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Isolating transformer in the UK

  1. Feb 28, 2008 #1

    f95toli

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    For a while now I have been having some problems with ground loops that are due to the diagnostics (in this case thermometry) in the my measurement setup.
    Unfortunately there is no "easy" way to solve this problem without breaking the connection to earth (=safety ground) and let some of the measurement electronics float or be grounded via the measurement leads.

    Now, the obvious solution to this problem is to use an isolation transformer where the "earth" on the secondary is not connected to safety ground on the primary.
    However, I can't seem to find a supplier of this type of transformer here in the UK.

    There are plenty of "isolation transformers" available from RS, Farnell etc but they are really "safety transformers", they won't break the connection to ground which is what I need. The only types that I have seen that DO let equipment float are intended to be used on boats ("shorecraft") and are not really suitable for a lab.

    Does anyone know of a company that sells these things here in the UK?
    I don't need a big transformer, something like 300VA would do, but ideally it should be portable (i.e. not meant for permanent installation) and have 2-3 outlets.

    (I should say that I used transformers of this type when I was still working in Sweden, so I am aware of all potential problems when it comes to safety etc).
     
  2. jcsd
  3. Feb 28, 2008 #2
    Try posting on CR4.globalspec.com. There's a bunch of UK guys on there who know all kinds of sources.
     
  4. Feb 28, 2008 #3

    berkeman

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    At least in the US, isolation transformers only isolate Hot and Neurtal -- the Earth ground connection goes straight through as you say. The purpose is to eliminate the potential for a ground fault at the output causing a ground current.

    Can you maybe post a drawing of your setup, so that we can offer other ideas for eliminating the interference you are seeing? Is it 50Hz noise that is getting into your sensor measurements?
     
  5. Feb 28, 2008 #4

    f95toli

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    No, there are definitly transformers that also isolate ground. I have, as I wrote above, used one before. We even have (at least) one where I work now, but it is a huge things (several kVA) and not very practical (I don't even think I would be able to move it into the screened room).

    I don't have a drawing at the moment. It is a very complicated system and it keeps evolving to I keep most of the "drawings" related to grounding in my head:rolleyes:

    My work mainly consists of doing very sensitive measurements (DC and microwave frequencies) at low temperatures (I am using a dilution fridge, the temperature is about 20 mK). I have been doing this for over 8 years now so I am reasonbly good at it.

    My problem at the moment is that the cable from AC-bridge that I use to measure the resistance of the RuO2 thermometers is creating a ground loop between the cryostat (which needs to be grounded to the screened room) and earth ground; I can't disconnect the shield of the cable from the bridge because then I pick up too much noise from the air which in turn heats the sensors (and in worst case the whole fridge) a few mK. The excitation voltage is 3 or 10 uV (resulting in a dissipation of a a few fW in the sensor, any more would heat it up).

    The loop means that there is more 50 Hz in there than I need, there are also some higher frequencies in there which I suspect are generated by vacuum pumps etc. But the main problem with the loop it that is also makes the actual measurements more difficult, the thermometery is after all mainly used for diagnostics of the fridge.
    The company that builds these bridges actually recommend that they be used with isolation transformers.

    I do have a nuclear orientation setup as well (Co-60 source +gamma detector) which I can use but it is very slow, it takes about 45 minutes to get enough data to accuratly measure the temperature (but the advantage is of course that it is a primary thermometer, so it is still useful) so it is useless for making measurements vs. temperature.
     
  6. Feb 28, 2008 #5

    berkeman

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    Wow, impressive. My next suggestion would be to use doubly-shielded cable, with the outer shield Earth grounded to the cryostat, and the inner shield bootstrapped to the sensor ground (driven by an active driver to the same potential as the average common mode voltage of the two (twisted) sensor wires). Kind of like we discussed in this other thread about very high input impedance ampilifier circuits and how to drive their guards:

    https://www.physicsforums.com/showthread.php?t=212030

    The outer shield will help protect you from 50Hz pickup and the higher frequency noise that you are seeing, and the inner shield will keep you from having ground loop noise couple into your sensor's balanced twisted pair.
     
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