Motivating current versus producing current

Discussion in 'General Electronics Chat' started by fullNelson, Jan 6, 2012.

  1. fullNelson

    Thread Starter Member

    Nov 14, 2011
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    Hi guys,

    Ive been reading Vol I on this website, in particular the series on electric shock. From this site and other sources I have come to understand current is induced in anything that has electrons that can be motivated with a voltage. That makes a lot of sense. I have electrons, the wire has electrons, if I become one with the wire (depending on how) I get shocked. I share the current.

    What confuses me is the ability for a bench-top power supply to be able to produce a current, rather than let voltage and resistance determine current in a system. So, if I have a voltage and a current, then by token I have already determined the resistance. Right? Taking this further I know the resistance through my body will vary by physical contact and a host of other factors, so how is it that a power supply can push a fixed amperage and fixed voltage through my body without changing based on the resistance of my body?
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Bench power supplies are often provided with current and voltage controls. These may be thought of as level limiters: the current control sets the maximum possible output current that will flow into a short circuit or a low load resistance.

    With a sufficiently low load resistance, the output current will be at the set value, but the output voltage will be below its set value. If the load resistance is increased, the output current will be (nearly) constant but the output voltage will rise until it reaches the set value, when it will stop rising.

    Further increases of load resistance will result in the load current reducing while the voltage stays fixed. The power supply cannot deliver more current than a given load will draw at the supply's maximum output voltage.
     
  3. fullNelson

    Thread Starter Member

    Nov 14, 2011
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    0
    Hey thanks for responding!
    Two questions based on this quote here:

    How low of a load are we talking about before the amps begin to change?

    Why would the voltage be below its set value? If anything I would think the current would vary, even with a slight resistive load.

    I am trying to build some knowledge around your statement to understand why the dials on the power supply are more like limiters.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This depends on the particular current and voltage settings. Knowing these, we can find the resistance using Ohm's Law.

    For a resistance of R ohms (symbol: Ω), a current setting of I amperes and a voltage setting of V volts we have R = V/I, so for example at a voltage setting of V = 10V and a current setting of I = 0.5A, we have R = 10V/0.5A = 20Ω

    Consider the extreme case when the output has been shorted. In this case the current control circuit is saving us from having an accident!. Obviously, the output voltage is zero at this point. If we now let there be a little resistance in the output circuit, again from Ohm's Law the output voltage rises to a value found by multiplying the ourput current by the resistance ( V= I x R but with different values from above).
     
    Last edited: Jan 6, 2012
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Think of these power supplies as being capable of working as a constant current source or a constant voltage source.

    Depending on the load resistance they work in either state.

    When current reaches the adjusted current limit the supply works as a constant current source --» output voltage changes with load resistance.

    When current is lower than the adjusted current limit the supply works as a constant voltage source --» output current changes with load resistance.
     
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