Almost There, But Smoking MOSFETS and Arduinos

Discussion in 'General Electronics Chat' started by Mark7, Jun 24, 2014.

  1. Mark7

    Thread Starter New Member

    Jun 24, 2014
    4
    0
    Hi,

    I'm entirely new here. It would be great to get a hand with
    this power supply challenge. The circuit is attached.

    It begins with a transformer, with 3 secondary wires and 2
    secondary voltages, 30VAC and 10VAC. I want to power a
    solenoid and an Arduino from these.

    I'm using a motor to bench test, the solenoid is sitting
    inside a door.

    The circuit works perfectly when the Arduino is powered
    from the computer. It will vary the motor speed using PWM.
    (Arduino ground and pin 10 are connected into the circuit
    as shown, but not the 5V VCC)

    But upon removing computer power and powering Arduino
    from the 7805, well... smoke, and fast.

    Tried using 100k pulldown on the MOSFET source, and 10k
    from pin 10 to gate, but still smoke. It might have worked
    for a second... difficult to tell.

    All the voltages are good, everything good, until changing
    the Arduino power source.

    Any thoughts? Would be much appreciated. Thanks.

    Mark
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
    2,435
    315
    I see the gurus looking at it.:D

    I can see the problem (PS) but can't figure out how to explain it.:(

    Can't wait to see it explained.

    You can't connect the bridges together (com.)in two places?

    I picture it without the jumper between bridge1- and bridge2-.

    You can no more connect these than the plus sides. Unless fed from separate windings.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,180
    1,800
    Is your goal to have 10VAC out of one bridge and 30VAC out of the second bridge? I don't think this will work.
    On the negative peak you'll get the brown wire trying to go to to different potentials.
     
    Last edited: Jun 24, 2014
  4. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Where is the "smoke" coming from?

    Are you sure the voltages form the two bridge outputs are as desired? My simulation shows both outputs going to the same high voltage. I believe your configuration will not work due to the interaction between the two bridges connected to a single tapped winding.

    Don't know that there's a good way around that problem. :confused:
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
    2,435
    315
    Do you have access to windings or just the tap?
    If not, half wave may be the best you can do.
    Or isolated solenoid drive power triggered thru opto.
    Yes, I think that should work!

    Remove jumper from bridge1- & bridge2-.
    5v supply as-is.
    30v bridge/sol. supply completely isolated from the rest of the circuit.
     
    Last edited: Jun 24, 2014
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,395
    1,607
    That is a good beginning but there are various ways to get 10 and 30 V from a transformer.

    Do you have a data sheet of that transformer or have you measured the voltages? Or both?

    Could you indicate what voltage appears across what windings?

    I believe what you want to do is possible but need that info to make a suggestion.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    That seems like a good solution. The top bridge should be connected between the center tap and the top of the transformer (assuming there's 30VAC between those two points) with no ground connection. The top circuit is then isolated by using a opto coupler to carry the PWM signal from the Arduino to the MOSFET gate. Depending upon the PWM frequency you may need an added driver circuit between the opto and the gate.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
    2,435
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    No points for neatness.:(
    Opto input reversed for sink might be better.
     
    Mark7 likes this.
  9. Mark7

    Thread Starter New Member

    Jun 24, 2014
    4
    0
    Wow! Thank you All for your insights.

    It will take a bit to get these concepts sorted in my mind.

    Will try and then reply

    Cheers!
     
  10. Mark7

    Thread Starter New Member

    Jun 24, 2014
    4
    0
    Thanks again gents for your insights.

    Ernie, the voltages are around 30 and 10VAC. (Lon Guyland, lol) Papa, as wired indeed it didn't work. Crutschow, as you say, the voltages ended up being the same, after frying the Arduino, the MOSFET and the regulator. Well, that's what used to happen... I got to work after seeing Inwo's diagram

    The optocoupler solution seems to work! Moving, wire by wire, the arduino over to get power from the transformer, I got to the final wire as before - the wire to the gate - and prepared myself for instant destruction. But alas, she just kept running along. The updated diagram is attached.

    Looks like the R value is off though, as max DC current per i/o pin is 40mA. I'm wondering whether 5V from the regulator can somehow help drive the opto, without adding another active component.

    IAC, now to test in the real circuit, with 60VDC, and the big solenoid!
     
    Last edited: Jun 27, 2014
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1,607
    LOL... I knew THAT from your first post. But across what terminals of the secondary?

    The two voltages both must share a common reference. Are the two voltages both referenced to the lowest terminal (the gray one), the center red one or the top black one?
     
  12. Mark7

    Thread Starter New Member

    Jun 24, 2014
    4
    0
    They were referenced with the bottom one; file attached..
     
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