no-brainer to most, I am not most

Discussion in 'General Electronics Chat' started by Mansfield_mike, Sep 17, 2008.

  1. Mansfield_mike

    Thread Starter New Member

    Sep 17, 2008
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    If I have a source putting out a constant DC voltage and amps and I want to use it to power say my laptop(although likely not my laptop). Let's say the constant power is 16V DC and 5 amps, and my laptop's power cord is say it converts my AC wall wattage to 16V DC @ 3.6 amps... what would I need to put in my power source so that unused Volts an Amps is converted to heat or something?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, it's not really quite that simple. There are a number of different types of power supplies.

    1) Unregulated supplies. "Wall warts" fall into this category. They're rated for an output voltage at a specified current, but they really need regulation.
    2) Constant Voltage regulated supplies. These will supply a specified voltage at up to a specified current, by varying the current flow.
    3) Constant Current regulated supplies. These will provide a constant current at up to a specified voltage by varying the supplied voltage.

    A constant current type supply would not be suitable for a laptop, as the actual load the laptop presents varies widely depending upon the charge state of the internal battery and how busy the CPU and accessories are (HDD activity, display activity, Ethernet port, etc.)

    So, what type of power supply do you actually have?
     
  3. Mansfield_mike

    Thread Starter New Member

    Sep 17, 2008
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    This is a theoretical proposed by a friend, the claim is if you had a constant voltage and a constant amp, you could just attach a resistor large enough between the positive and negative of the output and it would absorb and eliminate in the form of heat any voltage and currect not being drawn by the appliance or laptop, but have that power ready if the appliance or laptop needed it.

    does this seem true to you?
     
  4. niftydog

    Active Member

    Jun 13, 2007
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    A constant voltage would mean that the current through the resistor would also be constant - it would not vary with the requirements of the load. Therefore, no, it would not work as your friend suggests.

    A constant voltage supply varies it's current depending on the load requirements. A constant current supply varies it's voltage depending on the load requirements. If you theoretically have BOTH constant current AND voltage then you would need a variable resistance capable of redirecting the right amount of excess current.

    In the case you describe (assuming a conventional constant voltage supply) the excess current from the 5A supply is simply never sourced from the supply. The load only draws what it requires and nothing more, the power supply cannot force the load to accept more current.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Power supplies are rated for a given voltage at a maximum current. The load, whether laptop, toaster, or large hadron collider, will draw only as much current as it needs. If the load draws more current than the supply can source, then the power supply voltage will drop below the rated value. If the load draws less current than the supply is rated for, the rated voltage will be maintained within the operating limits of the supply.

    As for your friend's idea, you both need to read through this: http://www.allaboutcircuits.com/vol_1/chpt_2/index.html
     
  6. Mansfield_mike

    Thread Starter New Member

    Sep 17, 2008
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    I understand what you are saying, the device will only draw under the max load what it is rated to do, so what happens to the "extra" power from the source if it is not under load, and what component takes that extra power and get's "rid of it" in the case of a constant source?
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
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    There is no "extra" power from the source.

    Look at it this way: your car can go a lot faster than 30mph. But on many city streets, you keep your velocity to the 30mph limit. What happens to the "extra" speed? It stays in the gas tank! The power you don't use from the wall outlet stays in the coal, or in the damn, or wherever.
     
  8. Mansfield_mike

    Thread Starter New Member

    Sep 17, 2008
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    Let me see if I can clarify my retarded line of thinking.

    when you plug a laptop cord into the wall, the brick part of that power cord has 110V AC running to it, and the laptop has various needs depending on what is running. That power cord lays in waiting for the need to change, and I guess the question is, what lies on the DC waiting for the need to change, and how does it expel that energy not being used?
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    The energy not being used is not "expellled." The energy not being used is simply not being used. Someone else gets to use it across town. Or you get to use it next week playing a video game. The energy stays in the coal, or in the lake, or in whatever your local power company uses to make electricity.
     
  10. Mansfield_mike

    Thread Starter New Member

    Sep 17, 2008
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    so a generator that doesn't regulate it's speed by load, runs at a constant speed and outputs a constant amount of watts, if nothing is plugged into this generator, nor has batteries that it is charging, that output does nothing, just sits there waiting to be used?

    sorry that this conversation with my friend has spilled over into this forum, and I promise you I am not high
     
  11. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Read the good responses you have had again; and get rid of that common misconception about Amperes.
    Just like the battery rating in your car, that can deliver 400 Amperes to start it, when running and playing the radio alone with 2 Amperes , the other 398 are not needed to be discharged anywhere.
    The key word is capability of a power supply, that is its Ampere rating.
     
  12. Mansfield_mike

    Thread Starter New Member

    Sep 17, 2008
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    wow, ok I get it, even if a circuit is complete, everything is flowing, just because there is a max amp/volt available doesn't mean that needs to go somewhere... sorry for being slow
     
  13. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Yes. Please read the following: http://www.allaboutcircuits.com/vol_1/index.html
     
  14. loosewire

    AAC Fanatic!

    Apr 25, 2008
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    Did the generator run out of fuel. I'm trying to follow the post.
     
  15. Mansfield_mike

    Thread Starter New Member

    Sep 17, 2008
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    I was able to call my friend and tell him that he had shut down by the all about circuit guys! thanks for everyone's contribution!
     
    Last edited by a moderator: Sep 18, 2008
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