Transistor choice

Discussion in 'General Electronics Chat' started by The_big_dill, Jan 5, 2013.

  1. The_big_dill

    Thread Starter New Member

    Jan 5, 2013
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    Hi all, first post here,

    Glad to finally find a forum where i can read a ton of useful info and make sense of it :)

    I have been fascinated by LEDs and sensors for at least a year now and cannot get enough of the fun things i could do with them, along with controllers.

    However i have hit a roadblock where i have a 12V 6W strip of LED lights which i have no problem powering with a computer power supply, however i was looking to control the power it receives using a micro-controller (Arduino).

    The problem at hand is the choice of which transistor do i use?

    Arduino can supply a maximum of 5V and 20 mA, while this strip requires 12V and 2A (P=VI).

    I have been told to use "some kind" of FET and that they all have some activation voltage and some peak. These uncertain answers really confused me and discouraged me as i researched online and looked into electronics stores (people in the store told me they carried them, knew nothing about them though), haven't really gotten an answer for what to do next.

    I have several basic transistors, but none that can handle more than half an amp...

    Any guidance or assistance would be great!

    Cheers!
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,986
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    You need a "logic level" type MOSFET rated at 4A or greater. If you can switch the ground (common) side of the LED circuit then you can use an N type MOSFET with the LEDs connected between 12V and the MOSFET drain connection. The MOSFET source connection goes to ground and the MOSFET gate connection goes to the Arduino output control signal.

    When the Arduino output is high the MOSFET will turn on, connecting the negative side of the LEDs to ground and turning them on.
     
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  3. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Arduino can supply a maximum of 5V and 20 mA, while this strip requires 12V and 2A (P=VI).
    That's all it can put out ???? What the <snip> is it good for ???
     
  4. The_big_dill

    Thread Starter New Member

    Jan 5, 2013
    10
    1
    Its not a very high end micro-controller, its made for hands on experience and beginners like myself.

    It has solder-less connections on it, like a breadboard, making it good for experimentation.

    The chip on it is 8-bits, giving a reason for why people generally don't use it at the higher levels of experience.

    However it still fulfills the job of a micro-controller by taking inputs and giving outputs with a user friendly programming environment, all for $30.
     
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  5. The_big_dill

    Thread Starter New Member

    Jan 5, 2013
    10
    1
    Thanks for your assistance, i have 2 questions to follow this.

    1. How did you decide on the MOSFET and not any other FET? I assume its because of its wide availability and popularity.

    2. How much voltage do i know i need to apply to the gate with Arduino without over or under powering the transistor?

    Edit: i found an MJE182-1, rated at 80V and 3A, would that work?
     
    Last edited: Jan 5, 2013
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    You'll want to look for a "Logic Level" MOSFET.

    These are spotted by either having "Logic Level" in their title, or in the datasheet where RDSon is specified @5V Vgs. A widely used, but older example is the IRLZ44. There are newer devices available that are logic level with a lower RDSon and faster switching.

    There is another thread on this topic right now by a different user.

    MOSFETs are generally better for power switching when compared to JFETs, which tend to have good linear characteristics (vs. switching on/off).

    There are a few very common MOSFETs that are typically used due to their price and wide availability. Members here are from all over the world, so frequently mentioned parts tend to be mostly globally available.

    Datasheet Catalog is the best source for datasheets I've found, simply enter the part number and you'll get all the info you need.
     
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  7. MrChips

    Moderator

    Oct 2, 2009
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    We have to back up a bit here.

    You have a 12V 6W LED strip that you can power from a power supply ok.
    Now you want to use an Arduino. What do you wish the Arduino to do with the lights?
    I can only assume that you want the Arduino to turn on and off the LEDs.

    Hence you will need a power supply that is 12V or slightly higher than 12V, and a transistor that can handle about 25V and 1A. A MOSFET will work, and so will a NPN transistor.
     
  8. The_big_dill

    Thread Starter New Member

    Jan 5, 2013
    10
    1
    Wow, thanks, that simplifies my search by a lot... I guess logic level is the term 2 of you emphasized that is the key to my dilemma, on to reading i go!

    Thanks for your help!
     
  9. The_big_dill

    Thread Starter New Member

    Jan 5, 2013
    10
    1
    The Arduino has a electret speaker that senses sound waves, the chip on the Arduino is then programmed to use a certain threshold of sensor data to enable the LEDs.
     
  10. crutschow

    Expert

    Mar 14, 2008
    12,986
    3,224
    1. MOSFETs are designed for power applications. JFETs are mainly used for small signal use.

    2. Typically MOSFETs can tolerate at least 20V gate-to-source voltage without damage. There's no such thing as underpowering or overpowering a MOSFET. The voltage of interest is the amount of gate-source voltage needed to fully turn it on in switching applications. For a "logic level" device that's usually 5V (or sometimes 3V).

    A MJE182-1 is a bipolar transistor and requires a base current of about 1/10 of the collector current to turn on. The Arduino can't provide that much current for you load so you would have to add another transistor for more gain. That's why a MOSFET is preferred, since its a voltage operated device and easier to turn on from a low current source.
     
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