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Wireless Inductive Coil - How does the transistor make it work ?

Discussion in 'General Electronics Discussion' started by pratto, Oct 8, 2016.

  1. pratto

    pratto

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    Jul 2, 2015
    I have been reading atricles and watching Youtube videos about simple wireless power transfer, and I get it, except for one thing.

    Attached is a picture of the transmit part of the two coils, and it seems that somehow the transistor switches on and off to make a expanding and collapsing field around the coil, that a second coil will interact with.
    There are dozens of these circuits, but I can't find one that actually explains how the transistor cuts on and off. It is amazing the number of ways that the "designers" of these circuits use to avoid explaining how it works.

    Can anyone point me to an article that goes into detail about this (i.e., when powered on the first part of the coil expands which causes current on the base..., etc.). Please don't recommend the Instructables. They are the worst at avoiding understanding how something works.
     

    Attached Files:

  2. davenn

    davenn Moderator

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    you need to find a better circuit for a start, I don't see how that can work, it's missing components to make an oscillator
     
  3. pratto

    pratto

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    Jul 2, 2015
    hello davenn -

    Yet it does work as shown in the schematic. here is a Youtube link to the actual construction :


    And I built it about an hour ago. Attached is a picture of my rig working.
    Also attached is a better image from a YT video.

    I get that it is an LR oscillator, with a center tapped coil (transmitter), and that a separate coil (receiver) is placed in the transmitters field and thus makes a current to light the LED. But I still don't understand how the switching works. I know the transistor needs to be there to make an expanding and collapsing field, but just how does the circuit do this ?
     

    Attached Files:

    Last edited by a moderator: Oct 10, 2016
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Google "blocking oscillator"

    As shown it will probably destroy the transistor in short order.
     
  5. duke37

    duke37

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    Jan 9, 2011
    The circuit in #1 misses one critical component, this is the resistor. The coil produces positive feedback to make the oscillator. The coil is inductive and so the collector current rises linearly until the collector current reaches that which the transistor can supply with the base current supplied. At this point the feedback will turn the transistor off and the cycle will repeat.
     
  6. pratto

    pratto

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    Jul 2, 2015
    Yep. You guys noticed what I did not, the resistor is missing from the schematic. But as you can see in both pictures, it has been included in the actual builds.
    duke37, I will try and walk my way thru what you said, and if I get stuck I will reply again.
     
  7. pratto

    pratto

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    Jul 2, 2015
    duke37 -
    I can't figure out what you are saying "The coil is inductive and so the collector current rises linearly until the collector current reaches that which the transistor can supply with the base current supplied".

    Would you mind explaining it in different words ?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Google "blocking oscillator" for any number of examples and explanations.
     
  9. pratto

    pratto

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    Jul 2, 2015
    Thanks. Will do.
     
  10. duke37

    duke37

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    Jan 9, 2011
    The coil you have is in fact a transformer with the centre tap connected to the supply. The transformer works like a see saw, when one end goes up, the other end goes down.
    At switch on, some current will flow into the base and turn the transistor on, this will drop the collector voltage so the base voltage will rise turning the transistor on even harder. The collector current will rise in a linear manner due to the inductance. dI/dT = V/L
    At some stage, the transistor will run out of breath and the current will stop rising and the collector voltage will rise (the sky is the limit) so the base drive will fall and the transistor will turn off rapidly. The base voltage can go negative and the energy in the inductor can go into the base. Misuse of a transistor in this way can lead to degradation of the transistor (see thread by (*Steve*).
    Blocking oscillators usually have a capacitor to accept this energy and to set the off time. I am not clear how the off time is set in your circuit, I think it will be very short so the cycle repeats quickly.
    Since the inductance and stray capacitance are low, I would expect that the oscillation frequency would be very high, perhaps approaching 1MHz.
     
    Andre vanHeerden likes this.
  11. pratto

    pratto

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    Jul 2, 2015
    Steve -

    That was a helpful suggestion. I just didn't know what terms to use in my google searches. Now I'm reading about blocking oscillators, joule thief, flyback oscillators, and many variations. One thing that seems to be apparent, while blocking oscillators have been around for quite a while, there doesn't seem to be a simple explanation as to how they work.
    The Wikipedia explanation has convinced me that I'm not going to live long enough to understand it, so I'm going to take the cut & paste approach. I will make the basic circuit, try variations, and see what happens.

    My goal is to light an LED using (1) 1.5v battery with a current draw in the uAmps. An SMD LED would be best.
     
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