op-amp questions

Discussion in 'The Projects Forum' started by NeuroGrad, Jan 25, 2013.

  1. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Hi Guys,
    I am a graduate student and have to build an amplifier in order to power a coil which will in turn produce a magnetic field - in the end, we want the magnetic field to be as strong as possible. This is for a project I am working on in my lab. I have several parts that my advisor purchased, and I am not 100% sure how to use them to build this circuit. I have included links to the pictures/specs for each device. Could anyone give me information about how to use these devices, as well as suggestions for how to successfully build this circuit? The op-amp I am using is the opa549T. Also, what doesn't make any sense to me is my advisor didn't provide any resistors - in my mind, the best amplifier configuration for our needs is a non-inverting amplifier. Am I wrong? Is there a better configuration to be used to power the coil with the op-amp?

    1. http://www.mouser.com/ProductDetail/Mean-Well/SP-150-12/?qs=sGAEpiMZZMvOgUnvsteuZwCv71AyoIvd
    2. http://www.schurter.com/en/Components/Connectors/Power-Entry-Modules-without-Filter/DF11
    3. http://www.ti.com/lit/ds/symlink/opa549.pdf
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The maximum peak output current from the opamp is limited to 10A. Is that enough?
    With a high supply current and the opamp with a high current but not saturated then the opamp gets very hot. Can you cool it?
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What are you making (transmitter, medical device, SQUID, NMR/MRI,...)? What frequency will it be pulsing? Square wave, sine, narrow pulse? what is input and output voltage (current) requirement? What is DC resistance of coil?

    Get us some basic info to get us started. If you don't have it, ask your professor. Hopefully he had some sort of vision.
     
  4. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Hi Guys. Per the last conversation I had with my professor, we need to amplify the voltage and not the current. Essentially, this circuit I need to build will allow for the magnetic field produced by a coil to be turned on and off via a switch. My professor will not be around again until Monday, so I won't have an answer to the question of what frequency is the pulse being emitted, etc until then. I will have to measure the resistance of the coil, since it is something a previous lab member built.
     
  5. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    In a bit I plan on posting a picture of the design of the circuit. Also, I will include in this design questions about hooking up devices - my main issues are coming from not being too familiar with electronics.
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
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    The OPA549 seems to be a nice power op-amp for that application. I think it is the right choice! It has nice current limiting features, and can be used in any configuration. That means you don't need to use resistors if you want to use it as a voltage follower (unlike the LM675, for example). If you need to amplify the voltage, yes, you're better off with a non-inverting configuration.

    The datasheet has plenty of information on how to use it. Just use it as you would use a regular op-amp. You don't have any gain limitations, so, no problems there. You have to bias the current limiting pin, though. Also, using free-wheeling diodes at the output, as the datasheet suggests, is mandatory in your application.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    It would be nice to know its inductance as well, so that AC impedance can be estimated. Depending on what frequency you'll be using, the impedance will overwhelm the relatively small DC resistance. If it's a "regular" shaped coil, inductance can be estimated from various formulas such as the Wheeler formula. But of course measurements are better than estimates.
     
  8. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Ok guys, here is my crude drawing of what I think I need. http://i1353.photobucket.com/albums/q666/vmpunal/circuitdiagram_zps57d8ca90.jpg

    I went through emails from my professor, and it seems his original suggestion was to build this as a "voltage-to-current amplifier," and not as a non-inverting amplifier. My mistake. :( See this link for more information: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html

    You will see in my drawing that I have drawn the normal circuit diagram (sorry if some of the symbols are wrong), and have placed the names of devices throughout. I believe my drawing makes sense (let me know if I am wrong). Then, I have started drawing a diagram of the true connections between devices. You will notice some pieces are missing - this is because I am not sure where to insert them. Also, in case you are wondering, the fan in the circuit is to help cool the coil. Any advice?? Where do I place the coil and the fan in my drawing of the true connections in order for the circuit to be correct? And, do I ground in the port on the Meanwell power supply that has the ground symbol?
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What will this thing be used for?
     
  10. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    It's a lot to explain, but the short of it is that this coil will pull on microbeads when turned on and will cease pulling when turned off. :)
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    In your circuitdiagram_zps57 if the "fan" R2 is actually a resistor then the circuit will attempt to induce a constant current in the inductor of Vin/R2.

    Keep in mind a coil is an inductor and extreme voltage spikes *will* result from rapid changes in current as V still equals L * di/dt.
     
  12. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    That is what we would like to happen. The coil also will only be turned on seconds at a time. My question is, though, where do I connect the fan, the coil and the switch in the actual "connected devices" setup (the right hand drawing)? I really need help with that. :(
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    What are "microbeads" and what are they used for/what do they DOOO?
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    What are "microbeads" and what are they used for/what do they DOOO?

    Google has articles that say they are plastic particles in some soaps that srcub.
    But they polute waterways so they are being banned.

    Nothing to do with electronics.
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I am just going to take a guess before the OP responds, i think they are working with iron micro-particles (i.e. carbonyl iron powder) for magnetorheological fluids (MRFluids). A slurry of iron and oil changes viscosity in a magnetic field. The is the principle of the Delphi "magnaRide" system offered on Cadillac, corvette and, more recently, some Acura, Audi and Ferrari models. It can also be used in clutches.

    Interestingly, there is almost no other practical method to change viscosity over such a broad range. MRF can go from free flowing slurry with water-like viscosity to near solid. People have been trying for years to come up with medical applications. One company had an "active control" artificial knee joint that used MRFLUIDS but I don't know how successful it ever became.

    PWM can be used to control viscosity between full on/off states.
     
  16. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    The microbeads are iron microbeads, and the idea is we can pull on cells with them - biophysical experiments. :)
    However, to be able to pull on these microbeads with a constant force, I need to build the device I have been telling you guys about.
     
  17. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    There are dozens of papers published on various magnetic separation techniques of macrophages that have ingested carbonyl iron powder. Check the setup of the magnetic systems used in those papers.

    NdBFe permanent magnets will also produce a nice (strong) constant magnetic field for you.
     
  18. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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    Thank you. :) In terms of my drawing, however, do you have any suggestions of where I can place the switch, the fan and the coil?
     
  19. NeuroGrad

    Thread Starter New Member

    Jan 25, 2013
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