just checked :D. thanks again, that was completely new to me.

    and that picture was for reference purposes, but still thanks :)
     
  19. fezder

    fezder Well-Known Member

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    well, based on info i got here from you guys, i made my shoppings. Fuses and switch i ordered from locally, but due to high price of panel meters locally, i bought them from ebay. Case itself is pretty much ready, i covered microwaves inner case with plywood. Case must now be earthed, as the screws that hold plywoods in place have electrical connection to case, not surprise :D. i'll add little story to blog, when i can.
     
  20. fezder

    fezder Well-Known Member

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    ok guys, this project is now done and ready for use, only little furbishing left, but nothing that effects usability :) i'll add photos later.....and btw, any info about when blog feature in forum comes active again? or will it ever?...
     
  21. fezder

    fezder Well-Known Member

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    Hey again, here it is, done and ready for use :). Only small decoration things, like warning, fuse, and switch/meter labels are missing, which i'll add later. Also, other decoration thing, which will follow, is that i'll add some metal coversto corners of case. The specs are just as said, variac: 4A/250v about, and only isolation transformer: 10A/230v. mains braker is also 10A. And as you can see, i added safety-banana sockets above the wall socket, if alligator clips or similar are neede. And every metal is earthed. I can tell you this thing is HEAVY!. much heavier than i would have imagined!.
    But, please tell your opinions :).

    Ps. i would have added this to blog, but for now it seems blog is back online, only all entries are gone. hopefully
    situation will come allright :)

    . Isolation_variac (1).jpg Isolation_variac (2).jpg Isolation_variac (3).jpg
     

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