Isolation Variac or Isolation Transformer?

Discussion in 'General Electronics Chat' started by MrSoftware, May 13, 2016.

  1. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    I'm going to be playing with some busted UPS's, modern TV's with SM power supplies, etc.. so I'm thinking isolation would likely increase the potential service life of myself and my equipment. ;) Is there any reason not to get an isolation variac instead of a strait on/off isolation transformer? The variac functionality would be useful, especially when fiddling with UPSs. This is what I was considering:

    http://www.amazon.com/Variac-Variab...006NGC6HU/ref=cm_cr_arp_d_product_top?ie=UTF8

    Also would I need to make any changes to this to make it truly isolated? Maybe remove the ground pin from the plug?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    That isolated Variac should be fine if you want both isolation and a variable voltage.
    As an added safety precaution plug the device into a GFI outlet.
     
  3. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    Thanks, good idea with the GFI output. I just noticed the following in the fine print:

    Note - The grounding pass through and bypasses the isolation, which means the grounding isn't isolated. You may be able to use a 3 prong to 2 prong grounding converter

    Is there any reason this wouldn't be a safe thing to do, to achieve true isolation?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    If the transformer windings are isolated then there's no particular reason to isolate the safety ground, but you can if you want to.
    Either way should be safe.
     
  5. Jahnlee

    Member

    Jul 2, 2015
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    So did you decide to get the Variac? I read the reviews at Amazon and it seems there are conflicting claims.
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    So are you working on this sort of stuff in a damp basement with a concrete floor while barefoot and sitting on a metal stool in your underwear? o_O

    If not then get to know your electrical fundamentals being that if the area you are working in has no solid definable earth ground having an isolation transformer can lead to a false sense of security thus leading to you touching two points that do complete a circuit on the isolated side of the system thus leading to a direct hand to hand potentially fatal shock.

    Guess how I learned this? :oops:
     
    MrSoftware likes this.
  7. MrSoftware

    Thread Starter Member

    Oct 29, 2013
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    I haven't bought it yet (too busy), but I'm going to pick it up and see how it works. I saw those conflicting comments, and I'm guessing (hoping?) that some people received the wrong item and didn't double check?

    I'm less worried about killing me, and more worried about accidentally killing my scope. I won't tell you what gives me that fear, but I will tell you that a Rigol scope can take 120v through the ground for brief periods and still function OK. This only happens when the neutral, isn't really neutral. ;)
     
  8. Jahnlee

    Member

    Jul 2, 2015
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    A few of the reviewers who checked found it was not isolated and I think this is more likely the case since the fact that they asked to use a 2 prong converter is a giveaway. Do keep us posted in case you got it so we know the truth. Thanks.
     
  9. BR-549

    Well-Known Member

    Sep 22, 2013
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    If you are protecting a scope.....you must use isolation transformer.
    If you have old tube equipment that hasn't been on for years, or old electrolytics, one can use a variac to slowly bring them back to life, without an overhaul.
    They are also handy for testing old unmarked transformers.
     
  10. SLK001

    Well-Known Member

    Nov 29, 2011
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    You had a direct hand to hand fatal shock?
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Yes. You should try it some time. :rolleyes:



    I prefer to set my scopes up to be what's isolated. Solves a lot of other signal loop/ground issues as well.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    You were competing for a Darwin award? o_O
     
  13. dacflyer

    Member

    Nov 19, 2010
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    my set up.. i have a variac and a isolation transformer, then your truly safe..
     
  14. crutschow

    Expert

    Mar 14, 2008
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    According to the reviews of that Variac, it was verified by meter measurements that the output has no voltage from either hot or neutral to the safety ground.
     
    Last edited: May 20, 2016
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Depends how they checked it, if they used a voltmeter from Variac output to ground pass through they most likely got some kind of voltage reading with a modern high impedance DMM.
    It definitely states isolated from the pass through ground.
    Max.
     
  16. Jahnlee

    Member

    Jul 2, 2015
    36
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    Thanks Max.
    I am not sure what is the definition of 'pass through' ground. Could you help me understand what it means?
    There is a comment to the video review on Amazon that he got the full AC voltage reading.
    Maybe if TS gets the unit, he can help us confirm with a LoZ meter.

    Separately, if indeed it is isolated, then I am confused why they mentioned about using the 3 prong to 2 prong converter. I am under the impression that what they mean by isolated is to 'get rid' of the ground by using a 3-2 prong converter. Plus, it was also stated that "the grounding isn't isolated" - 'Note - The grounding pass through and bypasses the isolation, which means the grounding isn't isolated. You may be able to use a 3 prong to 2 prong grounding converter'.
     
    Last edited: May 21, 2016
  17. crutschow

    Expert

    Mar 14, 2008
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    I believe a "pass through ground" is simply a wire that connects the plug safety ground to the Variac socket safety ground.
    This does not affect the isolation (if there is) of the input hot and neutral from the two output power connections.
    There's no reason to use a 3 prong to 2 prong converter if the device is truly isolated since there can be no voltage/current between the output and the safety ground.
     
    Last edited: May 22, 2016
  18. Jahnlee

    Member

    Jul 2, 2015
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    Thanks Crutschow.
    I think the video reviewer at Amazon did mention that the plug safety ground is connected to the Variac socket safety ground.
    If I understand correctly from the link below, there is one situation in which this configuration will be an issue if the device under test also has a safety ground and that safety ground is also tied to the device's circuit ground. Connecting such a DUT to the variac would mean the DUT now has a mains referenced ground even if the variac outputs are isolated from ground. Touching a scope crocodile clip to the positive supply of the DUT under that scenario will create a short on the DUT even though the ground is isolated from the output of the variac. If that is correct, then yes, a 3-2 prong converter at the variac outlet will take care of this scenario. Perhaps that is what they mean by "the grounding isn't isolated." In this case, it is better to connect the plug ground to the chasis of the variac instead of a pass through, I think...

     
  19. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    I think that's how I wired mine. Safety ground is safety ground. Most of the things I needed the isolation for used a 2-promng polarized plug anyway. I was in my teens some 40 years ago and somehow managed a 3A Variac for free and a 1 KW isolation transformer for like $10.00.

    I added an Analog voltmeter and ammeter as well as binding posts to access the AC.

    Because the Variac was only 3A, wired a center-off switch to either use the Variac+isolation or isolation. Panel mounted fuseholder.
     
  20. BR-549

    Well-Known Member

    Sep 22, 2013
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    I guess my idea of an isolation transformer is quite different from most of the posters.

    I never heard of an isolation transformer for personal safety until I frequented this site.

    Isolation was for equipment protection.

    We were taught on tube circuits and were taught high AC and high DC voltages from the get go.

    The reason for an isolation transformer was to float and isolate load from a common ground.

    If you feed a ground thru an isolation transformer, you have defeated the whole purpose.

    An isolation transformer would allow an ac powered meter or scope to reference the negative lead to a high potential, without shorting the equipment thru the common ground.

    With battery powered test equipment, the isolation is built in and forgotten.

    Watch this 50 min. video....so no one gets hurt.

    http://hackaday.com/2011/05/02/isolation-transformer-101/

    When someone says "isolation transformer" be very wary, and make sure you understand THEIR context.
     
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