been looking for an answer for 40 days.

Discussion in 'The Projects Forum' started by fisal, Jul 4, 2010.

  1. fisal

    Thread Starter New Member

    Jul 3, 2010
    10
    0
    First hello ,this is my very first posting in any web site ever since I start using the internet 14 years ago ,i have been looking for an answer for 40 days and still counting.
    i am using a signal generator to drive a coil back and forth on a pole between 2 magnets ,but the current i am getting is too small 4mA with square wave voltage +- 20v.
    is there any way i can increase the current say to .5A or maybe 1A, but still using the signal generator, or if anyone know a way using switches to reveres polarity so I can use an external current source.
    plz check this link to say what I am trying to replicate but i dont wanna use any more power than i should .http://www.youtube.com/watch?v=Famr4YWBE7g&playnext_from=TL&videos=YBHThnM-Fzg
    thank you in advance.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    You could use a power opamp to amplify the current.
    A LM1875 will probably do the trick.
    See the datasheet for more info.

    Bertus
     
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  3. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    With a 20 V square wave and a 4 mA current, the load resistance is 20/0.004 = 20000/4 = 5 kΩ. To get 0.5 A, you'll need to raise the voltage to 0.5(5000) or 2500 V. Not a likely scenario...
     
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  4. fisal

    Thread Starter New Member

    Jul 3, 2010
    10
    0
    I think u r right ,i should make a better coil with much lower resistance ,thank but I first have to try an opamp idea cause it is much easier
     
  5. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    You could use a darlington emitter follower or a MOSFET to boost the current to the required level. The signal generator is unlikely to be able to supply so much current. You also need a diode to protect the transistor from the coil's back-emf.

    If you need to reverse the polarity on the coil, you will need an H-bridge circuit, like the ones used to drive motors. There are several of them here, some easier than others: http://www.epanorama.net/links/motorcontrol.html
     
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  6. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    someonesdad. I'm going to take a shot that the output of the signal generator is 20V peak to peak unloaded only, the voltage probably drops dramatically when connected to the coil. You would have to be using something like 40 gauge wire and a LOT of it to make a 5kohm coil.

    It really depends on the actual DC resistance of the coil and the reasons for you doing this. I don't know how big this experiment is but if it's small scale you can use the voice coil from a small speaker as the coil, which can be driven directly from an audio power amplifier.
     
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  7. DickCappels

    Moderator

    Aug 21, 2008
    2,650
    632
    reduce the inductance!
     
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  8. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    That's a good point, what's the frequency of the square wave?
     
  9. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    Guys, it's a signal generator. He needs something that is capable of delivering current. The power opamp idea would be easy to implement, and is probably going to suceed.
    The frequency should be pretty low, unless he tries to make the coil move up and down 1 million times per second :p
     
  10. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    A power opamp requires you to make a circuit, using an already existing power audio amp is problem solved without any messy design issues. There is however NO solution that can can make any sense till the TRUE DC resistance of his coil is KNOWN, the only statement of resistance so far was an estimate from possible incomplete information NOT directly measured.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Got that right. It took something on the order of 28' of 36 ga wire to measure 1 ohm.
     
  12. sceadwian

    New Member

    Jun 1, 2009
    499
    37
    With that much fine wire impedance may kick in far before resistance, especially at any appreciable frequency. I think the entire coil has to be rethought, then again we know so little about it. I'm giving this thread 5 days of silence if the original poster doesn't step back in with some real details to work with.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Well thats only 140,000 feet of 36 ga wire to get to 5k ohms ;)

    You will need an awful big spool.

    But whats 26.5 MILES of copper wire going for these days?
     
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