Load transient measurement equipment

Discussion in 'The Projects Forum' started by radia, Jan 29, 2015.

  1. radia

    Thread Starter New Member

    Jan 29, 2015
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    Hello all,

    I am trying to build a load transient board where the DUT has floating outputs. For that I'm using an NMOS which is driven by an op-amp (OPA27) and a 1Ohm sense resistor. The voltage across the resistor is fed back in the negative input terminal of the op-amp.
    The circuit in this configuration is working. However I wanted to isolate the input signal and for that I used an 1:1 isolation transformer (T6437-DL ). And it appears that I can't use this transformer at 10 kHz, and if I increase the frequency I have a problem with the op-amp. I tried everything I could imagine of, so now I think I need your help. I have attached image I took during tests.

    Thank you for your help,
    dD
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I don't know why, but I can't see your attachments.
     
  3. radia

    Thread Starter New Member

    Jan 29, 2015
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    I can open them, I try to attach them again here.
    T6437-DL -1.png T6437-DL -2.png
     
  4. crutschow

    Expert

    Mar 14, 2008
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    What is the difference between the two sets of pictures?
    Post your circuit.
     
  5. radia

    Thread Starter New Member

    Jan 29, 2015
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    In the first the frequency of the pulse is 10kHz and in the second it is 150kHz. Here is the circuit circuit.PNG
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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  7. crutschow

    Expert

    Mar 14, 2008
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    The transformer appears to be saturating at the lower frequency.
    I agree with ronv. An opto isolator would likely work better.
     
  8. radia

    Thread Starter New Member

    Jan 29, 2015
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    What I don't understand is why it is saturating, the transformer has a volt-time ratio of 134us and the pulse I am applying has a frequency of 10kHz and an amplitude of 1V.

    I thought at first to use an optocoupler too, but during the simulations I saw that they would be too slow and I wanted a rise/fall time less than 1 us. I have also the linear optocoupler HCNR200 but I didn't manage to make it work.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think the one I posted is less than a usec.
    It's hard to get to the data sheet. But you can copy the part # from Mouser into the Avco search box and get it fairly easy.
     
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