Transformer w/ no ground, danger of static discharge?

Discussion in 'General Electronics Chat' started by prodo123, Jul 11, 2014.

  1. prodo123

    Thread Starter New Member

    Jul 11, 2014
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    Hi! First thread and hopefully not the last :D

    I guess this is a continuation of this thread: http://forum.allaboutcircuits.com/showthread.php?t=32375

    I am in the exact same situation as the OP above. I moved to Korea and I have a ton of American electronics that need 110/120V output, such as a plasma TV. I bought a 2kW step down (220-110V) transformer online, and so far I can confirm it does its job. Currently the only thing connected to it is the plasma TV.

    But I noticed that while it accepts two American 3-prong grounded plugs on the secondary, the primary itself is not grounded and has a 2-lead plug for 220V. My knowledge of circuitry is still somewhat basic, but from what I know this seems like a floating transformer that Thyristor is talking about. And in theory, it should prevent any static charge buildup as there should be no ground leak.

    The problem in my case is that when I was moving the TV, I felt a strong static buildup on the metal chassis of the TV. Maybe it's the HDMI connection to the VCR, which is connected natively to the 220V, that is causing a leak. The VCR's DC power supply doesn't have a ground connection, either.

    So my question is: why is the TV getting a static charge buildup and should I modify the transformer so that the ground lead on the 110V jacks are actually connected to the mains ground so that it's no longer "floating?"

    **edit**

    Actually, I tested the TV now without the HDMI cable. There is no static when the transformer is off, but once it's on there is a noticeable charge on the chassis, regardless of the HDMI cable.
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I'm not sure of the electric regulations where you are, but my personal choice would be to connect your mains ground (with a new 3 pin plug) to the transformer metal body, then connect that with another ground wire to the secondary 3 pin plug.

    So the 2 wires to the secondary winding are not grounded, and the 2 primary wires are not grounded, but the mains outlet ground, casing, and 110v outlet grounds are all connected.

    I hope that's clear! :)
     
  3. prodo123

    Thread Starter New Member

    Jul 11, 2014
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    Sorry for the sloppy diagrams but I'm on my phone xD
    So what you're suggesting is the attached diagram which would require me to disassemble the transformer, solder a wire from the ground and have it connect to the mains ground (no big deal). Am I right?
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You should be able to do it with no soldering. You only need to connect the ground from the supply outlet to the appliance ground (as a summary). It is good practice for safety to route that ground to the Chassis (frame) of the transformer. Not completely required from my point of view because there is little chance that burned wires in the transformer will ever touch the transformer chassis. They will touch the core but the individual plates of the core should be electrically insulated from each other and no current will travel anyhow - in theory.

    Don't take the transformer apart - a simple tap screw hole is enough if you go that route.

    Just my opinion.

    Also, check local code on how Ground is connected at the fuse box to avoid any surprising results or if your ground is even necessary.
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Looks fine, assuming that is ok with your local regulations.

    Why the need to disassemble? If the transformer is enclosed totally in insulating plastic etc then don't bother grounding its frame.

    If it is not totally insulated and the metal frame can be touched, you SHOULD ground its frame, with a lug on the ground wire etc.

    Either way there is no need to disassemble or solder. :)
     
  6. prodo123

    Thread Starter New Member

    Jul 11, 2014
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    After reading a lot, lot more about transformers I decided it was best to disassemble and see the circuitry myself. And am I glad I did!

    First, my initial assumption that this is a plain old step down transformer was wrong: it turned out to be a step-down autotransformer. This would explain why the secondary did not have a ground wire: plugging it in the wrong way, especially with Korea's bidirectional power plugs, could cause a short and perhaps cause my electrocution. :eek:

    Second, I figured that as long as I don't plug it in the wrong way I'll be fine and bought a 3-lead power cable and a 3-terminal power plug. The hot wire leads to the coil and the neutral to the secondary and coil neutral; I had to remove the old 2-lead cable and solder the new ones on. I also took your advice and bolted the ground wire to the metal chassis. No ground wire from the secondary because the outlets were "cheater" outlets that just had the hole for the ground plug and no ground terminal. I thought with this, the only thing I'd have to do would be to find proper outlets and solder & ground appropriately. I also made sure which way to plug the transformer in for future reference.

    Luckily, the magnet core for the autotrans seems to be bolted to the chassis via a metal frame! I had unknowingly grounded the autotrans's core, which completely eliminated the static on the TV frame and any other leak.

    But then a new question arises: the autotrans is still configured to use a cheater plug. Is it okay to leave the secondary ungrounded (I assume this would reduce the issue of bidirectional plugs) or should I ground that to the chassis as well?

    Thanks guys! You've been a great help :D
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Autotransformer is not isolated from primary to secondary, so you can never gorund a winding.

    It's up to you if you provide a mains ground on the secondary socket's ground pin.

    Normally the rule is if the socket has a ground pin, that should be connected back to the mains ground pin. If the socket is 2-pin (ungrounded) then obviously no ground is expected or required.
     
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