help me transistor as a switch

Discussion in 'The Projects Forum' started by papero76, Jan 30, 2016.

  1. papero76

    Thread Starter New Member

    Jan 30, 2016
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    hello
    i'm italian, and i'm sorry for my english, i hope to be clear
    i need a advice
    i'm building a project and i 'd like use a transistor as a switch.
    this circuit put in series or parallel two batteries. I need very low energy consumption.
    what is the best transistor used as a switch?
    what is the resistance value?
    i attachment a drawing, in this drawing the optotransistor work when it has value 5V, but in other circuit part i need that, other transistor work when output NE555 has value 0V.someone has suggestions?
    thank you in advance.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    I am not going to rotate my monitor to look at the image.
    I have rotated, changed perspective and scaled the image using The Gimp :

    papero_img_20160130_190020-1.jpg

    Bertus
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Your circuit won't work, but, if we were to replace the transistor with a switch to achieve the series connection; how can the batteries ever be connected in parallel?

    Perhaps you should explain what, overall, you are trying to achieve?
     
    #12 likes this.
  4. papero76

    Thread Starter New Member

    Jan 30, 2016
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    i want replace variable speed dc motor and brush switch, because i want consume little energy
     
  5. hp1729

    Well-Known Member

    Nov 23, 2015
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    For bipolar transistors general rule is a base current 1/10 of collector current. This would be too much for the opto to deliver. At least two options, Darlington NPN or N-MOSFET.
     
    papero76 likes this.
  6. papero76

    Thread Starter New Member

    Jan 30, 2016
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    which n mosfet?
    what is the resistance value?
     
  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    re: which MOSFET
    What do you have on hand? Most any "Logic level" N-MOSFET, enhancement mode, TO-220. IRFLxxxx for instance.
     
    David Knight and papero76 like this.
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Ok, I don't understand that diagram, it appears to be nonsense. There are better methods for controlling motor speed than to use battery switching.

    There is little point discussing the merits of transistors vs mosfets without an inkling of a viable solution or design.

    Do you have any information on the motor?
     
    David Knight likes this.
  9. papero76

    Thread Starter New Member

    Jan 30, 2016
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    i'm sorry but the diagram has a good sense!
    motor turn 3 brush switch each brush switch close one time a contact and other time other contact.

    i have done a simple question
    how i can have a switch with electronic components?
    transistor
    mosfet
    moc
    or?
    the think very important is low consume energy
    i can't use relay for noise , consume energy and wear parts.
     
  10. David Knight

    New Member

    Aug 4, 2015
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    Please provide more information on the motor.
    From your post, I think you're driving a brushed motor.

    Use a low side n-channel MOSFET.
    Use a gate driver to turn it on and off fast to reduce switching losses.

    Your schematic lacks a method to turn the MOSFET off at all, except relying on gate leakage.
    The optocoupler will also be slow for a MOSFET gate driver.
    Your schematic also lacks a freewheeling diode, which is essential for a brushed motor.
    I think your schematic places the batteries in series with the transistor, which would be wrong.
    Which variant of the 555 timer are you using?
    What's powering the 555 timer?
    Your schematic leaves a lot of important questions unanswered.
     
  11. papero76

    Thread Starter New Member

    Jan 30, 2016
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    HI Mr Knight
    i don't want use motor and brush switch but i want replace them
    i have build ne555 for 800MHz signal
    if The optocoupler will also be slow for a MOSFET gate driver, do i can use other transistor?
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    The NE555 can never reach the mentioned 800 MHz.
    There are versions that can reach 2 MHZ at most.

    Bertus
     
  13. papero76

    Thread Starter New Member

    Jan 30, 2016
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    i'm sorry i've writed wrong
    800 Hz
     
  14. David Knight

    New Member

    Aug 4, 2015
    26
    5
    800MHz sounds fast for a motor.

    From your messages, it's hard to see what you want. You didn't answer all the questions.

    The circuit you use depends on the application.

    If you want bidirectional control, use something like this:
    http://www.microchip.com/pagehandler/en-us/technology/motorcontrol/motor-types/brushed-dc.html

    If you want unidirectional movement, use a circuit that looks like this: http://i.stack.imgur.com/pZM6G.png, with a chip like this to drive an n-channel MOSFET:
    http://www.farnell.com/datasheets/1669445.pdf
     
  15. SLK001

    Well-Known Member

    Nov 29, 2011
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    As blocco said above, your circuit WILL NOT WORK as a speed control. When the transistor is off, there is no current flow. When it is on, both batteries are in the circuit, making your circuit unneeded.

    And, what is a brush switch?
     
  16. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    This is painful, the solution could be very simple but you aren't making sense.

    Forget what you have already presented because it won't work, and instead provide details of the motor, available power-supply, and control requirement? If you can't do this, there is little point continuing.
     
  17. papero76

    Thread Starter New Member

    Jan 30, 2016
    8
    0
    yes the circuit must work so:
    When the transistor is off, there is no current flow. When it is on, both batteries are in the circuit
     
  18. papero76

    Thread Starter New Member

    Jan 30, 2016
    8
    0
    I'm sorry but i don't want use the motor, in the second drawing the motor and brush switch work like ne555 and i can replace it simply. i need a circuit that work as a switch
    for this reason i thought at transistor.
     
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