Adding voltage to a circuit

Discussion in 'General Electronics Chat' started by Tobias, May 19, 2008.

  1. Tobias

    Thread Starter Active Member

    May 19, 2008
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    I want to add voltage to the output of a potentiometer using a PIC and a digital POT or some other component(s). For example the output voltage of the POT is 2.3V and I want to add .1 to 2Vs to that signal. Any ideas?The voltage from the POT will rarely be a constant value. I have tried using the Pots output tied into one side of the digital pot and having 5vs on the other side of the digital pot. Doesn't work to good.Attached is a schematic for what I have built so far.
    Thanks in advance
    Toby
     
  2. triggernum5

    Active Member

    May 4, 2008
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    Why can't you just avoid dropping the needed voltage through your resistors/pot?
    Do you always want the extra voltage, regardless of the pot setting? Or is your problem that the range of the pot isn't ideal for your circuit? It seems like resistor adjustment would solve your problem..
    I apologize if I'm missing something, I don't mean to insult your intelligence..
    What exactly are you trying to create?
     
  3. Tobias

    Thread Starter Active Member

    May 19, 2008
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    I have a controller monitoring the mechanical turn pot. Depending on the voltage of the turn pot, the controller will perform certain functions. At times though I want to 'fool' the controller into thinking the mechanical pot is at a different location. So thats the reason for wanting to add voltage.

    It looks like I can do it with an Op-Amp set up as a non-inverting summing amplifier.
     
  4. triggernum5

    Active Member

    May 4, 2008
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    Would a simple transistor suffice? Cheaper than op-amps, and easier to salvage..
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    If you dont need to control the voltage you want to add by a PIC, just use a variable resistor in series with your turn pot configuration. if you want to use the digital pot connect the output of the digital pot to Vdd of the turn pot to control its overall voltage.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That is a rickety circuit, as any change in current by the load is going to change the voltage.

    Adding a switched resistor on either side of the 16.9K (always label your components) can add the voltage by making the drop greater below the pot. A 2N2222 in saturation can make the path to ground as you have it now - turn the transistor off and the extra resistor can be in circuit to add the voltage bump. Use a trim pot to adjust for the correct amount.
     
  7. triggernum5

    Active Member

    May 4, 2008
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    "Hey, the smoke test works!"

    Whats the smoke test? My electronics teacher always said that electronics need smoke to function.. If you let their smoke out, they just stop working..
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    The smoke test works by letting the factory smoke out. When you can't find why the fuse is blowing, you substitute a metal control shaft cut to size. When the smoke has cleared, the offending device will be very evident.
     
  9. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Here is what I have and it seems to work great in the simulation software. I am now bench testing and have a few questions. I can add voltage into the circuit on the bench yet it seems very noisy. For example Pot1 is outputting average 1.33v with a VPP of 240mv. The output of Op-Amp1 is average 1.48v with a VPP of 1.12v.

    I am using an AD5220 10k digital pot and a L272M Op-Amp. Here is a link to the basic info regarding the Op-Amp.

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=497-1387-5-ND

    This is my first attempt using Op-Amps. There seems to be a hell of alot of variations to choose from. What would cause the noise? If I am off on the correct Op-Amp, please suggest a replacement. Thanks
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Try adding small capacitors (say, 1nF-10nF) from the wiper arm of the pots to ground; and 0.1uF caps across both resistor networks at the pots.

    Add bypass caps across the supply pins for the opamps. You'll need a few different sizes, from 0.1uF to perhaps 100uF.

    If you're trying to build it on a breadboard, you might have a rough time.
     
  11. Tobias

    Thread Starter Active Member

    May 19, 2008
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    Thanks that has helped the noise problem alot.

    Is there circuitry I can use to limit the voltage out of Op-Amp3 to a max of 4.055v?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Basically, the way an op amp works is that the output of the op amp is adjusted so that the inverting input is the same as the non-inverting input.

    Right now for opamp 3, you have the feedback network set up so that the noninverting input voltage is doubled.

    You have op amps 1 and 2 set up for unity gain; whatever voltage is on the noninverting input will appear at the output (within the limits of the op amp, of course.)
     
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