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Inrush Current and back EMF spikes

  1. Aug 17, 2006 #1

    I was just wondering if someone could explain to me why there is a huge spike of current & voltage in a wire when you close and open an electrical circuit. I've noticed these 'kicks' or 'spikes' in wires when runing square waves through them (doesn't seem to occur with sine waves). I have heard refferences to Tesla working on ways to increase this effect by abruptly starting and stopping the current flow in wires. What do you think is happening here?

    Thank you,
    Jason O
  2. jcsd
  3. Aug 17, 2006 #2


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    Gold Member

    I'm glad you asked that, because I'd like to know as well and never thought to ask.
  4. Aug 17, 2006 #3


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    Staff: Mentor

    In inductive loads, like your air conditioner, it is because the inductive resistance doesn't exist until after electricity starts flowing and the inductive field is built, unlike with resistive loads, where the resistence is a property of the material. In lights, however, resistance changes with the temperature of the filament, so you do get a little inrush (which is why lights burn out when you turn them on).
  5. Aug 17, 2006 #4


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    Cool. Thanks, Russ.
  6. Aug 17, 2006 #5


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    [tex] v(t) = L \frac{di(t)}{dt} [/tex]


    [tex] i(t) = C \frac{dv(t)}{dt} [/tex]

    if there is inductance, a sudden change of current will create a big voltage. if there is capacitance, a sudden change of voltage will create a big current.
  7. Aug 21, 2006 #6

    Thank you all for the interesting information. I have an experiment to share with you. I created a post a while back about coils at right angles and how power can be induced in them (check out the post here: https://www.physicsforums.com/showthread.php?t=121122).

    The people who responded explained that magnetic fields at right angles won’t induce a current in the coil. (I attached a picture of a very specific setup to describe the situation on that post). So I created the coil setup and ran a sine wave signal into one. Of course, as expected, the second coil did not register anything. However, I switched the function generator over to the square wave signal and as I monitored the input signal into one coil. I noted that it was showing a lot of spikes on the leading edge of the wave. Then I checked the second coil and saw the same pattern of spikes showing up. The original square wave I inputted was only about 300 mV at the peak but I could see 900mV-1V spikes showing up in the second coil which didn't seem to respond earlier to my sine wave signal. The only explanation I have for this is a comment Tesla made about his sharp starting and stopping of DC current causing nearby metal objects to become charged. I've repeated this experiment many times and got the same result. What do you all think is the cause of the spikes showing up in the secondary coil, but not the sine wave signal? NOTE: I was running the function generator at about 1 KHz for the test.

    Jason O
  8. Sep 5, 2006 #7
    Inrush 'kick' from the earth's magnetic field???

    Hello All,

    In my continuing pursuit of understanding, I read about vacuum tubes and how their lifespan is reduced by the kicks produced during a cold start. In one book I got from the library called "Valve Amplifiers", towards the bottom of page 262, it talks about how the kick produced in the tube is due to an "interaction with the earth's magnetic field" in addition to the initially low resistance that causes it. I scanned in a copy of the page and posted it here so you can have a look. Have any of you ever heard of this? I want to find out more about this interaction with the earth's field.

    Jason O

    Attached Files:

    Last edited: Sep 5, 2006
  9. Sep 19, 2011 #8
    Historically (pre-WWII) "transmitting tubes" were among the most powerful tubes available. These usually had directly heated thoriated filaments cathodes that glowed like light bulbs. Some tubes were capable of being driven so hard that the anode would itself glow cherry red; the anodes were machined from solid material (rather than fabricated from thin sheet) to withstand heat without distorting. Notable tubes of this type are the 845 and 211. Later tetrodes and pentodes such as 817 and (direct heated) 813 were also used in large numbers in (especially military) radio transmitters
    RF circuits are significantly different from broadband amplifier circuits. The antenna or following circuit stage typically contains one or more adjustable capacitive or inductive component allowing the resonance of the stage to be accurately matched with carrier frequency in use, to optimize power transfer from and loading on the valve, a so called "tuned circuit".
    Broadband circuits require flat response over a wide range of frequencies. RF circuits by contrast are typically required to operate at high frequencies but often over a very narrow frequency range. For example, an RF device might be required to operate over the range 144 to 146 MHz (just 1.4%)
    Today, radio transmitters are overwhelmingly silicon based, even at microwave frequencies. However an ever decreasing minority of high power radio frequency amplifiers continue to have valve construction.
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