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A battery is a voltage source: it provides constant voltage

  1. Jan 11, 2012 #1
    Hello forum,

    a battery is a voltage source: it provides constant voltage, regardless of the load connected to it.

    there is also a current source: it provides constant current, regardless of the connected load.

    Is there something like a power source, something that outputs constant power, regardless of the load, or does it not make any sense?

  2. jcsd
  3. Jan 11, 2012 #2


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    Staff: Mentor

    Re: sources...

    True of an "ideal" voltage source, but real voltage sources have series resistance in their outputs that droop the output voltage at higher output currents.

    Also true of an ideal current source, but real current sources have output compliance limits that lower the output impedance as the output voltage increases (depending on the current source topology)

    The closest thing to a constant power device that I work with is a DC-DC converter. The output voltage is usually fixed (although it can be a fixed output current in things like LED drive DC-DC converters), and for high efficiency converters, the output power is very close to the input power, independent of the output current demand.
  4. Jan 12, 2012 #3
    Re: sources...

    Thanks you Bekerman.

    Do you understand the difference between maximum power transfer and maximum power efficiency?

    Take a battery (voltage source) +internal resistance r. The load has a resistance R.
    If the battery emits 100 W of power, the best we can do is to send 50 to the load if r=R (max power transfer). It means we can send 50 J of energy every 1 second to the load.

    In the case of maximum power efficiency, R>>>>r, say r~0, and we get the power efficiency close to 100%. But what does it mean? If the battery emits 100 W, it does not mean they all 100 W go to the load....There is some error in my thinking.
    the internal resistance can never be zero. The larger R is, the smaller the current. That seems to mean that the load may receive most of the power but that power is extremely extremely small.....

    Sociologically: if we share something you get a lot more than if you want it all for yourself: the load has the choice of sharing the same power with the internal resistance but both high powers, or get all the power to itself but very little.....

    by the way, do you do motocross? Do motocross riders use the clutch when they change gears or not?

  5. Jan 12, 2012 #4


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    Staff: Mentor

    Re: sources...

    Voltage sources generally have very small internal series resistances compared to the load resistances that they drive (otherwise they would not be a very good voltage source for that purpose). So r << R in most cases, and most of the power delivered by the voltage source goes into the load. The power delivered is determined by the source voltage and the load resistance R.

    While true that the most power you can get to the load is when R=r, that is not a configuration that is generally used with a voltage source powering a load. If you need that much power at the load, you use a bigger voltage source with an even lower internal resistance.

    I've never raced MX, but have ridden lots of MX practice. Great fun, and a real challenge. :smile:

    You generally don't need to use the clutch when changing gears. The clutch is used mostly as a "trigger" to maximize power to the ground coming out of corners, and to disengage the engine from the rear wheel when brake tapping in the air to lower your front wheel (adjust your pitch in the air).
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