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Inductive Voltage Help

Discussion in 'General Electronics Discussion' started by rdtsc, Apr 24, 2014.

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  1. rdtsc


    Mar 19, 2014
    Hello, I'm looking for information. I'd like to create PCB (A) with circular or square "coils" on the copper, from which an inch away, a second PCB (B) with similar coil(s), can induce enough current into (A) to raise a microcontroller pin to a logic-high state (1.8v.)

    This seems like it shouldn't be too difficult, as the power radiated need only be very little - just enough to rectify and charge say, a small cap / 1M pulldown resistance / Zener on PCB (A) to 1.8v. But where to begin? I haven't seen anything resembling a reference design anywhere.

    How to design the coils? Rough estimates are fine. All the math I've seen is very complex and for multiple-layer coils. A calculator like this, is nice, but what are the calculated units? Micro Henries? That is nice, but how do I go from an inductance value to "this will (or will not) work?"

    What rectifier diodes would be good for this type of application? I'm assuming they'd have to be fast.

    Could 100kHz work, or is the megahertz range necessary? Is resonance necessary?

    What driver circuit is good for PCB coils? The thread here, shows one such "driver" idea, as essentially a ZVS SMPS with an air-core transformer.

    Thanks for any insight.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The inductance is going to be low, so you need a high frequency.

    No, a rectifier diode isn't going to work, it will be far too slow. You'll want a diode with fast switching.

    The calculated inductance is in nH. See here for an alternative calculator.

    A resonant frequency would be good as you'll have lower losses in driving it.
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    You didn't say what your application is, but it sounds like you may want to sense the near proximity of two coils that can move with respect to each other. The Texas Instruments LDC 1000 Inductance to Digital Converter will do this using just one coil and a passive planar conducting surface placed within a few millimeters of the coil. An evaluation kit, which includes a two-sided PCB pancake coil with resonating capacitor, the LDC 1000, and a USB interface already programmed into an MSP430-series microprocessor is available from TI and electronics distributors for about $29. Here is a link.

    A go/no-go LED is provided on the evaluation board to indicate when the coil and conducting surface are "within range of each other". This is really a position sensor that excites the coil at low radio frequencies (determined by coil inductance and resonating capacitor) and then measures the eddy current losses in the nearby conductor. A range of about 50 kHz to several megahertz is supported. You may be able to use the evaluation kit as the "exciter" part of your two-coil system.
  4. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Oh that's a new beast. Been looking at that only last week. Very imprest can get
    about 50mm with plastic former coil of about 400uH. Very impresive results. Cost about twenty quid from Farnell.
  5. rdtsc


    Mar 19, 2014
    Hello, I am looking at the links shared, thanks for the replies. The application isn't exactly proximity sensing, but proximity control. When PCB (B) is activated and placed close enough to PCB (A), PCB (A) responds as defined by its microcontroller.

    So I guess this is wireless power transfer. But the amount of power need only be very little, perhaps 10uA. Air-core transformers are definitely a lost art. Is there any reference for such a design?

    So if the coils are "resonant", is there a baseline frequency response of the receiver coil? (Is it as simple as LC, or more complex due to the planar coil?) Reason I ask, is I don't want the device to "false" on things like nearby HAM or CB radios, etc.
    Last edited: Apr 27, 2014
  6. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Yes this is wireless power transfer and yes it is just the standard resonant formula. But you have to include the mutual inductance of the two coils. Planer coils are no more complex it's just when working out the inductance you need to consider the substrate you are using. False triggering will be taken care of by the tuned circuit but I would steer clear of high power signal as you only need 10uA choose a frequency far away from HAM and CB. The basic law of induction is E=-N(number of turns)*change in magnetic flux/change in time.
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