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Variable AC Isolated Transformer (showing wrong voltage)

Discussion in 'Troubleshooting and Repair' started by AcousticBruce, Apr 2, 2013.

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  1. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    I got this Variac in a package deal. I hooked my DMM up to the device and it reads quite a different voltage. For instance: In order get get 120V AC on the Variac I have to dial it to 140V on the LED display. Notice the two displays one being 140V on the Variac and 120V on the Fluke DMM

    https://www.dropbox.com/s/zu90uvfly9yvyvl/20130401_234442.jpg


    Why would something like this happen?
    Is there any way to fix it?

    Edit: I just added a straight link of the picture. Some found the previous one not to work.

    Edit: This is a Global Specialties 1504 ->> PRODUCT PAGE

    edit: There are better pictures later in the thread
     
    Last edited by a moderator: Apr 11, 2013
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,771
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    Nov 17, 2011
    Your picture is not showing.

    2 ideas:
    1) display on the variac not calbrated correctly
    2) display on the variac only correct under certain load


    Unlikey: fluke not calibrated correctly?
     
  3. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    1) This could be it and I would bet on this one. If you have any info on calibrating that would be awesome. I will do a search and try to find a manual for this.

    2) This is interesting, I made a computer power supply into a bench-top power supply which acted funny under certain loads. When a piece of equipment acts different under certain loads, is that a result of cheap equipment?

    The Fluke is calibrated and working great. So this is extremely unlikely.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,771
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    Nov 17, 2011
    1) No chance without manual or at least a schematic.

    2) Some power supplies, specially the switched mode type, require a minimum load for correct operation. That's not necessarily cheap (although a good quality ps can handle this with additinal internal circuitry). For a computer ps, for example, one can assume a certain inimum load in good faith and therefore forego the circuitry to make it stable under no load conditions.
     
  5. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    Contacted the company today by phone and I should be receiving a manual soon. We will see. I will let you all know what I find. Thanks again
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Third idea:

    Mains voltage is low.
     
  7. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    Very valid to think mains was low. I tested it with fluke as well as my bk precision bench top. Both read 120v ac from mains.

    Thanks for the idea.
     
  8. shrtrnd

    shrtrnd

    3,781
    499
    Jan 15, 2010
    I was thinking like Harald Kapp, the variac may be wired to read in a loaded condition.
    My second choice would be that the LED readout needs to be adjusted.
    Good to get the manual, and see what it says.
     
  9. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    What is that called? Unregulated?

    If that is called unregulated then is there a good reason to why you would use something that is unregulated vs regulated?
     
  10. shrtrnd

    shrtrnd

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    Jan 15, 2010
    I can't access the picture you sent of your variac.
    MOST variac's are 'unregulated', although some of the more expensive ones are.
    Yeah, basically a regulated variac has it's own circuitry internally to maintain a set
    voltage whatever the load applied.
    Most people USE a variac to get in the ballpark of a desired voltage, using a voltmeter
    across the load to get exactly what they want. All the money in construction of the variac is
    in the transformer windings, and it can get pretty expensive to start adding circuitry for
    regulation of that voltage, so cheaper is usually better for most applications (I think).
    What I think YOUR issue here is however, is that the LED readout might need to be
    tweeked to give you a more accurate read. There's probably a trimpot on the display,
    or feeding the display, that will do that. You just have to be sure you're tweeking the
    LED readout to the correct value.
     
  11. AcousticBruce

    AcousticBruce

    43
    0
    Apr 1, 2013
    https://www.dropbox.com/s/zu90uvfly9yvyvl/20130401_234442.jpg

    This is a Global Specialties 1504 ->> Product Page here with specs

    Perhaps it will be a simple one. I would be curious to know if this is regulated or not. Are there other words for regulated that I should learn? Such as: "Triple isolated continuous variable AC Output" What does this mean?


    Btw the reason I have this is because I went to buy this Tek 2235 and walked out with the Tek 2235 the BK Precision 1635 Benchtop PS and this Global Specialties 1504 all for $240 bucks. I think I did pretty good. Even though the scope is needing work, I probably did ok. Otherwise I wouldn't have bought a $500 dollar item without knowing what it is. It just seemed like a good deal.
     
