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XR2206 ss opamp coil driver

Discussion in 'Electronic Design' started by Phil Allison, Oct 26, 2005.

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  1. Phil Allison

    Phil Allison Guest

    "Mark Lindsey" <

    ** The MJE340 is a high voltage transistor - it is rated at 500mA max and
    only preforms well to about 200 mA Ic.

    Try an MJE15030 instead - good for about 5 amps Ic.



    ............. Phil
     
  2. Mark Lindsey

    Mark Lindsey Guest

    I'm not an expert, but I don't understand why I am having a problem
    with this single supply project.

    I have been wrestling for days with an op amp and discrete emitter
    follower to drive a coil from the XR2206's sine output. No matter what
    I try, the wave is distorted, usually on its upper half.

    Input to op amp can be adjusted from 0-2Vpp. Frequency is only 30Hz. I
    need to drive the coil to near full swing from a single regulated 12V
    rail, which also powers the op amp and 2206. The coil is an air core
    solenoid of suitable resistance so it draws about 1A as per the PS
    rating.

    The 2206 is no problem. But can someone please suggest an idiot-proof
    circuit and component values for the above op amp and emitter
    follower? Even a rough guess. Just something to get me on track.

    I am presently using a CA3130, BC337 and MJE340, but whatever works
    would be fine.

    Best regards,

    Mark Lindsey
     
  3. I think your driver transistors are way too wimpy. Take a look at the
    gain curve versus current for the BC337:
    http://www.ortodoxism.ro/datasheets/philips/BC337_3.pdf

    For a 1 amp peak load with something like 10 ma from the opamp, you
    need a gain of 100, at least. The gain of the BC337 starts to poop
    out past 200 mA. You can never try to operate transistors at the
    current given in the max. spec. Always look at the gain curve to see
    what they are really capable of. You also probably have a power
    problem with such small devices. At the 1 amp peaks, you have about 5
    volts across the transistor, which means at that moment they are
    producing 5 watts of heat. TO92 packs won't last long under those
    conditions (it is rated for well under a watt), even if you find one
    that will carry the current. You could parallel 4 or 5 transistors to
    get them operating in their high gain region, but you would probably
    need a small emitter resistor for each to balance them.

    There are some TO-92's that will deliver the gain and current, if you
    can keep them cool, like the Zetex ZTX869, minimum gain of 300 at 1
    amp and lots of current capability above that.
    http://www.zetex.com/3.0/pdf/ZTX869.pdf
    And the PNP version might be the ZTX749, with minimum gain of 100 at 1
    amp:
    http://www.zetex.com/3.0/pdf/ZTX749.pdf

    But for 30 Hz, where the opamp has lots of time to get through the
    crossover voltage (getting one half of the follower turned off and the
    other half turned on), you might consider a couple of 3 (or more) amp
    darlington transistors

    say, TIP120 and TIP125, with their gain curves that peak just above 1 amp:
    http://rocky.digikey.com/WebLib/On-Semi/Web Data/TIP120 - 122, TIP125 - 127.pdf

    , with high frequency feedback (small capacitor) around the opamp for
    stability, but DC feedback after the output of the darlingtons (for
    low frequency accuracy). Something in a TO-220 package would stand
    the heat batter than TO-92's.
     
  4. Phil Allison

    Phil Allison Guest

    "John Popelish"


    ** I think the OP has used the BC337 and the MJE340 connected as a
    darlington emitter follower.




    ............ Phil
     
  5. I never looked up what a MJE340 was before I launched the post.
    Obviously I latched onto the mental picture of a complementary emitter
    follower. Oh well, it was a good educational post for the archives,
    anyway, I guess.
     
  6. ehsjr

    ehsjr Guest

    Yup. And the MJE340 is still way too wimpy for either configuration,
    so your observation is correct. As Phil pointed out, its Ic is 500 ma
    max.

    Something like the TIP120 you mentioned would be nice. So your post
    was, as usual, both on target and a good educational post for the
    archives.

    Ed
     
  7. Phil Allison

    Phil Allison Guest

    "ehsjr"
    John Popelish wrote:

    ** JP never mentioned the MJE340.

    He posted under the misapprehension it was a TO92 device like the BC337.


    ** Nothing in JP post was actually on target.

    BTW

    Are you JP's self appointed guardian angel ?





    ............ Phil
     
  8. (snip)

    While the O.P. might have learned something useful from my answer, I
    misread his question and answered something else. I'll give him
    double his money back, if he wants it.
     
  9. Phil Allison

    Phil Allison Guest

    "John Popelish"

    ** LOL.



    .......... Phil
     
  10. ehsjr

    ehsjr Guest

    He referred to the transistors that the OP mentioned.
    "I think your driver transistors are way too wimpy." He proved
    the case with the BC337 and I added that the MJE340 is also too
    wimpy, as you showed. Thus his observation was correct, whether
    he considered the MJE340 or not.
    You disagree that the TIP 120 he mentioned would work?
    You think the op's transistors aren't too wimpy?
    Now that's funny! JP needs no help from me. But I think
    it's nice recognize a poster's contribution from time
    to time.

    Ed
     
  11. Phil Allison

    Phil Allison Guest


    ** JP has agreed he misread the OPs post.

    The BC337 was NOT being used to drive the load so all JP's comments were
    irrelevant.

    Just as all yours now are all irrelevant.



    ** No relevance.



    ** I though it was most patronising - actually.


    ** He would be far better off if you did not try.



    ........... Phil
     
  12. Rich Grise

    Rich Grise Guest

    You could try something like this:
    http://www.abiengr.com/~sysop/images/boosted_opamp.png

    where just about any general-purpose NPN/PNP pair would work, probably
    the 2N4401/4403 or whatever that famous pair is. You could also
    play around with the 470 ohm resistors, depending on how much of
    the output current you want the opamp to deliver. You also might
    want to play around with that network at the opamp output - this
    is an LM108, and the compensation is probably different. But this
    is the only example of this circuit I could find on short notice. :)

    I'm pretty sure it's also written up in The Book. ;-)

    Good Luck!
    Rich
     
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