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Driving an Ultrasound Transducer

Discussion in 'General Electronics Discussion' started by Mono, Feb 5, 2012.

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  1. Mono

    Mono

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    Jan 6, 2012
    Hello!
    Thanks to the help provided in this thread I was able to produce the pulsed ultrasound signal that I needed. (1.5 MHz sinewave, pulsed at 1kHz, 20% duty cycle, 21 V p-p)
    The general design ended being as follows: a signal generator that produces the sinewave, a 555 circuit that controls a 4066 which switches the sinewave and finally a LM318 that amplifies the signal to 21 V peak to peak.

    I was getting the correct signal by measuring with the oscilloscope without the transducer load. As I connected the transducer, the circuit worked for about 30s and the shut off and some smoke appeared. It seems no big damage was caused.

    What happened? What I have read leads me to believe that I need a transducer driving circuit (disclaimer: I am a total electronic noob). I thought the LM318 handeled the "power"part by amplifying the signal, but it seems that this is a current issue, right?

    So my question is: do you have any design advice for an ultrasound transducer driver for this kind of signal?

    BTW, on the LM318 datasheet on page 9 it shows a circuit for "isolating large capacitive loads". Will that suffice or do I need other completely different driver circuit?
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    ok
    what sort of transducer were you putting that signal into ? -- do you have a link to its data sheet ?
    maybe either a) it couldnt handle 1.5MHz or b) it couldnt handle 21Vp-p or c) combination of both

    a circuit layout/schematic of your project would help us guys to help you :)

    going by the SMA connectors in that DDS sig gene that your link provided it looks to be possibly a RF sig generator

    anyway lets see your cct diag
    and a complete description on what you are trying to achieve so we all dont have to play 20 questions :)

    cheers
    Dave
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Where did the smoke come from?

    Smoke from electronics rarely causes "no big damage", it's usually pretty catastrophic for the devices that smoke.

    I have a no smoking policy and eject any components that refuse to comply. :)
     
  4. Mono

    Mono

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    0
    Jan 6, 2012
    Dave:
    Here is a basic layout:
    Layout.png
    It worked perfectly until I connected the transducer.
    Also, the transducer is capable of operating at 1.5 MHz and 21 V p-p (one of the few things I knew from the beginning of the project)

    Steve:
    It is really that "I'm hoping" for no big damage.
    I have to go to the lab to remeasure everything (no oscilloscope at home).
    It seems the smoke came from the signal generator, but I have to really take it to the lab to know for sure.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The signal generator is one place I would have expected smoke *not* to have come from...
     
  6. davenn

    davenn Moderator

    13,798
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    Sep 5, 2009
    ok 2 questions to answer

    again .... what is the transducer do you have a link to its data sheet?

    and as steve asked where did the smoke issue from ?
    I would almost guess the Op-amp
    any device that issues smoke is usually terminal


    cheers
    Dave
     
    Last edited: Feb 6, 2012
  7. BobK

    BobK

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    Jan 5, 2010
    Voltage is not the only thing that determines the power output. The LM318 can only output about 20ma of current. At 21V p-p (about 15V rms) this corresponds to an impedance of 600 Ohms for the transducer. If it's impedance is less than that you the opamp is being overloaded.

    Bob
     
  8. Mono

    Mono

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    0
    Jan 6, 2012
    Hi guys. Thanks for your help so far!

    It actually WAS the 318 with the smoke. I have the signal generator mounted just over the zone the 318 was, so the smoke came from below. I also thought it could be the generator because it turned off when the smoke appeared, but luckily it was the 318.

    Dave, I can't find any datasheet for the transducer. It has no marks in it and it was given to me in the lab for this project. I think I should measure it's impedance before designing a driver. Will try it tomorrow at the lab.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The piezo device will have a very high resistance, however it has a significant capacitance and the majority of the current will be due to charging and discharging that current several million times each second.

    To achieve that voltage swing, you may need a driver capable of several amps.
     
  10. BobK

    BobK

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    Jan 5, 2010
    As Steve said, the resistance is not useful. You need to measure the impedance at 1.5MHz. You can do this by connecting your signal generatator thorugh a resistor and then measuring the voltage across the transducer. The resistor and the impedance of the transducer then form a voltage divider for the AC signal, and measuring the voltage across the transducer will allow you to calculate it's impedance at 1.5MHz.

    Use a resistor that is big enough that it does not lower the output of the signal generator much. Probably 1K would work.

    So, the equation would be:

    Vt = Vin * (Zt / (Zt + R)

    Where:
    Vt = Voltage across transducer (p-p)
    Vin = Voltage output by signal generator (p-p)
    Zt = Impedance of transducer
    R = series resistor

    Solving for Zt you get:

    Zt = R / ( Vin / Vt - 1)

    Bob
     
    Last edited: Feb 7, 2012
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Vin / Zt?

    edit: or is that solving for Zt, so Zt = ...
     
    Last edited: Feb 7, 2012
  12. BobK

    BobK

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    Jan 5, 2010
    Yep, should be Zt =

    Thanks for catching that, I have fixed the original post.

    Bob
     
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