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DPDT switch function using MOSFET's: how to?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by earckens, Mar 2, 2018.

  1. earckens

    earckens Member

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    Thks! And what do you think about the feasibility of linking the two bridges together as proposed?
     
  2. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Why did you request that I change the circuit to allow two signals so that direction could be separately energized, and then use a single signal with an inverter? :confused:
    My first circuit would do what you want without the inverter.
     
  3. earckens

    earckens Member

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    Euh? I have no idea how that would be possible?

    EDIT: now I see! Your first circuit, the inverter function is done by M2.

    EDIT2: so the drain of M3 can be connected to the drain of its duplicate if I want two bridges driven by one signal as shown in post #19?
     
    Last edited: Mar 6, 2018
  4. dave miyares

    Dave New Member

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  5. earckens

    earckens Member

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    When constructing this circuit, M1 stays in conductive mode. Exchanged mosfet's, same result?
     
  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    In my first circuit you just connect the the two IN1 inputs together to drive two bridges at the same time.
    Then it would appear the MOSFET is not an N-channel, or you have it wired with the source and drain interchanged.
     
  7. earckens

    earckens Member

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    Both M1 and M2 are N-channel, the same as in your second circuit. When powering up, M2 conducts once, then switches over to M2 which stays in conductive mode (frequency is about 1Hz). Source and drain are correctly wired, I checked and rechecked and..
    I have no clue at the moment; your circuit is very logical, yet here it does not work.
     
  8. dave miyares

    Dave New Member

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  9. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Post your circuit diagram exactly as you have it.
     
  10. earckens

    earckens Member

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    Hi crutschow, see attached. For M1 and M2 I use 1N60, for M3 and M4 I use FU9024 (datasheets attached).
     

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  11. crutschow

    crutschow Well-Known Member Most Helpful Member

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    If that's exactly how you tested it, it has no load.
    It needs a load to work properly.
     
  12. earckens

    earckens Member

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    sorry, I forgot the load: see attached
     

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  13. crutschow

    crutschow Well-Known Member Most Helpful Member

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    My simulation works okay with that load.
    Try connecting another 1kΩ resistor directly across the bridge output.
     
  14. earckens

    earckens Member

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    Same result. Where I am now I have no scope, next week I will. Measurements will be posted.
     
  15. earckens

    earckens Member

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    I put a scope on it: M2 stays conductive, the drain never goes high, the gate does alternate high and low (and M2 gets hot as hell).
    I don't understand this..
     
  16. dknguyen

    dknguyen Well-Known Member

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    To which transistor are you referring to when you say M2?

    Which gate are you measuring that alternates? Q9? If so, that's not a relaible indicator of what's going on with the other gates.

    Looks to me like the gates of Q11, Q10, and Q8 just go high impedance when you stop driving the gate of Q9 and it opens. That leaves the gate capacitors on the other three MOSFETs isolated retaining their charge which leaves them on. It appears to me that once you turn Q9 it will cause Q10 on, Q11 off, and Q12 off. And then it will stay that way even after you shut off Q9.

    MOSFETs don't just turn off when you disconnect their gate. You actually have to discharge the gate capacitor. You can actually test MOSFETs out of circuit this way by using the multi-meter measurement currents to charge up the gate-source cap and then moving the probes to other pins to test if the source-drain is conducting or not.

    Is that what you're seeing? I'm behind on this thread and only skimmed it.
     
    Last edited: Mar 13, 2018
  17. earckens

    earckens Member

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    I decided the check the 555 output. On the scope you can see some strange oscillations every low to high transition, it gets worse at lower frequencies. Is this normal behavior? I thought a 555 would give clean square wave output?
     

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  18. dknguyen

    dknguyen Well-Known Member

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    I assume this is a separate question from the transistor never turning off...

    Is it actually worse? Or are you just reading on a different time scale in each graph? Is the frequency of the ringing about the same between both except for the lower frequency it lasts a lot longer?

    Because that doesn't look like ringing to me. It looks more like your 555 can't decide between thresholds or something like that.

    Exactly what kind of 555 timer are you using?
     
  19. earckens

    earckens Member

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    As a result of M2 never turning on I decided to look at what comes at the gates of M2 and its opposite N-channel mosfet.
    At lower frequencies there are more oscillations before the pulse settles to high.
     
  20. earckens

    earckens Member

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    After startup the very first single pulse is clean, after that the oscillations start: see picture.
     

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  21. earckens

    earckens Member

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    I have a whole bunch of these 555, brand new; all the same behaviour..
     
  22. dknguyen

    dknguyen Well-Known Member

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    I just noticed that your 555 timer is not decoupled. Add a decoupling capacitor across the power pins. You might also want to do the same for the H-bridge itself.

    If those don't work, then add a low value resistor (100 ohms or less, maybe even 10) in series with the gate of Q9. I feel like they should not be required though and your problem is something else.
     
    Last edited: Mar 13, 2018
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