Isolation Transformers

Discussion in 'General Electronics Chat' started by bwilliams60, Aug 21, 2014.

  1. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    So I spend a lot of time diagnosing DC circuits and have been playing with AC circuits on and off most of my life and it sounds like I may have been on the lucky side at times. I have lately been reading about isolation transformers and the fact that we should be using them when we play with TV's, audio equipment etc so i would like to ask if someone could expand a little on these transformers and why we should use them. Also would like to know where the risk has been working on TV's and such with line voltage. I have taken 120 volt pokes along the way, but where is the real killer voltage that everyone speaks of. I would rather be here to ask the question so if you don't hear anymore from me, I guess I found my answer :)
    I also read oscilloscopes should not be plugged into an isolation transformer. Can someone expand on this as well.
    This would be a good lesson.
    If all else fails, point me in the direction of a good website or book and I'm just as good with that.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    It disconnects the equipment you are working on from the ground reference. Therefore, when you touch something on the old picture tube's high voltage coil you are essentially a bird on a wire instead of a path to ground. That was the theory as explained to me.

    Two transformers of identical design with the secondaries of each and sufficient VA rating to power your device is all that is needed, again, in theory.

    I have not had a reason to work on any HVoltage equipment so wait for additional advice, preferably from someone not named Smokey, Sparky, Flash, Ouch, ...
     
  3. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    LOL, point taken. I understand that part of it but let me expand on it a bit so others can make more sense of the question. Line voltage is 120V (for example) Neutral goes to earth. If you grab on to the hot wire and accidentally touch the neutral, you find yourself on your butt. With an isolation transformer, if you happen to touch what used to be 120V and 0V lines, it is still 120V correct. So why don't you still get poked on your ass from touching the same two points. I am reading a good book on this subject right now. Maybe I will find the punchline. Sparky, that kills me...
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Compare these two situations:

    In the left one without an isolation transformer, one end of the voltage source is tied to earth in the panel. If you touch L1 (120Vac line) and if there is a conductive path through your other hand or your feet to anything that is earthed, the current represented by I(R2) will flow through your body... (enough to stop your heart).

    In the right circuit with an isolation transformer, there is no conductive path from either end of the 120V voltage source to anything but the load. If you touch either end (L2 or N2), no current can flow through your body. Your body effectively "grounds" the circuit, so the voltage of the part you touch is forced to be at the same voltage as your feet. Your body provides the ground, but no current flows...
     
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  5. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Thanks MikeML. I think the part I keep thinking about is what happens when you touch both ends of the circuit at the same time. That would be L2 and N2 of the secondary winding. Would you not still be touching 120V and get lit up? I understand the ground path in the first diagram, I am just thinking you are still a conductor in the second scenario just like a light bulb if you touch both ends at the same time.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Yes, that is why you are told to work on live circuits with one hand in your pocket.

    Another big hazard with the first scenario is probing around with a scope probe that has a ground clip. The ground clip in a scope probe is connected to earth ground through the green wire in the 'scope's line cord. If the ground clip on your scope touches L1, instant explosion. In scenario2, the ground clip can go anywhere without hazard...
     
  7. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Thanks Mike. I think I get it but either way, it's all about keeping your hands in the right place. If you do keep your hand in one pocket, are there any other ways to zap yourself and I don't mean with any other body part. If one hand is your only contact point, you should be safe, correct?
    I also read somewhere about using GFI receptacles. If they are rated for 15-20A, how is that going to save your ass?
     
  8. #12

    Expert

    Nov 30, 2010
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    The GFCI will carry 15 or 20 amps to the load, but if some of the electricity finds another route, like through you, the GFCI shuts down the entire 20 amps. I don't know what the GFCI limit is, but it's low enough to save you. Somewhere in the milliamp range, I think.

    As for, "safe", I can name one thing that will still get you: high frequency. Once upon a time, I was "safely" working on a TV and, unknown to me, the plastic cap on the damper tube had a crack in it. The 15,750 Hz signal at several thousand volts stitched a line of carbon coated holes in my hand as I jerked away from it. Now, those old tube type TV's are so far back in history that you will probably need somebody explain, "damper tube" to you.
     
  9. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Thanks #12. Just to expand on the GFCI if you could, if it is designed for 15-20 amps, why does it shut down on milliamps. This is the part I am not understanding.
    As for television, I vaguely remember the damper tube. It has something to do with horizontal sweep and is connected somehow to the CRT. My brother built one of the first Heathkit color televisions and he took the time to tell me what each of the components was and how they could end your life suddenly.
    Is there anything out of the ordinary on plasma, LCD or LED Tv's or audio equipment for one to be aware of. And I mean other than capacitors, line voltage etc.?
     
  10. paulktreg

    Distinguished Member

    Jun 2, 2008
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    The GFCI monitors the current going to the load via the Live conductor and on it's return via the Neutral conductor. If these two currents aren't equal then it's going somewhere it shouldn't be and device shuts off the mains, typically at around 30mA difference.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    On the old CRT tubes, the voltage required on a colour was typically around 20kv in the right conditions, could hold the charge for days and give you a nasty zapp!.
    The liquid crystal display in your older lap top had a HV invertor that generated up to 1kv but would not sustain any appreciable current enough to do you harm.
    Plasma TV have a high voltage source also.
    Max.
     
  12. MrChips

    Moderator

    Oct 2, 2009
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    Looking at a number of articles, the trip current is 5mA for high-sensitivity GFCI (the ones commonly used in residences).
     
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  13. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    This is good information. I never knew that about GFCI receptacles. I learned something new today and it's not even noon. Thank you for that.:)
     
  14. #12

    Expert

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    This is a very crude drawing of the concept. It's basically, measure the current in hot and neutral, if equal, pass, if not, stop.
     
  15. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Thank you #12, I see much clearer now. I have several Superior Electric Company L21 Variable Power Transformers and I wonder if they are also isolation transformers. I can't seem to find any information on them. Any ideas?
     
  16. #12

    Expert

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    Measure them with an ohm meter. It's that simple!

    You could also post a photo. Many people here can tell by looking (if they aren't in a nice enclosure).
     
    Last edited: Aug 22, 2014
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it is anything like the typical Variac or modeled on the same, they are not isolated transformers.
    Max.
     
  18. crutschow

    Expert

    Mar 14, 2008
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    I recommend that you use a GFI outlet for all your electronic bench power whether you use an isolation transformer or not. It's cheap insurance in case you are working on a line powered device that has a faulty power connection. Even a soldering iron could have leakage to the tip which would be dangerous.
     
  19. bwilliams60

    Thread Starter Active Member

    Nov 18, 2012
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    Simple for you. What am I measuring with an ohmmeter? Am I checking for continuity from input to output or...
     
  20. to3metalcan

    Member

    Jul 20, 2014
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    If the primary and secondary windings have the same DC resistance, they're very likely to be 1:1, and hence an isolation transformer.

    EDIT: Missed that we were talking about Variacs, which are often autotransformers. Check for any continuity at all (a resistance less than infinity) from primary to secondary. If there is any, it's not isolated.
     
    Last edited: Aug 22, 2014
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