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Can I do this with 2 Mosfets, LEDs in series.

Discussion in 'LEDs and Optoelectronics' started by JPU, May 17, 2013.

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  1. JPU

    JPU

    281
    1
    May 19, 2012
    Hi

    I am back on an old project which I am struggling with. I am trying to control two high output LEDs. I use a 2009 micro puck wired as a Buck/Boost driver which allows me to have a battery 3.7V. This is increased to 7V by the Puck and the mA are held at 300mA.

    I am using a Picaxe chip to control 2 mosfets and I have written the code to take advantage of the Picaxe "touch" command. The problem is the micro puck must not have power to it if there is no LED connected. I have included a small drawing and I wonder if you have a suggestion of how I could introduce the second LED in series using the 1 extra mosfet? So that I can control the LEDs ie 1 on or 2 on. They must be in series. I have used switches to represent the Mosfets.

    Any ideas would be great!

    Thanks in advance
    JPU
     

    Attached Files:

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    1) I'm not sure that the wiring of that "puck" is right.

    2) place 2 LEDs in series and wire them from the 7V output to ground.
     
  3. BobK

    BobK

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    Jan 5, 2010
    I think the question is how to control, from the micro whether 1 or 2 LEDs are on, with the LEDs in series.

    You can wire them in series, then wire an N channel MOSFET in parallel around the one nearest to ground. Turn that MOSFET on, and only 1 LED lights, off and both light.

    Caveat: if the puck does not respond fast enough, the top LED might be giving a killing burst of current when the MOSFET is turned on.

    Bob
     
  4. BobK

    BobK

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    Okay, now I cam confused, I looked back at your other thread, and it seems you want one or the other lit, not both, but then why in series? Please tell us what states you actually want.

    Bob
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Concerning that caveat, if you limit the switching speed of the mosfet you will give the power supply time to react to the change in load.

    You could use a P Channel mosfet to turn off the "top" diode, and an N channel (as you suggested) to turn off the bottom one -- both, as you said, by shorting the diode. A logic level mosfet would be required in both cases and you would have a resistor from source to gate to keep them turned off until you pull the gate high (for the N channel device) or low (for the P channel device) to short out one or the other LED.
     
  6. JPU

    JPU

    281
    1
    May 19, 2012
    Hi

    Thanks, they are to be connected in series. The buck/toot is wired correctly and limits the 300mA. In this setup the buck boost a 3.7V battery to the requred 5V whilst providing a constant 300mA

    The LEDS are placed in series and I believe therefore both get the 300mA. (I hope this is correct) The buck most always have an LED connected when power is supplied or it will blow instantly (I know, I have blown a few).

    I simply want the user to be able to select 1 light on or both lights on or all off.

    I will give your N channel mosfet suggestion a go, thank you

    JPU
     
  7. JPU

    JPU

    281
    1
    May 19, 2012
    Hi

    I have tried to use the n channel mosfet as mentioned. By placing the mosfet in parallel with the last LED. However, it will not function as you have described. I think this may be something todo with the way the circuit is working as I did take a wire from a point before the LED and skip over the LED to a point the other side of it. The LED stopped emitting light but the same thing will not happen if I use a mosfet?

    I have included a copy of the datasheet which accompanied the buck.

    Would you have another suggestion or perhaps you can see that I am doing something wrong??:p

    Thanks

    JPU
     

    Attached Files:

    • buck.jpg
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  8. BobK

    BobK

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    Show us how you connected the MOSFET, and link to the datasheet for it.

    Bob
     
  9. JPU

    JPU

    281
    1
    May 19, 2012
    Hi

    Please note, I have to be treated as a simpleton when it comes to these circuits.

    Here is the circuit, I did try several configurations in case I had misunderstood the setup of the mosfet..but I couldn't get any useful results. I then rigged a completely new simple circuit (light a LED from a battery using the mosfet) without the buck to test and that worked as expected.

    So I assume its the buck that is preventing this working as expected.

    Here is the link to the PDF for the mosfet being used,

    http://www.irf.com/product-info/datasheets/data/irld014pbf.pdf

    Any help to get this circuit to work would be great. I would prefer not to use relays due to space limitations and battery life.

    Thanks for your help.
     

    Attached Files:

    Last edited: May 20, 2013
  10. BobK

    BobK

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    Jan 5, 2010
    You have the source and drain reversed.

    Bob
     
  11. JPU

    JPU

    281
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    May 19, 2012
    I have tried it the other way and no success....Should I be using a P channel?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You'll also need a resistor from gate to source to leak away the gate charge when you open the switch. Without this the mosfet will stay turned on, and the gate will be at risk of damage.

    It's always worth drawing the mosfet with the body diode showing. That will tell you if you've accidentally planned to connect it in a way that the body diode will always conduct. (in this case, the LED would never light because the forward drop of the body diode is less than the forward voltage of the LED).
     
  13. BobK

    BobK

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    Jan 5, 2010
    And, just checking, the two grounds are connected together, right?

    Try connecting the gate to the + and to ground alternately to make sure it is acting as a switch. When open, as Steve said, you don't know what it will do.

    Bob
     
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