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Define what is meant by Load Impedance?

  1. Oct 14, 2012 #1
    OK. I hope this question is acceptable here.

    I am studying to take my Ham Radio License Technician's Class exam and I am not sure about the topic of load impedance and load in general.

    I looked up "Load impedance" on wikipedia, I'm confused

    This sentence "when a high power appliance switches on, it dramatically reduces load impedance" , I can relate to this sentece. When a table saw is switched on in my home, I can hear the lights flicker. Not sure if a "flourescent" is the same as "incandescent" in terms of being affected by this effect but I hear the flourescent bulbs respond.

    Another sentence I don't understand is "If the load impedance is not very much higher than the power supply impedance, the voltage will drop"

    From what I picked up in my Physics II class (E&M) there is a general relationship that as current increases, voltage decreases not sure if that is only a specific case not a generalization such as the case of a capacitor but...

    I don't understand... can someone dumb it down for me?

    I think of a power outlet as an open pipe that constantly pours out water and the load could be a straight pipe connected to the original pipe vs. a pipe that is more complex / presents "load" by friction or attempting to oppose gravity such as an inclined pipe.

    I hope my question can be understood and answered properly.
    If this question is in the wrong place, I'd appreciate it if it was moved to the appropriate location or I was told to move it and I'll do it rather than delete it.

  2. jcsd
  3. Oct 15, 2012 #2
    IN the simplest form the impedance can be thought of as a resistance - and then keep in mind that the Source is not ideal - not a perfect and unlimited supply of power ( voltage AND Current) .
    So the Source has its own impedance therfore the current that can be delivered is Voltage divided by Resistance - but we have the resistance (impedance) of the Load + Source.

    For example - If the impedance of the source = the load - then 1/2 of the voltage would be across the Source's Impedance and 1/2 across the Load's impedance. In general an extreme case for your wall socket - but in a flashlight that is exactly what happens. As a alkaline battery becomes weak - it's internal resistance increases. So if you measure the voltage with No Load (open circuit) it looks to have full voltage, but apply a load ( like the bulb of the flashlight) and the voltage at the terminals drops way down.

    So your pipe analogy ( a much debated one here on PF) - is in this case about right. If at the end of the pipe (or a point in a closed loop) the flow is stopped - the full pressure is present at that point. As more fluid flows the more the pressure is needed to overcome the resistance in the pipe - leaving less pressure to be delivered where you want it.
  4. Oct 15, 2012 #3


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    A THEORETICAL way to measure the output impedance of a source is as follows (but don't actually try this as a practical experiment on a device with a low output impedance, for reasons that should be obvious!)

    Also note that this "simple" method only applies to a linear system.

    First measure the voltage V with no load connected to the output. Then measure the current I when the output is short-circuited. The load impedance Ro is V/I, which you can think of as the "internal resistance" of the source.

    When you connect something with impedance R to the output, effectively you have an "ideal" (constant) voltage source V, and the two impedances R and Ro connected in series. So unless R is large compared with Ro, there will be a significant voltage drop across Ro and the voltage across R will be less than V.

    If you want to design a system so that the output voltage is independent of the load (for example a stabilized power supply, or the output of an hi-fi audio amplifier) you want the output impedance to be much smaller than the load impedance. (For example the output impedance of a good quality amp, designed to drive 4-ohm speakers, would typically be of the order of 0.01 ohms.) On the other hand, if you want to get the maximum amount of power output from the device, you actually want Ro and R to be equal (this result the called the "maximum power transfer theorem"). That situation might apply in a system with very low output power, and/or a high value of Ro, for eaxmple a small solar cell used as a power source. You trade off getting the maximum power for losing half the output voltage.
  5. Oct 15, 2012 #4


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    The most simple way to describe the load impedance is: Voltage across load/current through the load. However, this does not apply to source impedance for obvious reasons.
  6. Oct 15, 2012 #5
    I need to start from the very beginning it seems.

    Thank you guys for the responses and explanations.

    I wanted to know at what levels the effect of "Dimming" or losing of power, what level can that happen to? Can it happen to a simple system of say 12.6volts, 2.2A's ?

    An LC filter doesn't do anything with this impedance stuff does it? AHh... I have to apologize, my knowledge is so poor, I hate to agree with that statement "if you don't use it, you lose it" but it seems in this case that it is true. I learned about this stuff in my E&M course in Physics II but now I can't even remember what I learned.

    There are some very distinct moments I learned such as eddy currents, current being induced through motion, magnetic field around a wire with current running through it... etc

    I hope that you guys can "slow down" for me and not get annoyed at my short coming.

    Again thanks for the help
  7. Oct 15, 2012 #6
    I suppose we can start with, in terms of P = IV

    What exactly does a voltage due and what does a current do in regards to let say, spinning a motor.

    I suppose I can say "The current induces a magnetic field" but then... what does the voltage do?
  8. Oct 15, 2012 #7


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    The voltage exerts the "push" that results in that particular level of current flowing through the complex combination of switches, circuit breakers, connectors, wires, brushes, and windings.
  9. Oct 16, 2012 #8
    Hello GA -- It seems you are asking a lot of related things at once. Is this a specific scenario you can lay out for us - or really just general questions. On one hand you mention studying for HAM - and load impedance - then a 12.6V 2.2. supply ( sounds like a wallwart) - then spinning a motor? The principals are really quite simple, when taken individually, and the same principals apply to complex systems, but difficult for us to know really what you are looking for.
  10. Oct 16, 2012 #9
    Why wouldn't it be?

    How come?

    Wikipedia is wrong! No matter what voltage or current is used to drive the load, the impedance remains the same if the frequency is constant. Impedance is determined by what the physical LCR is in the load, not by the voltage across it or the current through it. True, there will be a transient time while the magnetic field of the inductance or the electrostatic field of the capacitance are energized. During this time the voltages and currents will be different than they are during steady state, but this is not because the impedance changed. True again that the impedance can be calculated in steady state by the voltage, current and phase of the load, but that indicates, not determines what the impedance is.

    The load impedance competes with the source impedance for its share of the source voltage. It simply works as a voltage divider.

    Irrelevant! The load impedance is determined by the physical load components, not by the voltage or current applied to it.

    Why dumb it down? Just get a good explanation.

    Don't get wrapped around the axle with hydraulic analogies for electrical problems. Do hydraulic engineers use electrical analogies to explain and understand hydraulic principles? The basics of electrical principles don't need analogies to understand.

    Well, was it?

  11. Oct 16, 2012 #10


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    Ratch, concerning what wiki says about the load impedance lowering when a heavy appliance is switched on I would say is pretty much right on. It may vary depending on where the load impedance is 'viewed from'. The load impedance as seen from the transformer feeding the residence is constantly changing as we turn various appliances on and off in the house.
  12. Oct 16, 2012 #11

    Surge voltage and current do not determine the load impedance. The impedance has to be defined during steady state, otherwise it could be just about anything. I've never heard of a heavy inductance being called a variable impedance.

    That is not the same thing. Adding loads to a source in parallel will lower the impedance after the circuit reaches steady state, unless resonance is involved.

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