Looking for a low-frequency amp circuit (2 hz to 20 hz)

Discussion in 'General Electronics Chat' started by manu de hanoi, Aug 20, 2016.

  1. manu de hanoi

    Thread Starter New Member

    Aug 20, 2016
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    Hello
    I have a similar request to this old thread:
    http://forum.allaboutcircuits.com/t...quency-audio-amp-circuit-1-hz-to-25-hz.29597/

    I need to drive an electro magnet with a variable sine in the 4 to 20 Hz range, with a power up to 1000w. (the solenoid will be designed later trying to match the amplificator accepted impedance)
    I dont want to buy a lab power supply (too expensive), VFDs are cheap (100 USD) but none can supply a single phase load ( they are all 3 phases or 2 phases supply).
    Then I've looked at the audio amplifiers but they dont seem to go below 20Hz.
    I dont need "HI-FI" signal but rather a way to get a cheap and powerful amplification in the 2Hz to 20Hz range, it could be controlled for example by a line audio signal (but I dont know if a sound card can supply such frequency) or by an IC. Could you point me towards a chip or a circuit design ?

    Thank you very much
     
  2. bertus

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  3. wayneh

    Expert

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    Just throwing out an idea. It seems to me you could build a constant-current DC power supply, and place your sine-wave signal into the feedback loop so that the output current is following the signal. If you need current in both directions, you could build it like an H-bridge.

    I believe even a fairly generic design would be able to react and achieve control much faster than even a 20Hz wave, and therefore produce a controlled current sine wave at the output.

    I obviously haven't worked out the details but this is the strategy I might start with.

    I've never built a class-D audio amp, but I don't see why such a design wouldn't be able to go down to 2Hz. It varies a high-frequency PWM signal to reproduce the input wave. With adjustment for a lower range of frequency, it should work?

    If that's the case, you might be able to buy a cheap class-D audio amp module off e-bay and modify it to suit your low frequency range.
     
  4. manu de hanoi

    Thread Starter New Member

    Aug 20, 2016
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    thanks , but these projects are complicated and the tutorial is full of death threats because of the high voltages.


    the solenoid has inductance that would mess up the feedback wouldnt it ?Is it ok to drive an electromagnet by current instead of voltage ?

    isnt there out there a cheap and powerful power amplifier (opamp?mosfets? I dont even know what's what) that could handle such frequencies without worrying about High fidelity ?
     
  5. bertus

    Administrator

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    Hello,

    The voltages are needed to reach the wanted 1000 Watts.
    What solenoid would require 1000 Watts?

    Bertus
     
  6. manu de hanoi

    Thread Starter New Member

    Aug 20, 2016
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    a solenoid driving a permanent magnet oscillating, the magnet drives a compressor
     
  7. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Most decent higher powered audio amplifiers will do frequencies down close to 2 Hz even if they are not rated for it.

    Most dedicated subwoofer drivers with full DC coupled input and output stages can even go sub 1 Hz! ;)
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Methinks a geared AC or DC motor, driving the compressor directly (if a rotary type) or via a crank (if a diaphragm type) would be far more efficient. The force a solenoid exerts is a very non-linear function of armature position.
     
  9. laceholes

    Member

    Jul 26, 2016
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    Why not build a 12V astable multivibrator using transistors and buffer its output level up to what you need?
     
  10. manu de hanoi

    Thread Starter New Member

    Aug 20, 2016
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    Hello, I dont need a linear response, and motors and shafts pose sealing issues, with a solenoid I can put everything inside the medium (steam)

    Hi do you mean using a 555 and some capacitors for smoothing the square signal ? Yes it's feasible, I worry more about amplifying that signal to drive the electromagnet (power about 1000 watts)
     
  11. wayneh

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    A MOSFET switch could easily give you a square wave at whatever frequency you want. Your first post said sine wave, so I think everyone ruled out a square wave as an option.

    The next approximation is what cheap inverters use, a square wave with a brief lull at zero between steps. If you squint, it resembles a sine wave.

    Then there are increasing complexities to get closer and closer to a sine wave. The audio folks talk about low levels of total harmonic distortion, <0.01% THD.

    Define what you need.
     
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  12. tcmtech

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    I work with mechanical stuff as often as I do electrical and electronics and I am not buying the shaft seals excuse, steam application or otherwise. :rolleyes:
     
  13. manu de hanoi

    Thread Starter New Member

    Aug 20, 2016
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    Solenoids dont like abrupt change in current I read, that's why I opted for the sine, but I dont need a perfect sine or a high fidelity amplication.
    EDIT:About the mosfet switch, so I would have a powefull DC, then shop it with the MOSFET right ? I could also smooth it with beefy capacitors. But is there a convenient way to inverse the voltage ? I need the voltage to reverse to pull the electromagnet in the other direction, so far I am not using springs for the electromagnet reverse, the solenoid will drive a permanent magnet in both directions for the vibrations, I hope this will work. Should use a h-bridge for that ?