  12. shrtrnd

    shrtrnd

    3,781
    499
    Jan 15, 2010
    That is an expensive variac. You can email Allied or Global Specialties and ask them
    but I'd be shocked if it isn't a regulated voltage.
    Make sure you've got an accurate DVM, and I'm sure you can tweek the read-out to
    be accurate is heck.
    The vaiacs I use are around $100, and only have circuit protection.
    You have a little work to do on your instruments, but yeah, you got a great buy.
     
  13. AcousticBruce

    AcousticBruce

    43
    0
    Apr 1, 2013
    Variable Isolated AC Power Source - Teardown and Troubleshoot

    I had another post about this but it was closed... so here is a better one with more info.

    Here is the manual with the schematics HERE

    It is a 1504 Isolated Transformer Variac that is showing the wrong Voltage. So I opened it up and on the display circuit was 1 variable resistor so I tweaked so the display said 120V while the Fluke also said 120V.

    So the update is now the unit shows 120V perfectly but as I go above or below that voltage it changes exponentially the farther away you get from that voltage.

    Here are some measurements for the Fluke and the Variac at various volts.

    (I will update these when I get home tonight - this is from memory)
    Variac - Fluke
    120V 120V
    185V 150V
    12V 2V

    Does anyone have any idea what this could be???


    Here are the pics of the inside.. MAN this transformer is awesome!!!

    [​IMG]

    If you look at the top left of the display circuit you can see the variable resister I adjusted.

    [​IMG]

    There are about 4 more variable resisters as you can see from this view.

    [​IMG]
     
  14. AcousticBruce

    AcousticBruce

    43
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    Apr 1, 2013
    Oh I see... closed was the wrong terminology. I just have no ability to edit the first post for some reason. I figured it was closed or something. I would rather it be added to this anyway.

    How can I edit my first post? Is it locked?
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It's better that you DONT edit your posts to add more information.

    I, for one, certainly don't expect it. People will probably not read it again to see what you've added.

    Also it makes the thread disjointed if you edit earlier posts to give information asked for in later posts.

    Ian disabled editing of older posts for this and similar reasons.
     
  16. AcousticBruce

    AcousticBruce

    43
    0
    Apr 1, 2013
    Well the pictures do not work on the original post. I would like to add these to the original.
     
    Last edited: Apr 11, 2013
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    See post #15

    You're just wasting our time now. Can we get on with the problem?
     
    Last edited: Apr 11, 2013
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, start with the minimum voltage the device will output (knob all the way anticlockwise).

    What does your meter read, and what does the display indicate?

    Turn it up bit by bit and report at interesting places (e.g. if the reading remains at zero for a while, tell us when it first becomes positive, or if the output voltage stays zero for a while, do a reading when that changes.)

    get 5 to 10 readings from 0 to the max voltage and let us consider them...
     
  19. AcousticBruce

    AcousticBruce

    43
    0
    Apr 1, 2013
    Ok fair enough.


    Variac - Fluke
    10V - 54mV (lowest possible)
    11V - 1.5V (one bees dick notch up)
    12V - 3.8V (Again just a bit of a notch)
    15V - 7V
    20V - 12V
    35V - 28V
    50V - 44V
    75V - 71.5V
    100V - 98.6V
    120V - 120V
    130V - 130.5V
    140V - 142V
    150V - 152V
    160V - 163.5V
    163V - 167V (max rotation)

    Here is a picture of the variable windings.

    [​IMG]
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
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    Jan 21, 2010
    OK, I can see 5 pots.

    I suspect you adjusted the one on the rear of the display.

    I would recommend that you attach the probes to the voltage input for this display module. I'm not sure which ones they are, but they'll be the ones which change as you change the output voltage :)

    See if the voltage you measure here is the same as the display (it won't be because I think you adjusted the pot).

    But it would be useful to know if the voltages you see here are the same. i.e. at the minimum value for the variac, do you get 54mV, 10V, or something different again.

    Probably best to repeat the measurements for the same values on the display. You can set then probe, set then probe... and so on.

    The unfortunate thing is, once you've changes one setting, you're in unknown territory. We really need to know what the trimpots adjust before you change them.
     
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