under voltage circuit using op amps

Discussion in 'The Projects Forum' started by shaunAES, Feb 1, 2011.

  1. shaunAES

    Thread Starter New Member

    Jan 26, 2011
    7
    0
    Hi Guy i'm new at this electronic circuit stuff. I need help to built a under voltage circuit using op amps. It's for powering LEDs @ 3.3V but when the 12V battery discharges too much it must cut off the supply to the LEDs.
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Questions...
    1. At what voltage do you want the LEDs to cut off?
    2. What is the current (load) the LEDs are drawing?
    Regards,
    Ifixit
     
  3. shaunAES

    Thread Starter New Member

    Jan 26, 2011
    7
    0
    Hi I need help with building a under voltage circuit using a op amp. The supply is a 12V battery which is going to turn ON LEDs at 3.3V. Before the battery discharges too much it must cut off the supply to the LEDs. Please help me with this circuit.
     
  4. shaunAES

    Thread Starter New Member

    Jan 26, 2011
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    1. The cut off is less than 3.3V

    2. I dont know the current load but it is 2 30W LEDs.

    Regards

    shaunAES
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Two 30W LEDs rated for 3.3v operation would require a minimum of ~9.1 Amperes current if they were wired in series. That's a mighty hefty load. You will require a LARGE battery to get much of a run-time out of it.

    You will also need a switching-type driver, as resistors for current limiters would result in far too much wasted power.

    Do you already have a driver circuit for these LEDs?

    What is your application? Why do you need LEDs that consume so much power?

    In the meantime, look in the General Electronics Discussions forum in the Tips & Tricks thread, post #38 (I think) - if you discharge a 12v battery below ~12.05v @ 77°F, you will shorten the life of the battery considerably.
     
    Last edited: Feb 1, 2011
  6. shaunAES

    Thread Starter New Member

    Jan 26, 2011
    7
    0
    These 30W LEDs are for a solar streetlight. Its a 12v solar system and the LEDs are 3.3V. All I need is a undervoltage op amp circuit to switch the LEDs off when the battery discharges too much.
     
  7. shaunAES

    Thread Starter New Member

    Jan 26, 2011
    7
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    I am using an inverter to get 3.3v
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK, what inverter are you using? Part number and manufacturer, or datasheet please?
    You really need an inverter that will power them in series, otherwise you'll have a much higher current requirement.

    What is the battery or batteries you are planning on using?
     
  9. GT82

    New Member

    Jun 30, 2010
    2
    0
    I would spec the battery first. If you use standard deep cycle lead acid solar batteries outdoors they wont last more than 6 months. Have a look at the battery spec sheet for charging above room temp, higher the temp the fewer cycles you get. We use gel batteries for our solar systems, far more expensive but will last you 3 - 5 years as they can charge at high temp. If you have to use standard lead acids then bury them in stead of putting them on top of the pole. check out Victron: http://www.victronenergy.com/batteries/gel and agm batteries/

    For your cut off the batteries are flat at 12V, as SgtWookie says. let them drop below 9 or so and they will no longer accept charge. You will also need hysteresis as if you disconnect your load at 12V when you go open circuit your battery voltage will rise slightly turning your op amp circuit back on, this will cause your circuit to flicker indefinitely. Check out MC34064, you can build a small resistive circuit to adjust the hysteresis and use it to drive a FET to disconnect.

    Lastly it is always good to design for 3 days automony as this will limit your depth of discharge and increase your reliability in bad weather but increase your cost.
     
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