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Help with an inductive heating circuit!

Discussion in 'PCB Layout, Design and Manufacture' started by Timmsinc, Jul 18, 2018.

  1. Timmsinc

    Timmsinc

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    Jul 18, 2018
    hey guys first post, I need to make an inductive heating circuit using pancake coils. Attached is a circuit design but when I breadboard it nothing ferrous heats up on the coils. When I put an led at the end of the circuig it lights up so I’m getting power throughout the circuit but I’m not getting any high frequency switching !! Any ideas where my circuit has gone wrong?
     

    Attached Files:

  2. duke37

    duke37

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    Jan 9, 2011
    I do not understand your circuit, redraw it in a standard way

    Ferrous is not a good definition of the heated material. Stainless steel comes in two flavours, austenitic which is non or very little magnetic and ferritic which is magnetic and will heat much better. Carbon steel is ferritic and magnetic.

    Austenite has a face centred cubic crystal structure and is the high temperature phase which can be stabilised with alloys
    Ferrite has a body centred cubic crystal structure and is the room temperature phase.
     
    Timmsinc likes this.
  3. Minder

    Minder

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    Apr 24, 2015
    With those L/C components I assume you are aiming for quite a high frequency?
    Google Royer HF circuits for some ideas.
    M.
     
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  4. Timmsinc

    Timmsinc

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    Jul 18, 2018
    Attached is a more succinct sketch of the design being used.
    As for the ferrous material the circuit is intended to heat up hand tool wrenches.

    Thanks!
     

    Attached Files:

  5. Timmsinc

    Timmsinc

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    Jul 18, 2018
    Thank you for the reference, I've found a circuit quite similar to mine on there, which leads me to believe I have something wrong in my physical breadboard...
     
  6. duke37

    duke37

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    Jan 9, 2011
    I may be wrong, it has been known:)

    The new drawing is neater but still the same.
    The circuit looks like a multivibrator with the diodes as feedback. Capacitors would normally be used here.

    One drain goes to the power supply via a choke and has no effect on the heating coils.
    The other drain passes current through the coils but the series inductance will limit its amount

    I would think that both fets should work in a similar way.
     
    (*steve*) likes this.
  7. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Show us your coil.

    Most such inductive heating coils require the object to be INSERTED into the coil - yes, you will get power 'close' to the coil (sufficient to light an LED) but if your coil is what I think it is then you won't get anywhere near the maximum power transfer required to do decent heating.
     
  8. ramussons

    ramussons

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    Jun 10, 2014
    Should'nt the supply (thru' the choke) go to the junction of the 2 coils? Else, what is the function of the upper FET?
     
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  9. Timmsinc

    Timmsinc

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    Jul 18, 2018
    Unlike most of the coils used in this design I am not using a helical coil. The plan is to use a pancake coil and have magnetic slugs to direct the magnetic flux upwards towards the object. Right now the coil is just a template but we are using an O-scope where the coil connection is and aren't seeing any high frequency waveforms. We looked over our physical circuit and it seems our diodes were in the wrong position so we are going to test that later today!
     
  10. Timmsinc

    Timmsinc

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    Jul 18, 2018
    Yeah that came to our attention thanks! We now have the choke center tapped in between the coils.

    thanks !
     
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    I think the slugs would heat up and consume energy intended for the target.
    Is this a school/college project?
     
  12. Timmsinc

    Timmsinc

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    Jul 18, 2018
    I thought that the slugs just redirected the heat but you may be right. Yes this is intended for a college final project, I have lots of stuff I wanted to add on to it but I can't get the basic circuit to heat. It is very frustrating.
     
  13. Timmsinc

    Timmsinc

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    Jul 18, 2018
    UPDATE -- I the circuit works! I had the diodes hooked up to the wrong pinouts on the mosfets... rookie mistake. Using a variable power supply we had the source at 20VDC at 1.18A and we're able to produce a 140KHz slightly distorted AC waveform across the coils. It heat up the metal 10 degrees in about a minute... Any ideas on a current limiter to kill the power after a certain temperature is achieved?
     
  14. duke37

    duke37

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    Jan 9, 2011
    You have some funny units, perhaps you mean 140kHz, not 140KHz.
    Please post the latest circuit. Do both fets contribute to the output?
    A thermocouple or a varistor could detect the temperature. You will need a matching circuit to stop the oscillator, how this is done depends on the specific circuit. I do not think you need a current limit, that may be difficult with fets which are either fully on or fully off.
     
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