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Vss vs. Vdd vs. Gnd

Discussion in 'Troubleshooting and Repair' started by RBraverman, Dec 16, 2017.

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  1. RBraverman

    RBraverman

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    Dec 16, 2017
    Hope this is the right place ... and no laughing please, at least not to my face ...

    I have not played with circuits for a very LONG time, and now I find myself confused on what should be an easy point.

    I'm replacing the display LCD in my wifes favorite radio, which is about 12 years old. Part of the pin-out is:

    Vdd

    Gnd


    The replacement is using VSS/VDD/V0 -

    Vss can not be ground - correct? Everything I've read claims/states Vdd is/would be Gnd and Vss supply.

    It's crazy, as the more I read the more this easy transformation becomes difficult! AND it shouldn't be.

    Anyone care to hold my hand?
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    Can you show photos?
     
  3. RBraverman

    RBraverman

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    Dec 16, 2017
    Thank you,

    I guess one of the radio's wiring harness and one of the replacement LCD?
     

    Attached Files:

  4. kellys_eye

    kellys_eye

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    1,146
    Jun 25, 2010
    Vss and Vdd are your power lines. Vo is a variable voltage taken from a preset resistor wired across the supply and is used to control the LCD contrast - without which you won't be able to see the digits.
     
  5. RBraverman

    RBraverman

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    Dec 16, 2017
    So Vss and Vdd are interchangeable - in this application anyways?

    Then what is ground in/on the new LCD?
     
    Last edited: Dec 17, 2017
  6. Bluejets

    Bluejets

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    894
    Oct 5, 2014
    Connections for Vdd and Vss not normally interchangable.

    This instance shows whomever made the first photo unit perhaps is Chinese maker and Chinese/English confusion set in.

    Try this diagram for you new unit.
    I would guess the Vdd on your old one is positive supply and Gnd is negative supply. Best to check with a meter.

    Follow the diagram carefully as the layout is left>> right and yours is right>>left for connections.

    Vo on new unit may be Va on old unit (voltage adjust perhaps?)
     

    Attached Files:

  7. RBraverman

    RBraverman

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    Dec 16, 2017
    You ARE correct - that was what I kept getting led astray by, in other posts as I searched.

    TWO in a row - I like you.:)

    Now then everything works DANDY except this, the radio - via it's menu's, has an option to adjust the backlight brightness, which NOW adjusts the character brightness - the backlight stays full on, which it would as it's got a Vdd connected to it.

    I can't remember how this DID work, but If I swap the Vdd from pin 15 and Va/Vo from Pin 3 the display is dark. I can leave it full on, but it BUGS me.

    ************ UPdate ******************
    Just realized I have a portable radio by Grace the same manufacturer and the character light and backlight are adjusted in tandem when adjusting backlight, but jumping Pins 3 15 do get me that adjustment.

    I tend to ramble.
     
    Last edited: Dec 19, 2017
  8. davenn

    davenn Moderator

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    1,884
    Sep 5, 2009

    Vss = +V rail
    Vdd = -V rail
    GND = 0V rail

    this is indicating a split power supply rail requirement
     
  9. Bluejets

    Bluejets

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    894
    Oct 5, 2014
    15 (anode ....+) and 16 (cathode...-) are the LED connections. They are completely independant from the other cabling in the new LCD. You have obviously connected the character adjust on the new unit to the backlight adjust of the existing wiring. If this is the case you will have to change that and then provide some manual adjust as shown in the diagram I posted earlier (ie pot adjuster) for the character adjust. 10k value normally
     
    Last edited: Dec 19, 2017
  10. RBraverman

    RBraverman

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    Dec 16, 2017
    AGAIN THANK YOU!

    I see no other way. If Pin 3 (LED on old LCD) with Pin 15 (nonexistent on old LCD) are swapped - blank display.

    As noted I wouldn't know what to change.
    I believe this is as correct as possible, even though the backlight adjustment adjusts just the character illumination - not both. I'm wondering if the old LCD Pin 3 was a control?
    The pot would easily work (plumb it out the back) - I actually thought of that (yeah I know, surprise), but 15 minutes of rummaging through whats left of my parts (after the movers had their pick before they delivered everything) found nothing that would work. Nor is there any place ANYWHERE near me to buy something.
    I've put a ~300 ohm in line with the LED - Pin 15 - and it's dimmed both displays to a decent level.
    I may just leave it that way.
     
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    Pin 3 needs to have the adjust as shown in post #6 to display characters.
    However, if you are happy with the way it is then that's ok.
    Good that you actually get it to work.
     
  12. RBraverman

    RBraverman

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    Dec 16, 2017
    I don't understand your post, it's either the way I posted MY last reply, or I'm am lost. I say this, because this seems to happen to me a lot to me - everything makes sense then something like this. So if you would kindly bear with me ....
    If I put full 5V to Pin 3 - displays blank. So what did I not get in your post about adding a pot there?
    Other than adding a pot to the Pin15, which would adjust both character and backlight, I'm lost in regards to what you are saying.
    It seems to me that whatever the LED lead was/is coming from the radio controlled (which never has a voltage higher than +1.8) controlled a hmm ... what, a ... "bridged" character/backlight circuit in the OLD LCD?
    So in other words, I've already got a "pot" on Pin 3 controlling the Character Illumination (within the software of the radio), and I need another, physical "pot" to control the backlight via Pin 15.
    Thank you! AGAIN
    I've still got it laying out on the bench until I've exhausted my possibilities.
    I appreciate your responding, what your saying may just well be Waterford Crystal to everyone else - just not me, and I just need to know if my memory is getting worse.
    Thank again.
     
    Last edited: Dec 20, 2017
  13. Bluejets

    Bluejets

    4,216
    894
    Oct 5, 2014
    Backlight and character adjust are 2 completely different items. Normally one applies power to terminal 15 and ground to 16 to turn on the backlight. This may be different in your original where you say it is software adjusted. I have never seen this done however it is easy to do via pwm from any microcontroller. Terminal 3 is a character adjustment ( normally a 1 off adjustment hence the suggested trim pot). Character adjustment via a pot applies a variable voltage via the pot which results in " no character visible at minimum to a blank white square at maximum". By your applying 5v to this terminal you are getting the latter as you described.
     
  14. RBraverman

    RBraverman

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    0
    Dec 16, 2017
    OOooooooooooook, here's the outcome;

    Vo - as it comes from the radio, isn't a variable 5+, it's a variable ground - something I've never seen before. Now I realize that is how a pot's wired, but in order to get this work the way it should/as in the original, Vo needed to be grounded to give full brightness to the characters (in relation to the LCD). When the LCD lead, as it comes from the radio, is connected to Pin 16 (Vss) and Vdd to Pin 15 it works perfectly.

    I've never seen that before, but that's the way it is - now if I'd only been able to get a proper sized LCD and C/B - I can't get this IN the radio. I measured side-to-side, not height
     
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