Regulating current

Discussion in 'The Projects Forum' started by MrBear, Dec 3, 2012.

  1. MrBear

    Thread Starter New Member

    Dec 3, 2012
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    How do you regulate current without regulating Voltage?
    PWM???
    As i understand it, you vary the output voltage by varying the PWM cycle.

    12V source @ 50% PWM = 6V
    So it is not possible to vary only the current.... U=I*R

    But then i am thinking - the power-supplys from school, they have 2 buttons, one fore U and one for I..... how does that work?
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    In DC circuits, Ohm's law says that current, voltage and resistance are interdependent. Holding any of the three constant and changing one of the others forces a change in the third. To your original question, regulating current...... With a fixed load resistance, current will be defined by the applied voltage. If you want to reduce the current to that load, inserting a resistance in series with it will reduce the current by, in effect, reducing the voltage across the load. Part of the voltage source will be "used up" by the series resistance. In your school lab supply, independent control of voltage and current LIMITs are available. That simply means that when the preset value is reached, the power supply will not allow more to the load. As an experiment, if you attach a fixed load to your lab supply and start lowering the current limit control, a point will be reached at which the voltage will also start reducing. That is what is called "fold-back" control. The unit senses that the current is trying to exceed the control value and the only way to prevent it from being exceeded is to reduce the voltage available. Likewise, if you have the ability to change the load value, if the load tries to pull more current than the control value, the voltage will fold back to prevent the excess current.

    Gather up some resistors that will provide a measurable load on your school supply and experiment! That's the best way to learn.
     
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  3. #12

    Expert

    Nov 30, 2010
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    Save a copy of that answer, Bill. It's a good one and we get a lot of similar questions.

    How to regulate current without regulating voltage. The basic question is almost, "How can I break the laws of physics". The answer is, you can't. Something has to give. Another answer might be, change the load resistance. That will change the current without changing the voltage, but the word, "regulate" seems to indicate that this answer is unsatisfactory. Go with what Bill said.

    ps, the schools long ago changed E=I*R to V=I*R. Are they now teaching that U=I*R?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    To summarize, you can regulate the current, or you can regulate the voltage, but you can't do both at the same time. :rolleyes:
     
  5. MrBear

    Thread Starter New Member

    Dec 3, 2012
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    Thanks BillB
    @ Crutschow.... To summarize, you cant regulate current directly, but regulating the voltage (by adding resistance to the system) the current changes.......?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Read what he said again. You can control current "I", or you can control voltage "V". Once either is under control, and "R" is fixed, there are no degrees of freedom left - you are controlling both voltage and current.
    V = I • R
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Perhaps the word "regulate" needs to be clarified.

    When you regulate the voltage it stays constant with a change in load resistance (within the current limits of the regulator). The current changes as the resistance changes to maintain a constant voltage.

    When you regulate the current it stays constant with a change in load resistance (within the voltage limits of the regulator). The voltage changes as the resistance changes to maintain a constant current.

    So that's what is meant by the statement that "you can either regulate the voltage or regulate the current but not both at the same time". It's in the context of the variable being a changing load resistance.
     
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  8. MrChips

    Moderator

    Oct 2, 2009
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    I think we need a sticky on this one. It keeps popping up.

    Just like:

    decoupling capacitors
    LED current limiting resistors
    voltage at vs voltage drop vs voltage across
    neutral and ground
    single supply and pseudo ground
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    That one is made already, It's a sticky in General Electronics chat Here (also in my sigline)

    I could write one on current/voltage regulation, but I believe crutschow or BillB3857 would do it better, as shown above.
     
  10. #12

    Expert

    Nov 30, 2010
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    I use my blog space to do things like this, then I point 'em to my blog.
    I have one for the RC time equations and one about measuring inductors.
    You could write a sticky like the one on decoupling capacitors...maybe "ground, neutral, common, and bond". The Mods are very helpful with those.
     
  11. MrBear

    Thread Starter New Member

    Dec 3, 2012
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    Okay... that helps so much.

    Lets play with an example to see if i got this right...

    I hook up a 24V motor with brushes and permanent magnets to a battery in series with a variable resistor.

    The motor spins.

    When i turn on the resistor (thereby regulating the voltage) the motor will reduce in speed since speed is proportional with voltage right?
    But what about the torque then? The torque is proportional with current. So by regulating the voltage down, the motor will draw more current and thereby produce more torque - but at a lower speed?

    And voltage is the only thing i can play with, since R is constant?!?
     
  12. wayneh

    Expert

    Sep 9, 2010
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    This is fine as a thought experiment, but in the real world you'd need a big and very expensive variable resistor to accomplish this. There are better methods.

    At the point where maximum power is being dissipated in the resistor, where maximum heat is being produced and must be removed safely, the resistor and the motor will be drawing equal amounts of power. So if you're motor is anything other than tiny, this becomes a large rating required by the resistor.

    When you increase the resistance in series with the motor, the voltage AND the current across the motor will drop. Total current flows through both equally (Kirchoff's law, I believe) and the voltage drop across each is just the current times the resistance of each (Ohm's law). Total voltage drop is the source voltage, and the current is that voltage divided by the total series resistance (Ohm's law again).
     
  13. wayneh

    Expert

    Sep 9, 2010
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    For the motor, you mean? Sort of. But the apparent resistance of the motor will vary depending on the load on the motor and how fast it's turning. If it's starting from a stop, or has been stalled by a big load, its apparent resistance (or, its impedance) will be much lower and it will draw more current until it gets spinning again.
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    DC motors are an entirely different ball game. Permanent magnet, series wound, shunt wound, compound wound?? All will behave differently with a variations in power being supplied and load variations.
     
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