High output current Op amp

Discussion in 'General Electronics Chat' started by bassplayer142, Jan 30, 2011.

  1. bassplayer142

    Thread Starter Active Member

    Jan 2, 2007
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    I have a DAC connected to a microcontroller with a precise signal. I am using it to power an induction load which is basically like a galvanometer. I don't know yet what the current is needed to drive this load but obviously an op amp will not do. I still need a clean linear response so is there any high output current op amp or other circuit to power the load. I'm guessing the load would be somewhere around 500mA but I don't have the tools in front of me. Even a simple schematic would be great. Thanks!
     
  2. Papabravo

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  3. kubeek

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    something like this or with a bit more complicated output stage should be enough.[​IMG]
     
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  4. russ_hensel

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    Jan 11, 2009
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    Frequency of that op amp seems a bit high. There are power op amps around. It is also not hard to buffer with emitter followers which can be included in the feedback loop. Google op amp transistor booster may work. opa510 peak 10 a LH0101 may be available, google may find more.
     
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  5. bassplayer142

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    Jan 2, 2007
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    Thanks for the replies so far. I found that the galvanometer does not really get driven by voltage but current. Therefore the circuit will have to be a transconductance amplifier. I will try the second circuit first considering it seems pretty easy.
     
  6. Jaguarjoe

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  7. Ron H

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    You really need to find out what the current requirements are.
     
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  8. Ron H

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    What "second circuit" are you referring to?
     
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  9. SgtWookie

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    The second circuit that he saw, which is the first one in this thread; Kubeek's bare-bones schematic.
     
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  10. Ron H

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    I hope someone fills in the missing pieces for him.
     
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  11. SgtWookie

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    I have a feeling that his test run will be interesting. :)
     
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  12. kubeek

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    Ok ok, just a tiny glitch :)
    (the resistors should be lower for high currents and low gain transistors)
    [​IMG]
     
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  13. russ_hensel

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    Jan 11, 2009
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    May be wrong but diodes seem to be in wrong direction. Basic circuit will work to some extent even without the diodes and bias resistors. Who cares about cross over distortion? Also not clear that he needs bipolar drive. Also no need to use a current device like ota. Garvanometers are ohmic so are not really current or voltage driven they are both. Finally a galvanometer is usually a low current device, so may not need any buffer.
     
  14. kubeek

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    The diodes are the right way, they create the voltage drop needed for lower crossover distortion.
    Single ended version for positive voltages seems to vork too, beware the opamp then should be a rail-to-rail version for it to work properly and even then it could need a base resistor for the transistor.[​IMG]
     
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  15. SgtWookie

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    R1 and R2 really need to be constant current sources.
    D1 and D2 need to be thermally coupled to Q1 and Q2, respectively - otherwise thermal runaway could occur.
    Even if the opamp has RRIO, (rail-to-rail inputs and outputs) the output from the driver is limited by the Vbe's.
     
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  16. kubeek

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    Sep 20, 2005
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    True, that is the practice for high-power amplifiers.
    But if we are talking about simple boosting of the opamps output to say 500mA then it is not worth the fuss and properly heatsinked transistors in TO-220 or even TO-126 case like BD139 will be enough.
     
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  17. bassplayer142

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    Jan 2, 2007
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    I thought there may have been something wrong, "I threw the schematic in pspice last night". I'll look into the new one and also those high power op amps. Any way for a really clean response without too much overkill. And yes, I need to get a rating for the current draw. I don't think its a lot because there is no real load on the galvanometer (it is driving a mirror). Thanks everyone so far!
     
  18. Ron H

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    The current may be milliamps or less. If it is, most of this thread has been wasted effort.
     
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  19. bassplayer142

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    Nope, I tried using multiple op amps the and the input dropped significantly indicating an overdraw.
     
  20. Ron H

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    I doubt that inductance is an issue here. Can't you run a simple experiment with DC to determine how much current is required?
    What is the minimum and maximum frequencies involved?
     
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