pulse transformer

Discussion in 'The Projects Forum' started by fls_design, Apr 6, 2015.

  1. fls_design

    Thread Starter New Member

    Apr 6, 2015
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    Hi everyone: I have been designing a circuit that will output a very low level pulse. The pulse train is two pulses, with each pulse at 10 mV for 50 uSec, and -10V for 50 uSec. This is repeated every milliSec. The outpus needs to be isolated, which (at least in my current attempts) has employed a transformer. I have all the drive and controls worked out and prototyped. I can't seem to get a good pulse across the transformer. The circuit runs off a single AAA alkaline cell with a boost controller to produce 3.3V. If everything works, I may reduce that to about 2V.
    Can some one suggest a way to get to a solution?

    Thanks in advance.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,432
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    Can you supply a part number and specs on the pulse transformer?

    Have you considered using an opto-isolator?
     
  3. fls_design

    Thread Starter New Member

    Apr 6, 2015
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    Thanks MrChips:

    The problem with an optoisolator is the need for some kind of power on the isolated side. I have none, and can't put one there in this project.

    I have tried a number of transformers:
    The 100o series from Murata,
    The 786 series from Murata,
    Miniature SMT transformer T6437-DL from coilcraft.

    One of the controlling factors for a pulse transformer is the VuS product, how long it is designed to holdup a particular voltage in uSec (on the primary). Based on the frequency of the square wave I am passing, that would be 50 uSec multiplied by the voltage on the primary.
    Also, I am using a center tapped primary on the transformer, with each leg driven to ground by a pulse on a 2N7000/2N7002 transistor.

    On the output side, I can resistor divide down to get the voltage levels I need.

    I did neglect to mention that the output load is a nominal 600 ohms. I had thought to use an audio transformer, but cannot find enough info in the specs to be able to make a decision.
     
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Sure. First step is to post your circuit here so we can get a look at it; then maybe we can figure out what's going wrong.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,100
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    What sort of load is on the output pulses? In other words, how much current does it need to sink at -10V?

    Ah, I see. Cross posted.
     
  6. fls_design

    Thread Starter New Member

    Apr 6, 2015
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    You are right:
    I have posted the output circuit I have been working with. The NPN transistor is currently a 2N3904. BV1 is a AAA alkaline battery. VCC is 3.3V from a step up controller. The transformer CT can be attached to any of these at this time, and will stay attached to the source that actually works.
     
  7. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Hmmm... all I can think of, looking at the datasheet for the T6437-DL (as an example) is that perhaps you are exceeding the V*T rating of the transformer; if that's the case, the only remedy I can see (other than finding a transformer with higher V*T) would be to drive the transformer from as low a voltage as possible, and then not attenuate its output so much. You're looking for output pulses of only +/- 10 mV, correct?
     
  8. fls_design

    Thread Starter New Member

    Apr 6, 2015
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    I see a typo in my original post. the output goes from +10mV to -10mV
     
  9. fls_design

    Thread Starter New Member

    Apr 6, 2015
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    I agree with your initial finding. That's why I tried using the op amp current source (it is compliant when there is no pulse, with about 100 mV on the output.) That is also why I tried the T6427. It has a much higher Vt rating. However, it is designed for 10 KHz operation, and it does not do so well at 1 KHz.
     
  10. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Do you have any scope traces you could post? It might help if we could get a better idea of the waveforms you're actually getting from this circuit...
     
  11. fls_design

    Thread Starter New Member

    Apr 6, 2015
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    I will put them together and post. It may take me a couple of days though, I am on vacation and my lab is not available (also don't want to upset my wife too much by working).

    I will do that next week.

    So far you have all been extremely helpful and I appreciate that a great deal. Thanks.
     
  12. OBW0549

    Well-Known Member

    Mar 2, 2015
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    You're posting here about pulse transformers WHILE YOU'RE ON VACATION?????

    That's just... just... just WRONG!!!
     
    wayneh likes this.
  13. fls_design

    Thread Starter New Member

    Apr 6, 2015
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    Ah well, More to come.
     
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