Need help some designing variable Voltage and Current circuit.

Discussion in 'The Projects Forum' started by Midnighthsuky, Aug 6, 2015.

  1. Midnighthsuky

    Thread Starter New Member

    Aug 6, 2015
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    Hello. So I want to start off by saying I've never tried to design my own circuit from scratch before.So I'm trying to build a variable current and voltage circuit that will be run from a 2s (7.4v) li-po battery and controlled via an arduino nano. In a simulator it works pretty much perfectly but as I have a lack of expertise I need some help adding diodes and what no to eliminate any unpleasantness occurring. Thanks in advance.

    in the circuit the 2 variable voltage inputs will be were the arduino analog outputs attach.

    Link to the circuit on falstad.com: http://www.falstad.com/circuit/circ...400+560+448+0 x+1106+235+1165+238+0+24+<+Out
     
  2. TheButtonThief

    Active Member

    Feb 26, 2011
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    It's a little unclear exactly what help you're looking for.

    I'm afraid I'm just as inexperienced as you when it comes to diodes that eliminate any unpleasantness occurring.
     
  3. Midnighthsuky

    Thread Starter New Member

    Aug 6, 2015
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    I'm just worried because most circuits seem far more complex than what I've come up with so I guess I'm just looking for confirmation that it will work as intended.
     
  4. TheButtonThief

    Active Member

    Feb 26, 2011
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    Depending upon what your load may be, it looks ok to me.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is the circuit supposed to do?

    In real life (not a simulator), the voltage at V(out) is unstable, unpredictable.
     
  6. pwdixon

    Member

    Oct 11, 2012
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    No idea what this circuit is supposed to do, it doesn't really make that much sense.

    Simulators are great the forums are full of people using simulators and finding the real thing doesn't do what they want.

    You need to say what you are really trying to do because I think you are trying to generate a voltage and I imagine the transistor is supposed to bleed off some of the output current and divert it away from the load somehow.
     
  7. Reloadron

    Active Member

    Jan 15, 2015
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    So my guess is you want a current source? You want to take an analog voltage output from an Arduino or other uC and generate a current? Should that be the case what are the voltages and currents actual values? If this is what you want a Google of op amp constant current source circuit will yield plenty of results. For higher currents the op amp will typically drive a transistor.

    If you want a simple turn key off the shelf solution Calex makes the Model 930 Programmable Current Source which I have used in numerous industrial applications. They work great and never fail. The 930 can easily be configured in several different ways.

    Ron
     
  8. Midnighthsuky

    Thread Starter New Member

    Aug 6, 2015
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    Sorry for not being clear. I'm trying to power a heater coil for an atomiser (basically a resistor) so I need to be able to alter the voltage and current going to it. The op-amp is there to alter the voltage and the transistor to control the current, well that's the intention anyway. I want the range to be from 0-(as close as the amp will allow) for the voltage and 0-(as high as possible) for the current.
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Why didn't you state this in the beginning? If it is for a heater coil, you only need to control the current. Consider V=IR; V and I are not independent of each other.
     
    Last edited: Aug 6, 2015
  10. crutschow

    Expert

    Mar 14, 2008
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    You can't control both the voltage and the current at the same time, it's one or the other.
    So which do you want to do for the heater?
     
  11. ScottWang

    Moderator

    Aug 23, 2012
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  12. Midnighthsuky

    Thread Starter New Member

    Aug 6, 2015
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    I'm aware that current and voltage are linked by blood but I got the impression that by using limiters i.e the op-amp and transistor they could be controlled more or less individually.

    Well no because the idea is that the coil will be removable and therefore different coils with different resistances will be added most have values between 0.8 and 3.2 ohms
     
  13. #12

    Expert

    Nov 30, 2010
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    Even if you equip the circuit with both kinds of limiters, only one will be in control at a time. It's the nature of the beast, the, "linked by blood" characteristic. Most of the time, the voltage controller will be in charge. When the current limiter becomes the limiting factor, the voltage will be forced lower than the voltage controller would normally place its value.
     
  14. Midnighthsuky

    Thread Starter New Member

    Aug 6, 2015
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    Ok then. Here are two example of what im trying to recreate (what is known as a vape box mod) unfortunately neither has particularly well documented circuitry.

    The first example I found and what inspired me to attempt this: http://ukvapers.org/Thread-VapeShield-VV-VW-VA-Mod-Board

    A schematic of an early test version of the one above: http://ukvapers.org/attachment.php?aid=22595

    and another one with similar features: http://breaktru.com/smf/index.php?topic=615.0

    So I guess my question is now: How did the respective makers of these "mods" achieve variable voltage and current?

    BTW thanks for all your help and patience so far.

    (wow just realized I wrote the title wrong, good English on my part)
     
  15. crutschow

    Expert

    Mar 14, 2008
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    A power supply with current and voltage control is designed as a variable voltage supply with a variable constant current circuit in series.
    The supply delivers a constant voltage output until the load reaches the constant current limit. Then the voltage will drop as necessary to keep the current from exceeding that current limit value as the load resistance gets smaller.

    That is why it is stated that you can control voltage or current but not both at the same time.
    They are not independent variables as they are unavoidable related by the load resistance.
     
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