How do I work out the transistor needed?

Discussion in 'Homework Help' started by Jez365, Dec 22, 2016.

  1. Jez365

    Thread Starter New Member

    Dec 22, 2016
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    Hi, I need help with determining the transistor needed to power about 17 solenoid valves with a Raspberry Pi which produces 3.3V. I have made this diagram with the circuit:
    [​IMG]
    (There will be 17 connections to GPIO pins on the Raspberry Pi, 17 transistors and 17 solenoids)
    I need to know how to work out the transistor needed. Also, will any 12v power supply suffice? And do I need a resistor in this circuit?

    Thanks for any help.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    19,130
    6,148
    You need a transistor that can handle 12V @ 1A.
    A logic-level MOSFET such as IRLZ44 would be suitable.

    The 12V power supply has to be capable of delivering at least 10A.

    You need a 1kΩ resistor in series between the Rpi output pin and the gate of the MOSFET. This is to protect the Rpi output pin in case of a catastrophic failure of the MOSFET.

    You will also need a reversed biased diode such as 1N4001 across the solenoid.
     
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  3. Jez365

    Thread Starter New Member

    Dec 22, 2016
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    Thanks so much for the help! Here are the parts I'm going to buy:
    Transistor: IRFZ44N IRFZ44
    Power supply: 12V 10A 120W
    Resistor: A 1kΩ from this pack
    Diode: 1N4001
     
  4. Jez365

    Thread Starter New Member

    Dec 22, 2016
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    Is this diagram correct?

    [​IMG]
     
  5. MrChips

    Moderator

    Oct 2, 2009
    19,130
    6,148
    Your circuit diagram is not correct.
    The resistor should go where the diode is shown.
    The diode goes across the solenoid, cathode lead (band on the diode) connected to +12V supply.
     
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    8,285
    1,447
    Like this, you can omit R2,,

    MOSFETDCMOTORSWITCH.jpg
     
  7. Jez365

    Thread Starter New Member

    Dec 22, 2016
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    Ok, I think I have worked it out now. After some more research, I have made some changes:
    [​IMG]
     
  8. bertus

    Administrator

    Apr 5, 2008
    19,917
    4,144
    Hello,

    If the red line is the + of the powersupply, then you have the diode the wrong way around.

    Bertus
     
  9. Jez365

    Thread Starter New Member

    Dec 22, 2016
    12
    0
    Oh, I missed that. I have fixed it:
    [​IMG]
     
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