Inductive Charger Circuit (Help)

Discussion in 'The Projects Forum' started by Neom22, May 1, 2012.

  1. Neom22

    Thread Starter New Member

    May 1, 2012
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    Hi guys, I'm new here just registered.

    I got a circuit that im attempting to build as a college final project of an inductive charger, for cell phones or batteries with an output similar to an USB port, 5v with like 700mA.

    So, i got a problem, i've built it like the following diagram but when i plugged it in an AC power (127v 60Hz) the diodes on the bridge fried.

    [​IMG]

    [​IMG]

    As you guys can see, in MultSim everything runs without any problems and the output on the second inductor is just what i wanted, around 700mA with 5v.

    Do you guys think that the problem could be on the circuit building it self or the project have flaws?

    Can you guys help or share any suggestions?

    Thanks a lot!
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,103
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    I guess I don't see why Q1 wouldn't just be on all the time, with nothing to limit current except internal resistances of the diodes, coil and Q1 itself. OK, it's supposed to oscillate. But is it? What's the "on" time percentage?

    Actually this is a mains voltage project, and therefore off limits?
     
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  3. Neom22

    Thread Starter New Member

    May 1, 2012
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    It does oscillate but its a very minimal value both on the voltage and current on the multimeter after the second coil.

    It should be on at all time since theres no on/off button, until i unplug it from the ac source.

    Since the first diodes failed what do you think it could be?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Putting unidirectional pulses through a transformer can cause it to saturate and conduct high current, which is what may have blown your diodes. Transformer saturation will not show in the simulation if the transformer has an ideal model.

    I don't understand why you have that circuit at the transformer input. :confused: If you need to limit the current then put the bridge and current limit circuit on the output of the transformer, not the input. That way none of the circuit will be connected directly to the mains, which is safer.
     
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  5. Neom22

    Thread Starter New Member

    May 1, 2012
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    I think you're right, it maybe be getting saturated when i plug it in. The diode bridge is on the main coil because the second coil will be attached to an cell phone cape, the second coil and it circuit must be as small as possible.

    I do need to limite the current and voltage, but i dont know how much. It must be enough limitation so it can create an field large enough to be absorved by the second coil and provide it 5v with at least 500mA.

    Since V1 is a random power plug of my house or any 127v source what do you would recommend to decrease the current and voltage?

    Thanks for the awnser by the way!
     
  6. crutschow

    Expert

    Mar 14, 2008
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    If you need the limiter circuit in the primary, then place the coil in series with the bridge input, not the output. That way there will be bipolar AC going through the primary and the coil shouldn't saturate. You can still regulate the current by having the limiter circuit across the bridge output.

    But normally you build the primary with sufficient inductance to limit the primary current to an acceptable value. Don't see why you want the limiter, which just wastes power and generates heat. Look at how electric toothbrush inductive chargers are constructed, for example.

    You will also need a bridge rectifier at the output coil to efficiently rectify the transformer output voltage. If you want maximum efficiency, use Schottky diodes in the bridge.
     
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  7. Neom22

    Thread Starter New Member

    May 1, 2012
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    Im looking for an lower value of current and voltage at the second coil only because of the devices that im trying to charge, like an cell phone. It must be something around 1A and 5V because of the specs that most phone batteries has.

    This is actually what im trying to build http://www.energizer.com/inductive/product-overview.aspx

    The product itself is made of basically 2 coils, one in the phone cape and one at the pad.

    But, back to the changes you told me to do, i didnt understood it all, you would you be able to draw or show me the changes at the circuit? I would really appreciate if you could.

    By the way would be the simulation at MultiSim be somekind of broken or incorrect? It shows me the correct output at the end of the second coil.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    I recommend that you redesign the device with enough primary inductance so you don't need a current limiter on the primary.

    At 60Hz you should have an inductance of at least 3H which would limit the magnetizing current to <100mA.

    The device you are referencing likely uses an electronic inverter circuit to convert the line voltage to a higher frequency (likely in the neighborhood of ten to a few hundred kilohertz) so that the coils can be smaller with less magnetic material (Read this for example)

    What did you use for T1 when you built the circuit and the diodes fried?

    The MultiSim used an ideal transformer which is why it showed the desired (not correct) output. To simulate correctly you would need a non-linear transformer model that includes saturation effects of the transformer you used. Have you studied transformer theory?
     
  9. Neom22

    Thread Starter New Member

    May 1, 2012
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    Since you mentioned the coils, now i really think that is the problem. I didnt calculate those properly and i dont know it indutance, i've just made one with 100 turns on a 1 mm ferrite making it a total diameter of 2,5mm.

    I've found a similar project on inter with these values:

    Coil 1= 100 turns, 25 SWG, center tap, over largest possible ferrite E-core
    Coil 2= 50 piled turns, 20 SWG, 2 inches diameter, air cored
    Do you think it should work? Since it said, over the largest possible ferrite what do you think it should be?
    Im using some calculators around to see if i get something solid about the coil but yet nothing.
    Once again, thanks for you patience!
     
  10. crutschow

    Expert

    Mar 14, 2008
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    I don't know offhand whether that will work. What did that "similar project" drive the coil with?
     
  11. Neom22

    Thread Starter New Member

    May 1, 2012
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    The project says that i just need to plug an AC source on it but it does not say the voltage and current.

    Looking back now i think this is why it is getting fried, i was just plugging the diodes right on the 127v source (my house power plug). I think i should decrease this power to something around 8v or 10v with 1A.

    Do you think thats the problem?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Without knowledge of your transformer characteristics, it's not possible to say.
     
  13. Neom22

    Thread Starter New Member

    May 1, 2012
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    0

    Well, I've built again the same circuit but now using and AC/AC transformer of 9v and 1A.

    Nothing exploded this time and i got some voltage readings before the first coil, something about 3v. I think im on the way to get this working now but i didnt tested the coil yet im not sure how to build ir correctly.
     
  14. Neom22

    Thread Starter New Member

    May 1, 2012
    8
    0
    All right, long time since I last posted here but I've made it out to work!
    Heres the deal.

    This is the schmatics of the circuit

    http://imageshack.us/photo/my-images/607/circuitfinal.jpg/

    And these are the real appearence of it

    http://imageshack.us/photo/my-images/854/materiais6.jpg/

    http://imageshack.us/photo/my-images/802/materiais5.jpg/

    http://imageshack.us/photo/my-images/846/materiais4.jpg/

    http://imageshack.us/photo/my-images/20/materiais3.jpg/

    So, for power source im using an electronic reactor for 12v halogen lamps, wich supplies with 12V, 2A max and boosts the frequency to around 33kHZ-40kHZ. In my research i discovered that the higher the frequency the lower is the loss on the transfer of power from one coil to another.

    The coils im using are 70 turns with 20AWG wire on the primary at 7cm of diameter. And the secondary with 6 leds attached on it has around 100 turns with 25 AWG wire with 4,5cm of diameter.

    The circuit works fine and the leds light up stronger as it gets closer to the primary coil, but i have some problems. Im not getting much voltage on the secondary and neither current. On the lowest distance possible im getting around 5 volts and for the current i couldnt measure it.

    Anyone has any idea to increase the current aswell the voltage on the circuit? Should i increase the turns on the primary coil? On the secondary?
     
  15. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    You may need to increase the turns on both.
     
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