Help finding this type of step down transformer

Discussion in 'General Electronics Chat' started by Management, Jul 25, 2011.

  1. Management

    Thread Starter Active Member

    Sep 18, 2007
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    I need to measure 120V and 240V AC but I need it transformed down in the 10V range. Maybe where 10V is like 300V.

    Anything like this out there? Not something huge.

    Any help would be greatly appreciated. Thanks in advance.

    EDIT:

    I'll be a little bit more specific. I have a center tap transformer that has 3 terminals. L1, L2 & N. L1-N & L2-N are 120V AC and L1-L2 are 240V AC. I need 3 transformers.

    Thanks!
     
    Last edited: Jul 25, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, it looks like you have two completely separate requests in there; one before the edit, and one after it.

    For measuring voltage up to around 240v, you might use a small 6.3vac transformer with a 240vac primary. The output voltage will be somewhat higher than 6.3v unless there is a load on it.
    [eta]
    Found this 230v to 6v 0.08vA (13.3mA) xformer at Digikey:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=567-1002-ND

    As for your 2nd request - I don't know what to make of it. Did you want to measure current, or voltage?

    Why don't you draw a schematic of what you want to do?
     
    Last edited: Jul 25, 2011
  3. Management

    Thread Starter Active Member

    Sep 18, 2007
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    My apologies. I only have one request and I only want to measure voltage. I removed that sentence. Mixing stuff up in my head.

    I just need to measure the difference line voltages and need to find the transformers.

    Diagram Attached:
    [​IMG]
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    ch0 is measuring L1 to L2 which is 240v and will require a different ratio than the other 2 because ch1 and ch2 are measuring L1 to N and also L2 to N, which are only 120v.
    If you buy name brand transformers you should be able to learn what the turns ratios are so you can have accurate voltages to your recorder or meters. The current requirement will be low so they will be small and pretty cheap. Digikey, Mouser, and Newark will have a huge selection.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You have connected the transformer secondaries together so that now the configuration is an autotransformer. This is not safe, as the secondary is no longer isolated from the primary.

    You show the neutral as ground; which technically it IS grounded at the service panel. However, once Neutral exits from the service panel, consider it as carrying the same power as L1 or L2. It is the mains power return, not to be used as a safety earth ground. There is a separate wire provided for earth ground.

    You should use actual earth ground for the secondary side of the transformer, not the neutral line. If the neutral line happened to open, you would have mains voltage on the secondary side of the transformer.

    In particular, look at your CH0 output. What should be connected to earth ground is connected directly to L2 instead. That is not safe.

    This is more like how you should have it connected. Note that nothing on the primary side is connected to the secondary side - except for earth ground.

    [​IMG]
     
    Last edited: Jul 25, 2011
  6. Management

    Thread Starter Active Member

    Sep 18, 2007
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    I should have just used a Orcad model to should the schematic instead of an edited picture.

    Neutral will be connected to earth ground on the transformer.
    No primary inputs will be connected to secondary inputs.

    The purpose of the transformer is for isolation and stepping down.

    Any other transformer options? When I select one I will confirm with an Orcad diagram the true connection so you all know and can raise issues you may have with it.

    I appreciate the help and the focus on safety. That is why I like this forum.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    No, DO NOT connect neutral to earth ground. Neutral and ground must be kept separate once the wiring leaves the service panel. Failure to keep them isolated can cause an unsafe condtion, and on GFCI-protected circuits will cause a fault the moment any current is drawn from it.

    I didn't realized you'd replied - I had edited my post and added a schematic after you had already posted.
     
  8. Management

    Thread Starter Active Member

    Sep 18, 2007
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    Thank you for all your responses.

    A little clarification. I shouldn't use the transformer term so loosely.

    The voltages that I am measuring are on the secondary side of a 13kV transformer. That transformer's neutral from what I can see is connected to the canister which is connected to earth ground at the pole.

    The grounds on the secondary side of transformers that I want to buy will not be connected to neutral. That would go into a voltage measurement circuit and connected to that circuits ground.

    The diagram of what you posted is exactly what I wanted to show but didn't have my desktop (working on my laptop).

    What I need is to find those step down transformers. I'm not working in a service panel. Thanks though for all the advice and much appreciate the safety suggestions.
     
  9. Management

    Thread Starter Active Member

    Sep 18, 2007
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  10. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, those transformers are what I linked to in my earlier post.

    However, I don't know why you are doing ANYTHING with the secondary side of the power company's transformer. It is not fused/breakered at that point - except for extreme overloads. It may take several hundred or more amps current to trip a fuse on the transformer.

    You should not connect anything to the mains power until after it has gone through a service panel that has been installed/serviced by a licensed electrician.
     
  11. Management

    Thread Starter Active Member

    Sep 18, 2007
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    No worries, I am not doing anything with a power companies transformer. I think that is illegal and somewhat dangerous to climb up a pole and jeopardize killing oneself and cutting power to many people.
     
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