TL431 vs ZH431

Discussion in 'General Electronics Chat' started by keyboardcowboy, Mar 3, 2013.

  1. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    I have this LIIOn charger schematic using LM317 and TL431, i couldn't find TL431 so i bought ZH431 instead. The problem is that the circuit does not work with ZH431, frustrated i desoldered a TL431 from a pcb and tried it. The circuit started working, charged the battery and stopped as it should..
    The strange thing is the circuit works with ZH431 if i insert a multimeter to measure current from OUT pin of LM317 and resistor R1 when the meter is removed the circuit stops working..Is there any difference between ZH431 and TL431. I have read the datasheets and could not find any.:confused::confused:

    [​IMG]
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    The ZH seems to have a much lower minimum required cathode current to work. But that doesn't explain much.
    What do you mean by "it does not work"? Can you put some voltage measurements in your schematic when working with the ZH and when not?
     
  3. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    by "not working" i meant that the battery doesn't gets charged. LM317 stays cold, have tried leaving it for long period of time no increase in battery voltage but with TL431 battery voltage rises, LM317 warms up and i have already charged a battery with it. i have 4 ZH and 1 TL, none of the ZH work..i will post voltages asap
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Doesn't the LM317 need 10ma minimum from out to adjust to work properly?
    I think R3 needs to be 120 ohms.
     
  5. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    The 317 chip needs a 10 ma load. You can dump the current to anywhere...like ground.
    It's like a leaky faucet. 10 ma is going to leak out of the regulator. If your load is not as much as 10 ma, the output voltage simply rises until 10 ma can escape to somewhere or it runs out of voltage.
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    You don't need to do that for this circuit. The 10mA maximum bias current out of the adj terminal is supplied by the current through the ZH431. The ZH431 data sheet shows the regulator circuit with no R3, so it is not even needed.

    So, since the circuit in the data sheet is basically the circuit the OP is using I'm at a loss to see why it's not working. :confused:

    Keyboardcowboy, have you checked the pinout to be certain that it is correct? Is it the same package as the TL431?
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The 10mA in question is the minimum load current required in order to guarantee regulation. The ADJ pin current is 100uA max.
    The load current is absorbed by the battery until the CV stage is reached. I would change the 1k to 120Ω to guarantee regulation, but I don't see that this has anything to do with circuit not working with the ZH431.
     
  8. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    But the 1K resistor is there to limit the current through the 431, since it is a shunt regulator when the required voltage is reached on its reference pin it will conduct to maintain the set voltage on its cathode pin like a zener. Wouldn't using a smaller resistor cause a high current to flow through it..pinout of both 431's is the same
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Yes, 120Ω will cause about 11mA to flow through the 431. This is well within the operating current range of the 431, and it is high enough to guarantee regulation in the 317.
     
  10. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    Ok i will try 120 ohm and report back but its a bit strange that the circuit operates with 1K resistor when TL431 is used.
    The input voltage is 12v and i am charging a standard 4.2v 1500mAH battery, LM317 is set at 1.3 amps therefore it should get quite hot which only happens when a TL431 is used. it is ice cold when ZH431 is used (same battery).
     
  11. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    Also how did you come up with 11mA?
    The max voltage at the 431 cathode would be 4.2v which gives 4.2/120=35mA
     
  12. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    120 ohm NOT WORKING
    1.2K NOT WORKING
    12K STARTS WORKING

    Using ZH431

    @ 12K LM317's ADJ PIN sinks 0.2mA
    @ 1K 0.9mA

    Using TL431
    Works with 1K resistor and LM317's ADJ pin sinks 0.8-0.9 mA

    Using ZH431 and 1K resistor (Battery Connected)

    ADJ pin 2.93v
    output=batt voltage

    Using ZH431 and 12K resistor (Battery Connected)

    ADJ pin 2.90v
    output=4.29

    Using Tl431 and 1K resistor (Battery Connected)
    ADJ pin 3.37v
    output=4.22v

     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    In an LM317, the voltage between OUT and ADJ is 1.25V, which is called the reference voltage. When the battery is charged, no current flows into it. Ignoring the current that flows through R4 and R5, the current through the 431 is

    I(431)=1.25V/(R1+R2+R3) + Iadj. Iadj is around 50uA. Do the math.:)
     
  14. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    understood, any idea why 12K works?
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Nope. Maybe you have discovered why ZH431 is not recommended for new design.
    Why don't you use TL431? Did you by 100 of the ZH431s?
    BTW, TL431 will probably not work with 12k.

    What are you using for a load? A partially discharged battery?
     
  16. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    after the ZH431 did not work i did buy the TL431 but i was just curious why it was behaving this way. The battery is dead not completely discharged
     
  17. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    The LM317 is suppose to source current from the ADJ pin but in my case it is sinking current, i can tell this by the negative sign on my multimeter when i measure the current by connecting the red wire to the ADJ pin..why is that so?
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Post the schematic, showing the ammeter in the circuit.
     
  19. keyboardcowboy

    Thread Starter New Member

    Dec 31, 2012
    25
    0
    This is how i connected the ammeter

    [​IMG]
     
  20. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    I believe it is physically impossible for current to flow into the ADJ pin on the LM317, unless it is damaged.
    I added a diode to protect the LM317. See attachment.
    If you want to use the 2-cell arrangement, your supply voltage should be at least 13V.
     
Loading...