Isolation transformers

Discussion in 'General Electronics Chat' started by electronis whiz, Mar 9, 2015.

  1. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    I know if you use isolation transformers to work on stuff your less likely to get shocked (at least in theory).

    I'm curious though about a few things. I have an old isolated power conditioner made by RTE Deltec, and an old TV ISO tap. I tested the TV ISO tap if I put a meter on the output and the other meter lead on chassis, or ground pin hole in receptacle on other side i'd get like 50V on the neutral and like 150 on the hot to gnd. The power conditioner is not registering on neutral, but hot acts similar.

    I did the test with both digital and analog meters. Analog showed less voltage in most the cases except measuring output neutral to hot. So it seems it is isolated, and voltage is there, but thinking its low amperage.

    I plan to use the conditioner as a way to isolate and protect my bench power strip. I opened it to see what shape it was in, and noticed the neutral and GND are bonded. I'm thinking that for what I want to do I should remove that bonding.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    According to the code there the only one place the earth ground and neutral should be connected is in the service panel.
    The exception is when an Isolation transformer is used and you require to re-reference the local or secondary neutral to earth ground, then it should be done right at one output terminal of the transformer, from then, two conductors are used, one a neutral and one an Earth Ground.
    In general control circuitry, it is optional whether to set up the local neutral are allow the two separate fused conductors as L1 L2..
    Max.
     
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  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    I have a problem with your post based on safety issues.

    A major reason an isolation transformer is used is that some equipment/appliances rectify the AC voltage from the mains (without a transformer.) In this case, it is dangerous to touch the internal chassis or to connect the ground of a scope to it. You should only "float" the equipment you are working on. The rest of your test equipment should be grounded in the traditional method. Working on a non-isolated piece of equipment/appliance is very dangerous and should not be attempted without evaluating the best strategy to safely do so. Isolating your bench strip will not protect you or your equipment from a transformer-less circuit. Ground your bench and equipment properly and float the unit under repair.
     
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  4. DickCappels

    Moderator

    Aug 21, 2008
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    There are isolation transformers and there are isolation transformers. Leakage current is one of the most important specifications because is it the current, not the voltage that can injure or kill. I doubt you will see one with a leakage current specification of zero, since a digital voltmeter has a very high input resistance, measuring "leakage voltage" with only the meter's input resistance doesn't really tell you very much about the device being tested.

    When checking with your voltmeter as described in your post, you are looking at is the leakage current and effectively, measuring the AC leakage current by using a high resistance shunt -probably 10 Meg Ohms in the case of a digital voltmeter. The preferred way to measure AC leakage current is to put a resistor and sometimes a capacitor in parallel with the voltmeter's terminals, as shown in the illustration below. That way, you will know the maximum current that is available from the electrode or chassis being measured.
    [​IMG]

    When using the 1.5k/0.15 uf shunt shown above, divide the voltmeter's reading by 1.5k to obtain the leakage current. The capacitor has a high reactance at 50 and 60 Hz and can be ignored. The pdf document at the URL below explains the concepts and procedures associated with grounding and AC leakage testing.

    www.psma.com/ul_files/forums/safety/estguide2.pdf
     
  5. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    Okay I kind of see what your talking about. with the isolation. I don't currently have any test equipment that runs on mains. Only thing I run on the strip is a florecent lamp, a pair of bench supply, a variac, a monitor, solder iron, what ever I may be working on testing.

    My real concern would be that since the neutral of the output on the conditioner is bonded to the chassis if that would be an issue for a real isolation transformer. I understand what the isolation is supposed to do by making the output unable to ground out to earth. To me having the neutral bonded to GND and those bond at the breaker box it seems you loose all if not a portion of the isolation protection. Would it be best to leave as is or should I disconnect the bond wire? It would still be grounded through the 3rd prong back through to the breaker box.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Without pertinent details of the equipment you have it is hard to give a definitive reply, but to re-iterate, any direct mains fed equipment should not normally have the neutral connected to a earth ground inside the equipment itself.
    I cannot think of any reason it would?
    Max.
     
  7. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    ^ I thought seemed a bit odd, but is isolated so thought perhaps it was normal for something like this. I'll pop the unit open get a few pics and post them in a bit.
     
  8. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    Picture of entire device inside. Input circuit is on the right line cord goes to fuse, switch then transformer primary.
    Left side is the output side and they connect to what seems to be a solid state relay of some sort. The PCB I think is just something that measures the input based on one of the secondaries, and triggers the relay to adjust the saps that hook to the hot side of the outlet.

    As you can see on the left back there is a normal 3 pin wall socket. There is a short wire from the neutral connecting to the GND which then connects to the chassis and the earth pin on the input. With this setup it don't seem 100% isolated. esp with the neutral binding. I've seen some isolation things that have 3 prong in a direct connected 3 prong outlet and isolated 2 prong outlets. Make me wonder if the ground should even be connected to earth on the output isolation side.
    IMG_20150310_110617.jpg

    IMG_20150310_110704.jpg

    IMG_20150310_110717.jpg
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The fact it is on the socket appears to me to be a re-referenced earth grounded neutral as I spelled out in the previous post.
    Which is OK according to the code.
    You are isolated from mains supply but referenced to earth.
    Max.
     
  10. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    I thought that isolation was supposed to remove all possible earth reference. which is why I wondered if I should lose the bond and or GND on output.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It all depends on the application, When using a 1:1 transformer, the intent is usually for total Galvanic isolation, in this case the secondary would not be re-referenced to ground.
    When using a transformer in order to obtain a common service or control circuit e.g. from 480v to 120v or 240v, then it is common to ground one side of the secondary in order to retain the earth safety feature.
    Max.
     
  12. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
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    Okay for something like isolating say an LCD monitor you need to test the PSU and want reduce chance of shock by touching a point I assume galvanic should be best. I know isolated or not if somehow you torched both inputs that you'll still get shocked.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Yes if you want safety in that respect or if using ground referenced test equipment. 'scope etc, then isolated would be best.
    You often see 1:1 120v/120v transformer on a test bench for this reason.
    Max.
     
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