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Transistors & MOSFETS - What am I doing wrong?

Discussion in 'General Electronics Discussion' started by smirks, Sep 26, 2014.

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  1. smirks


    Sep 26, 2014
    Evening all,

    I am banging my head against the wall with this problem.

    What I am trying to do should be very simple - just turn on/off a 12v solenoid with a transistor or MOSFET controlled by my arduino.

    I have read at least 50 blog posts or forum threads on this subject, but just cannot find the solution.

    My electronics experience - beginner.

    I have tried numerous different transistors and MOSFETS, following advice found during my research, but to no avail.

    My test circuit -

    - 3 Blue LED with resistor.
    - 12V adaptor supplying 2A current
    - Blink sketch with pin 13 running to appropriate pin on tran/mosfet
    - All lead positions treble checked and LED lighting up if supplied power direct.
    - Common ground between 12v supply and Arduino
    - Tested - TIP31, IRL8113PBF & some other transistors that I can't find at the moment (desk is a bit cluttered)
    - Transistor/MOSFET all N series and placed after the LED from the 12v supply

    - Circuit is like this but without the capicitor (and with various different components tried:


    Problem -

    - 90% of the time with transistor/mosfet in position LED simply stay on at what looks to be half brightness, as if there is a power leak getting through the component. They do not Blink though.
    - Nearly rest of time LED are either at full brightness (not blinking) or just do not light at all.
    - I did get the test circuit to work with the TIP31. The LED flashed on and off as expected but again were at a much lower brightness than I thought they would be. I then switched in my solenoid (which again works when given power directly) which did not work at all.

    I have spent a fair bit of money on different components now, all to no avail, and simply can't find the answer. All the problem threads I've read end with the initial poster having their problem resolved but the answers don't work for me.

    So, does anyone have any ideas at all?

    Even better, and for my eternal gratitude, could someone list the exact parts and circuit diagram I need to follow to get this to work? The solenoid I need to get to work is this one -

    Many, many thanks indeed in advance.

    Have a great day!

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Hello Smirks,
    The circuit you posted above seems absolutely correctt (even including the freeweheling didoe, good).
    Your problem issues obviously somewhere else. Where is the LED you're speaking off? Could you please add the arduino's pin (13) and the LED to schematc as well as any other components that are connected to this pin?

    There are 2 simple tests you can do to verify the circuit step by step:
    1) Remove the transistor, relay etc. (ecerything shown above). Connect only an LED plus series resistor to the arduino's pin 13. Does the LED blink as expected? If yes, the arduino and your program are o.k. If not, check your program, check the arduino. Try another output of the arduino to see whether something interferes maybe with pin 13 or pin 13 is defect.
    2) Take above circuit without the arduino. Connect a piece of wire to the "output pin" (which is an input for this circuit). tap the wire to V+. Does the relay turn on and off with the tapping of the wire? If yes, this part of the circuit is working correctly. If not, check all connections here, something is wrong. You may also check the input current (from V+ through the base resistor). It should measure as (V+ - 0.6V)/1kΩ give or take a mA.
    KrisBlueNZ likes this.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    yup, would have said the same as Harald .... the circuit shown is good

    and what is this LED you are talking about ?

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    A TIP120 is a darlington. The base current will be about (Vcc - 2.5)/1000 (in amps). Nite that Vbe is specced to be up to 2.5V!

    I was worried about the gain given the relatively low base current, but this device has a gain of about 1000, so you should be able to switch half an amp with a Vcc of 5V.

    However, the voltage drop across the transistor is likely to be in excess of 1V (specced to be between 2 and 4V for Ic of 3A to 5A_. This may make a difference if your solenoid voltage is low.
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