Not getting desired 700mA in the marked area..

Discussion in 'General Electronics Chat' started by corefinder, Nov 20, 2012.

  1. corefinder

    Thread Starter Member

    Oct 6, 2011
    55
    0
    Hi, this is simple LM393 based led driver circuit. I made this circuit and applied 6V DC to it. But not getting the desired current of about 700 mA across the Leds. I put 47 H inductor and removed R2 ,VR1 and R3 and directly connected -ve pin of ic1 to end of R1. Can anyone help me know where iam wrong and how should i get 700 mA through LEDS...Thanks in advance !!
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    How are the Vf,If of LED?
    How many LEDs are you series with?
    How many voltages cross on those LEDs?
     
    Last edited: Nov 20, 2012
  3. corefinder

    Thread Starter Member

    Oct 6, 2011
    55
    0
    specification of single led
    Vf is 3.6 volt
    If is 350 mA
    and i am using a pair of two leds in series and voltage across leds is less than 6 volts maybe 5.8 volts. Leds are glowing but current coming is only 80 mA but led circuit can take upto 700 mA.
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Difficult to say without knowing about Q3.

    Also isn't that a buck configuration? Then you can not reach enough forward voltage.
    You should use the LEDs in parallel, or use a booster, or use higher input voltage.
     
  5. corefinder

    Thread Starter Member

    Oct 6, 2011
    55
    0
    Q3 is MJE2955 transistor. I dont know its specs and yes its a buck configuration but why forward voltage is so important as leds are glowing but current is very less. Yes maybe voltage across each led is less than 3.6 volt but why current is not flowing as it should be.
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    In the schematic is it drawn as MOSFET.

    But that is secondary.

    If you look in the LED datasheet, you can observe the LED dI/dU is rising very sharply. The region between the threshold voltage and the voltage where nominal current flows is typically only a few 100mV.

    If the voltage is less than this nominal voltage, the LED will glow of course, if the voltage is higher than the threshold voltage.

    Power LEDs, even regular LEDs can be used by controlling the voltage very tight. In most applications however, current control is prefered.

    A buck converter never can deliver 7.2V from 6V input voltage.
     
    corefinder likes this.
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If Q3 is MJE2955, it's a PNP middle power transistor.
    R8 chnage to 4.7K, C connect to the minus pin of D3, E connect to +6V.
    Because the voltage is too less, so the LED use in parallel is better, but you just don't forget that every LED series a resistor to limit the current.
    You also can add MC34063 to increase the 6V battery raising to 20V or 30V, and using the LED in series.

    MC34063 datasheet.
    http://www.datasheetcatalog.org/datasheet/motorola/MC34063A.pdf

    MC34063 Universal Calculator
    http://sourceforge.net/projects/mc34063uc/

    MC34063 Calculator Online.
    http://www.bobtech.ro/tutoriale/com...e-mc34063a-mc34063-step-down-step-up-inverter
     
  8. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    Isn't so easy from 6V to 30V with that small chip, having a NPN switch. You'd need a good inductor.

    When you limit the current, and when the LED just lights up a little, why would you want to add a resistor?
     
  9. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    The 20V or 30V just for the transistor or mosfet of the LED needed, then R8 should raise more, maybe 10K or more.
    How many voltages should be raising to, it depends on the numbers of LED, it maybe not need that high, but according to the calculation of MC34063, if you want to rasing to 30V is Ok, specially the LEDs just need the current is not much.

    If you want to arrange the LEDs in parallel from 6V, then you will need the resistor for each LED, it just avoid to damage the LEDs.

    If you don't want to change the transistor and change the circuit, and use 6V for power, then use the LEDs in parallel is a good way.
     
    Last edited: Nov 21, 2012
Loading...