Best Way to Add a AC Signal with DC Signal?

Discussion in 'The Projects Forum' started by lekhanhvinh, Oct 18, 2010.

  1. lekhanhvinh

    Thread Starter New Member

    Oct 18, 2010
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    I would like to add an AC signal with a DC signal to create a power supply (DC+/-||AC||)

    - AC signal - variable frequency from 1Hz to 1MHz Sine-wave. VAC = 500mV p-p, IAC = 1A p-p
    - DC signal - variable voltage from 1V to 4V, current is 15A.
    Purpose AC signal can run on top (superimpose) of the DC signal.

    The problems:
    - If I am using a regulator (which I never has experience with) at the output, then it might cut-off the AC. If I do not use a regulator, the the output might not be stable.
    - I research how to superimpose this by using a high current op-ams (up to 20A DC) but it is very expensive around $300 on Digit Key.

    How can I overcome these two 2 problems?
     
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    What are you trying to build?
     
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  3. Audioguru

    New Member

    Dec 20, 2007
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    A regulated DC power supply has a very low output impedance. It tries to cancel any change in its output voltage.
    Since the output impedance is very low than it takes a lot of power to feed AC on it and then the power supply and the AC amplifier will both get hot because they are "fighting" each other.

    You could make a regulated DC power supply then capacitor-couple AC onto its reference voltage.
     
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  4. DonQ

    Active Member

    May 6, 2009
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    A 'regular' voltage regulator operates by comparing the output voltage, usually after passing through a resistor divider, to some constant voltage. This keeps the output at a constant voltage. This circuit is called a regulator.

    The difference between a regulator, and something like a DC servo controller, is that the controller compares the result to a voltage that may be varying (say... like a DC control voltage with an AC voltage superimposed on top of it.)

    Some servo controllers (not talking about RC servos, real servos) control many amps. Using one of these, with a reference voltage created by a normal op-amp summing of your AC and DC voltages, will do what you want.

    The simplest form is just a regulator with a reference voltage that is input from your source rather than a fixed voltage reference. All the same variations of linear, switching, bipolar, mosfet, etc. that are available for regulators are also available for controllers.
     
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  5. lekhanhvinh

    Thread Starter New Member

    Oct 18, 2010
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    A strange power supply that can supply the DC power but with small portion of AC power to power up some strange-load which is currently no-specs.

    I guess I can use regulator to keep the DC before summing this regulated DC signal to AC signal.
     
  6. blueroomelectronics

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    Jul 22, 2007
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    Just seems every "strange" power supply leads to something completely useless like an HHO generator.
     
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  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The easiest way to maintain isolation between the AC signal source and the DC circuit is to inject the AC signal into the circuit with a transformer. Drive the transformer with a rather larger sinewave, and step down the voltage in your transformer. This will give your AC gen a high impedance for its output and keep a low impedance(stepped down secondary) in the DC side. I'm not sure what else you would need for support circuits since you haven't shown a schematic to work with, but the secondary could be wired in as a series choke coil and the AC injected 'on top' .
     
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  8. wayneh

    Expert

    Sep 9, 2010
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    There are audio amp circuits based on voltage regulator ICs. Would one of those be workable here? I think normally the DC portion would be decoupled out of the signal.

    Or, couldn't you just provide a bias voltage to a large power transistor so that it's always conducting a large current, and then apply your AC signal to the base? I was playing with making a single-transistor speaker amplifier and built such a beast. The always-on DC current made it useless to me, but it sounds just like what you want.
     
  9. lekhanhvinh

    Thread Starter New Member

    Oct 18, 2010
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    This is the first time I really build something that is complicated. Most of circuit I built before is very simple (retificer, low-pass, high-pass filter with RC circuit, current amplifier).

    I intend to use a large current op-am that can handle 15-20A current to create a summing circuit to add the regulated DC signal to non-regulated AC signal but this kind of expensive.

    Regarding the output of the regulated DC signal. Assume that I have fully working regulated DC power supply that can provide a stable 4V@15A DC output (60W). If I want to adjust the voltage from 1 to 4V@15A. How could I do this? Should I just put a variable resistor in series with the regulated 4V@15A signal?
     
  10. blueroomelectronics

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    Jul 22, 2007
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    So is it for HHO generation?
     
  11. Audioguru

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    Dec 20, 2007
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    Then the voltage is dependant on the current and therefore is not regulated anymore: High current causes low voltage. Low current causes high voltage.

    You need to modulate the reference voltage of the power supply (with your AC signal) instead.
     
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  12. lekhanhvinh

    Thread Starter New Member

    Oct 18, 2010
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    I used WEBECH from national semiconductor to generator the circuit:
    - I can get 4V@15A DC fixed no
    - I can get 1-4V@5A DC variable -> only 5A
    :confused::confused::confused: cannot achieve both
     
    Last edited: Oct 20, 2010
  13. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I had a similar thought until I re-read the OP's spec. The transformer / choke would have to handle a frequency range of 1Hz to 1MHz.
     
  14. lekhanhvinh

    Thread Starter New Member

    Oct 18, 2010
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    What you mean by "modulate the reference voltage of the power supply"? Did you mean that I need to vary the input voltage?


    At some point I want to turn off the AC signal just let the DC signal out. In this case, I only deal with the DC signal. And I only want to vary the DC voltage and DC current only. LM27402SQ from National Semiconductor can create fixed voltage of 4V@15A.
     
  15. Audioguru

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    Dec 20, 2007
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    A regulated power supply has a voltage reference and an amplifier. The voltage reference has low current. If you modulate the voltage reference with your AC signal through a coupling capacitor then the output DC voltage of the power supply will have the original DC plus will have your AC on it which is what you want.
    But most power supplies are designed with poor high frequency response so the modulation won't work above about 10kHz.

    The LM27402 buck converter power supply IC switches its external output Mosfets on and off at a high frequency (200kHz to 1.2MHz) so it cannot easily be modulated. Maybe its switching duty-cycle can be modulated.
     
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  16. lekhanhvinh

    Thread Starter New Member

    Oct 18, 2010
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    Thanks for all answers.

    I have question regarding how to vary the output current and output of the current
    V: 1-4V, I = 10-15A, could I implement this right after the regulated output signal or it must be done within the regulated circuit?
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    I don't see an inexpensive way to do what you wish over such a wide range of frequencies.

    If you would limit the upper frequency range to ~50kHz, you could perhaps use a modern push-pull switching PWM current mode regulator operating at 500kHz as a class D amplifier.

    If you need more than 50kHz output, it's going to cost you an arm and a leg, and maybe your firstborn child, too.

    Power opamps won't "cut the mustard", as they are generally quite limited in their GBW.
     
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  18. lekhanhvinh

    Thread Starter New Member

    Oct 18, 2010
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    Thank you very much.

    I have decided to use a power op-ams [an expensive one:(] that can handle 2MHz signal to sum them up. Cannot find any better solution right now.
     
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