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Parallel/Series DPDT With MOSFETs Help?

  1. Feb 19, 2017 #1
    Hi everyone, semi beginner here looking for a little help wiring this system. I am looking to use MOSFETs to handle higher current levels and a DPDT switch with toggle between on+off/series/parallel configuration. My idea for the wiring is here:
    IR3YAyX.png
    However I am not sure how to go about wiring the MOSFETs in with the DPDT, any help with a wiring diagram or education is greatly appreciated, here to both do and learn!
     
  2. jcsd
  3. Feb 19, 2017 #2

    rbelli1

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    Can you explain what you are connecting in parallel and series? It it the batteries or a pair or loads? Why is a switch no good for this?

    BoB
     
  4. Feb 19, 2017 #3
    Im connecting them in parallel and series so the person can pick which type they would like to use. This is for a friends birthday present so the series option would be for 8.4v, 30-40 amps, the parallel option would be 4.2v 30-40 amps. Its an electronic cigarette/vape where the user can build a coil to a specific resistance in order to benefit from both options (lower for parallel vs higher resistance builds for series)

    The DPDT switches I have found that are small enough to fit within the ~3x5x1 enclosure max out at 20 amps so I was looking to use MOSFETs to handle the current/load.
     
  5. Feb 20, 2017 #4

    rbelli1

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    That circuit will be a lot more complicated than just hooking up some wires. Also with those kinds of loads on lithium ion batteries it can get real explody real fast if you don't know what you are doing.

    BoB
     
  6. Feb 20, 2017 #5
    Correct and he knows what hes doing in terms of controlling amp draw via resistance coils, my concern was using gates on MOSFETs to handle the amperage through the DPDT switch
     
  7. Feb 22, 2017 #6

    Baluncore

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    Here is a circuit that you might consider for the job. I offer no guarantee. Any circuit will become explosive if you get it wrong.
    Notes:
    1. Mosfets have an intrinsic substrate diode, sometimes paralleled by a Schottky diode. The design must account for that ever-present diode in all situations.
    2. Controls A and B select parallel or series operation. They should be changed while the batteries are disconnected. Never set A and B high = Vcc at the same time. Always tie B low = Vee when changing A.
    3. M1 is a P-Channel mosfet, all other mosfets are N-channel.
    4. R1 is the heater element.
    5. Press the “fire” button to turn on the heater. Wear safety goggles during testing.
     

    Attached Files:

  8. Feb 22, 2017 #7
    OK, by your schematics I could understand what the OP wants.
    Just one remark: switches has an inherent protection against unwanted shorts, what's the distance between the poles.
    FETs has nothing like that. There should be some kind of safety delay for switching between the states.
    Even better if the 'stable' state is the 'all off', and any switching can be done only through that state.
     
  9. Feb 22, 2017 #8

    Baluncore

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    I considered pull-up and pull-down resistors to safe voltages, but then that current drain would be continuous. It will take a CMOS flip-flop and a couple of gates to build a latch capable of cleanly conditioning the switch, and so managing the A and B gate voltages safely. But designing that is another problem; working out how to minimise the investment in mosfets was enough of a problem.
     
  10. Feb 22, 2017 #9
    I think some mechanical solution might be OK. One SPDT to control the FETs...

    Maybe this is the case when the 'solution' is just more complicated than the original problem?
     
  11. Feb 22, 2017 #10

    Baluncore

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    Can you control A and B safely with just a switch, without pull-up resistors that waste battery power ?
     
  12. Feb 22, 2017 #11
    If the control is a switch, then I think you would need just G-S resistors?
    But it's possible that SPDT won't be enough...

    Ps.: it would be good to know the expected current. If it's not out of the range what an average switch can handle, then I think the best solution for the problem is to forget it. It can be done with FETs, but it's not trivial. Sometimes the good old switches are just better.
     
  13. Feb 22, 2017 #12

    Baluncore

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    Which is too much current for a small toggle switch.
     
  14. Feb 22, 2017 #13
    True. I missed that part about the current.

    It's harder than I thought. Since there is no fixed voltages in the system and the poles are actually separated by default, it's hard to find an adequate gate drive voltage. Otherwise your schematics fits the bill (with some additional G-S resistors).


    One more issue. The 4.2V suggests that it's a Li-ion battery. Is the 40-50A fits with the maximal current of the battery? Lead-acid can supply some insane currents for limited time, but Li-ion is a bit sensitive for shorts.
     
  15. Feb 22, 2017 #14

    rbelli1

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    How about going hard series. Then add PWM to the "Fire" control. Instead of high/low/off you get anywhere in between. That is probably easier to make safe and needs only one FET.

    Make sure that your control knob goes to 11. That one gets hotter.

    BoB
     
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