    I totally welcome suggestions on that field too, problem is I need also need something airtight ( the steam may be below atm pressure) and food safe (no lubricant). I did look into materials like carbon ptfe for a compressor piston ring, but seeing how much it cost I dont feel like experimenting. I'd also need a piston shaft seal/bearing, and possibly a coating on the piston cylinder wall. I dont have the money to experiment all that.
    For oil less rotary compression, the equipment is seldom in the 1000watt range and often cost prohibitive (screw compressors,lobe compressors...)
     
    Last edited: Aug 21, 2016
  14. crutschow

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    It's true that abrupt changes in solenoid current can generate high voltage spikes, but abrupt changes in voltage are fine (even a square-wave) since that will generate no transient voltages.
    The solenoid inductance just acts as a low-pass filter, so it will convert the abrupt change in voltage to a smoothly changing current.
     
    Last edited: Aug 21, 2016
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  15. wayneh

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    My reply exactly.

    And yes, reversing the current is easy. It's more complex than just chopping DC but it's a problem solved long ago. Look up an H-bridge motor control circuit. The robot folks use them, and they're in just about anything with a reversible motor. It would not be hard to drive an H-bridge to make a square wave at your frequencies.
     
    Last edited: Aug 21, 2016
  16. manu de hanoi

    Thread Starter New Member

    Aug 20, 2016
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    Thanks you all very much for helping me solving this issue and especially wayneh for taking the time to drill down the problem
     
  17. Alec_t

    AAC Fanatic!

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    1000W is one heckuva solenoid! How much pressure and what mass flow rate is the compressor rated for? What type of compressor?
     
  18. laceholes

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    Jul 26, 2016
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    thanks , "but these projects are complicated and the tutorial is full of death threats because of the high voltages." What are these voltages and why are they high? Is the electromagnet already existing? When you say "electromagnet" and you mean "solenoid" what does it do? You say it oscillates and drives a compressor. What does that mean exactly? Does the moving part of the solenoid physically move a mechanical air compressor. If so I agree with Alec-t that a mechanical drive using a motor and a crank for getting reversals sounds best. When you say "reversals" I'm assuming you don't want to drive the compressor in reverse but to give a push/pull effect to the piston, Doesn't the compressor already have the necessary crank? Most compressors are reciprocating piston types. What type is yours? You then introduce steam into the equation. Where can the steam go? Can it touch the solenoid? If you could start again and explain exactly what you've already got and what you want to do with it this would help me understand better.
     
  19. manu de hanoi

    Thread Starter New Member

    Aug 20, 2016
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    -it's a new type of compressor that I am designing, it's not "rated" yet, but according to the theoretical compression work I need (using this https://en.wikipedia.org/wiki/Isothermal_process#Calculation_of_work ) we should be around a 1kw power supply.
    -I already built a piston compressor with a crank, it's just too big and clunky and on top of that it doesnt fulfill all my requirements: food safe, no lubrication that could mix with the steam, airtight (+/-0.5atm relative pressure), high temp (steam).
    -If you know any compressor on the market that could satisfy these requirements (and cost less than 1500 usd) please let me know. There are elegant new micro turbine compressors developed in Switzerland that come close to fulfilling my demands but they cost from 6 k to 90 k usd (almost had a heart attack), they use new motors running at 200k rpm and.......they use air bearings (air leaks) that cant run on steam or ball bearings that would leach oil and rust on steam .

    -The solenoid can withstand the heat (i must source the suitable magnet wire rating) and will be encapsulated in a ptfe(inside)/steel assembly(outside to help with magnetic flux)

    My next question on the device is (http://physics.stackexchange.com/qu...nside-a-solenoid-for-an-electromagnet-shuttle) :
    Should I use a permanent magnet inside a solenoid for an electromagnet shuttle?

    I'd like to make some kind of vibrator/speaker but much stronger. I though of using a static tubular solenoid with AC current and inside it a mobile permanent magnet cylinder (magnetized along the axis of the cylinder). One end of the permanent magnet would be tied to a membrane and make it vibrate.

    -Will the magnet core rock back and forth in a much stronger way than a classic iron core would?
    -Will the magnet core require a spring, like an iron core would ?
    -Any comments on the conditions such device would work or not are appreciated.
     
  20. ronv

    AAC Fanatic!

    Nov 12, 2008
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    If you are careful you could probably make a diaphragm pump that runs off of mains voltage.
     
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