Problem during switching of 2N2906 for LCD display

Discussion in 'The Projects Forum' started by CrackJack, Sep 23, 2010.

  1. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    HELLO everyone,
    I am having a LCD display which shows various data on it... One channel on the LCD is used to increase/decrease the contrast of the light.. And to increase/decrease this, I have:
    1. Analog Potentiometer --> turning knob
    2. Digital Potentiometer --> Using +/- button on the front panels

    The analog potentiometer works very well, but the digital potentiometer does not work keeping the rest of circuit same.. Initially I thought that
    the AC source would be producing some noise in the circuit, so introduced a filter.. So, the power section of the circuit is fine...

    However, when I use a digital pot, the LCD behaves crazy.. I do not have a circuit diagram yet... But will post one soon.. I think the problem is with a transistor 2N2906 which is used as a switching transistor. ( The transistor has -22V as VDD)
    Can anyone please suggest me any better switching transistor??

    Please be sarcastic if you do not understand my problem
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Sure you don't mean 2N3906?

    Post your circuit. We'll wait.
     
  3. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    SgtWookie, its 2N2906 http://www.datasheetcatalog.org/datasheet/philips/2N2906A.pdf

    Actually, the circuit prior to this 2n2906 is a Single Push Button Controlled Potentiometer FN6779 http://www.intersil.com/data/fn/fn8205.pdf

    I think there is a case that analog and digital signals are getting mixed, since the buton controlled pot is a digital pot, and the signal provided to it is an analog one,
    do you think I should use a buffer in between the transistor and the pot... SImple Op-amp as a buffer..

    I know you must be guessing a lot for the circuit diagram, so am I... waiting for it..
    ;)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I don't see your circuit posted yet.

    Don't expect any meaningful answers until you do.
     
  5. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    Hello Everyone,
    please see the attached file for the circuit diagram of the
    power supply unit that I have...

    The problem that I am facing is this circuit works in normal temperature conditions, but when I take this to a high temperature-- above 100°F, the contrast simply turns dark... When it turns dark, it is observed that the output of the transistor increases,

    Is there any problem with the maximum temperature capacity of the zener diode? Does it breakdown at 100°F? Can anyone please suggest?

    I also thought that since the chip is digital control, and rest of the circuit is analog, we might need an opamp as a buffer... dunno yet.. :confused:

    The zener diode is BZX79-C5V1,113 in case it is not visible
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    100*F shouldn't affect anything but the LCD itself, they're rather temperature sensitive.

    The only other thing is that you're running something extremely close to the edge. I had a cheap Christmas light sequencer that worked fine until it got really cold outside, turned out they had a transistor just barely biased and the cold reduced its gain just enough to stop it from turning on.
     
  7. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    So, you think i should go for a trial and error basis and check when the LCD gives up???
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Try some sort of selective heating/cooling method to see if you can isolate down to an area or hopefully pinpoint something. Hair dryers or just a soldering iron near something warms things up. Freeze spray or a non-conductive solvent such as carb cleaner will cool down when air is blown across it and it evaporates.
     
  9. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    Hi,
    I did try with a hand blower/hair dryer... and i saw the similar effect, increase in temp causes the contrast to act waywards and weird...
    it became dark...

    What should I do now?
    :confused:
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Try and pinpoint the heat to see if it's the actual display or something in the circuit itself that's changing.
     
  11. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    Hello Everyone,
    Today I observed something new... The temperature of the entire circuit and the room were kept constant., but still the output of the circuit (measured across the transistor) changed...

    We measured the voltage across the zener diode (which should be -5.1V) and it read -6V instead....

    Could zener be a problem? I think the potential problem areas would be the zener diode or the decoder chip (X9511) since, they are the only active elements...(I left out transistor as it does not affect the output voltage)

    Please do correct me if I am going wrong anywer...
    Thanks in advance..
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Your transistor doesn't appear to be used as a switch, as you said. It appears to be an emitter follower. What is it supposed to do?
     
  13. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    No, the transistor is not acting as a switch... I think it boosts up the voltage, i am not very sure as i have not designed this circuit...

    it might also work as a switch as I don't see any biasing resistors around it..
     
  14. Ron H

    AAC Fanatic!

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    What sort of load is the emitter connected to?
     
  15. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    the transistor is driving a LCD screen... I dunno the exact part no, but the
    output of the transistor goes to a pin 10 of the LCD screen input, which requires -22V for proper operation of it..
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Where did you get the idea to use a PNP emitter follower to drive the display?
    It might work, or it might not, or it might be temperature sensitive, depending on what sort of load the transistor sees. A display datasheet is necessary to resolve this issue.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    I believe he stated that he did not design the circuit.

    Why someone would design it that way is anyone's guess.

    But, he needs to find the datasheet for the particular LCD display. Without that, we'll have a hard time figuring out what the operating range of it is.
     
  18. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Sounds fishy to me, I'd double check that and if you can duplicate it again you might have a bad zener.
     
  19. CrackJack

    Thread Starter Active Member

    Aug 7, 2009
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    @marshallf3, we duplicated the effect of heat, when we blowed a hair dryer to it... But again, it was not everytime, that the temperature affected the performance of the circuit, 2 days before, we had everything constant, temp of room was steady at 72°F, but still there was a change in the output voltage across the transistor...

    @SgtWookie, the LCD part no. is Hitachi SP14Q002; I am wondering, that if it would be a problem that all all parts in the circuit diag i provided were analog devices; except the digital decoder chip(X9511), should i use op-amp as a buffer after it; so that the output of the op-amp goes into the
    transistor we are talking about...
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    Well, the maximum operating temperature is 50°C (122°F).

    Datasheet:
    http://www.hitachi-displays-eu.com/doc/sp14q002-a1.pdf

    However, the LCD supply voltage is in a range of 20 to 22v. Since you're using the PNP transistor in an emitter follower configuration, you're going to lose ~0.7v across the BE junction even at pretty low current. But, the supply to the digital pot is limited by the 5.1v Zener, and as the output is configured as an emitter follower ... wait a minute; you posted the schematic as a .jpg instead of a .png, so it's very fuzzy and hard to read.

    Re-post it as a .png image.
     
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