constant current generator

Discussion in 'General Electronics Chat' started by pradeeba, Mar 11, 2009.

  1. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    Hi all,
    I have developed a constant current provider using HV9921 for LED lighting. The output current of HV9921 is limited max to 20mA. At 20 VDC to 75 VDC output current is 15mA constantly. After 75 VDC output current starts increasing around 115VDC input voltage resistor blasted. my current requirement is 20mA constantly from 85 VDC to 220 VDC. can anybody tel me what is the reason for this?
     
    • CCC.zip
      File size:
      5 KB
      Views:
      26
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    So where is the data sheet for a HV9921?
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    R4 doesn't really serve a purpose other than to waste power.
    I don't see a reason for D5, either.
    C2 seems quite large. 0.1uF should be OK.
    L1 seems very small. Increasing it will decrease the ripple and also decrease the switching frequency of the HV9921.

    You should have a 0.1uF cap between the junction of D2/D4 and -ve.
     
  5. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    Approximately how much should L1 be increased?
     
  6. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    I have increased L1 to 32mH. now from 20 to 58 VDC input the output current is 20mA after 58 VDC starts increasing... can you tel me how much should be the inductor value?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Offhand, I'd say for a single LED, 10mH or higher. If you're going to use several LEDs in series, increase the inductance by another 5 to 10 mH per LED, depending on the Vf.

    The larger L1 is, the lower your ripple will be; but your parasitic capacitance increases.

    The datasheet goes into a fair bit of detail on that.
     
  8. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    If i use an optocoupler(MCT2E) in the place of LED then how much is the value of inductor?
     
  9. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    as per formula given in the datasheet the calculated inductor value is 7.8mH. I tried with 7.8mH then also output current starts increasing after 58VDC. what is the problem?
     
    Last edited: Mar 12, 2009
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Are you following the guidelines in the Layout Considerations section?

    Is your inductor a high-Q type? Are you winding it yourself, or are you using a purchased version?

    If your inductor is going into saturation, you will wind up with increasing current at higher voltages.
     
  11. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    Am using general purpose PCB to conduct this experiment. I have wounded Inductor myself.My inductor is not saturating, current is kept on increasing beyond 20mA for increase in input voltage
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    There's a problem.
    Why don't you try the layout in the datasheet?
    What are you using for a core?
    What are the dimensions of your inductor (core type, core diameter, length, number of turns, etc.)
    How do you know this? Do you have an oscilloscope?

    So you still have a problem.

    Try starting out by using the suggested layout in the datasheet.
     
  13. italo

    New Member

    Nov 20, 2005
    205
    1
    Most LED are with a breakdown voltage 5-10 volts. So if you connect a 75 v of LEDS some may leak more then others. If this is the scenario then you may find out that poof one after the other will fail. there is a solution add a very large resistor across each LED all of the same value so the leakage will be the same for all. the value well it depends on the LED doesn't
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Are you thinking Vf or Vr?
    Must be Vr, because most common LEDs have a Vf of somewhere between 1.3v (infrared) to around 4v (blue and white).

    Vr is generally somewhere between 5v to 7v.
    Did you even look at his schematic, or look at the datasheet? :rolleyes:

    He is powering ONE LED.

    You have no idea what you are talking about.
     
  15. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    From the notes, a design running < 85V is slightly different than one designed for higher.

    What is the purpose of the 1N4001 diode after L1 in your schematic?

    If the current increases, the inductor is going into saturation. What is the core and number of turns, or, if you have an LCRZ meter, what is the value and Q of L1?
     
  16. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I think that reference was for AC vs DC; AC input requires rectification and filtering - or perhaps I'm missing something?

    You meant D5, a 1N4007, right? I don't see any condition where it might become forward biased either.

    That's my train of thought as well. Additionally, in the datasheet under the layout considerations, numerous cautions are made along with specific recommendations. That's why I suggested above that he start with the recommended layout.
    I have a feeling that pradeeba is using an inappropriate core, and/or may not be isolating the core from the windings by using tape or a bobbin, and/or may be using so many turns that the self-capacitance of the inductor has risen quite high.

    Ronald Dekker's "Flyback Converters for Dummies" page is a good resource for projects like this:
    http://www.dos4ever.com/flyback/flyback.html
     
  17. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    For the value of 2.1mH the parasitic capacitance is in the order of nF. if i increase the inductor value then it will be in the order of pF. what do you suggest?
     
  18. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    Hi,
    can anybody help still am not getting constant current output. i have changed the inductor value to 33mH. still same result............... am using axial leaded inductor now,
     
  19. pradeeba

    Thread Starter Active Member

    Feb 27, 2008
    104
    0
    Am getting constant constant current after connecting 175mH. is this value is correct? will this high value inductor affects my circuit?
     
Loading...