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Weird Voltage divider issue

Discussion in 'General Electronics Discussion' started by SparkyCal, Aug 16, 2020.

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  1. SparkyCal

    SparkyCal

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    14
    Mar 11, 2020
    So, I have been building this same circuit, a number of ways (see circuit here:https://www.electronicspoint.com/forums/threads/a-stretch.293212/

    I used a LM741 op amp and it worked. Did the same circuit, this time incorporating an additional resistor as suggested by AudioGuru- and it worked.

    Now I built it using the 4558D op amp. It worked but then when i tried it the next day, it did not. I found that the voltage was not being properly divided by the voltage divider. I found a faulty resistor and replaced it (one of the 10 K ones R33)

    Tested it and it indeed dropped the voltage to roughly 4.5 volts. But minutes later, it went back to close to 9 volts. How can that be?
     
    Last edited by a moderator: Aug 17, 2020
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,015
    2,506
    Nov 17, 2011
    As usual: show us a circuit schematic with component designators and values. It is real hard to imagine what you're doing. Remember: you'r sitting in front of your circuit, we don't.
     
  3. SparkyCal

    SparkyCal

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    Mar 11, 2020
    Sorry Harald. This might help further define the issue. I don't need help with the entire circuit. For whatever reason, i have a mental block which causes me to sometimes build the voltage divider correctly, and sometime not. For the ones I have done correctly, they are already glued into stompboxes and it would be messy to take them out to look.

    In light of that, I have attached a schematic in which i have circled in red, the part of the circuit that I keep building correctly, and incorrectly. I think i need to look at it in a different way.

    I am wondering if someone might be kind enough to draw this part of the circuit in a clearer way that would make it easier for me to interpret. It could even be a hand written drawing on a napkin.

    Here is is
     

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,015
    2,506
    Nov 17, 2011
    That cannot be helped by looking at the schematic. One would have to see the actual build. From the schematic I can see no obvious reason why R33 should go short circuit.
    Check the connection between R33 and R33 and also of the other side of R32 to GND (0 V).
    The schematic is clear to me. It shows all components, all connections, no crossings that could be misinterpreted. Which part would you not understand?

    Cheers,
    Harald
     
  5. SparkyCal

    SparkyCal

    362
    14
    Mar 11, 2020
    Hi Harald:

    It's a mystery to me because the night prior, the entire pedal was working. The next morning, i try it again and it was not working. I checked the voltages to the pins, and it seemed like the voltage was no longer being divided. I replaced one of the resistors which I thought may have been faulty. The meter showed the voltage properly divided. But that only lasted on measurement. When i tried the pedal, it reverted to full voltage without it being divided.

    In light of this, I'm not sure that i mis understood the schematic, or perhaps something else is causing the voltage divider to fail. Not sure if that is even possible.

    Perhaps I need to go back to the breadboard to ensure the divider is being assembled properly. as mentioned, have made this pedal a few times, and never had this problem on the others. They still work.
     
  6. SparkyCal

    SparkyCal

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    Mar 11, 2020
    I drew a pic of how I hooked it up. The dotted circular area is what i am particularly unsure of
     

    Attached Files:

  7. bertus

    bertus Moderator

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    Nov 8, 2019
    Hello,

    When using a breadbord, you can have bad contacts that are intermittend when a part is moved.
    The contacts in the breadboards are strips that can go bad when a to thick wire has been forced into it.
    Read the attached PDF and have a look at the image I post.
    Some breadboards have a break in the power lines in the middle of the board:

    breadboard-connections.jpg

    If you want to make a drawing of a breadboard, you can use Pebble, a third party program from picaxe:
    https://picaxe.com/software/third-party/pebble/

    Bertus
     

    Attached Files:

  8. ratstar

    ratstar

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    Aug 20, 2018
    Id say it would be R32 that is not connected, by what you said what was happening.
     
  9. Martaine2005

    Martaine2005

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    May 12, 2015
    This implies you are NOT using the breadboard.
    How did you connect the components?.

    The resistor you replaced, did you measure it with your meter?.
    Was it faulty?.

    As already mentioned, using thicker wire or thicker legged components in the breadboard will cause bad connections for thinner legged components.

    Martin
     
  10. SparkyCal

    SparkyCal

    362
    14
    Mar 11, 2020
    You're right. i did not use a breadboard. I used a PCB. The resistor did appear to be faulty after measuring it. When i connected a new one, i got the half voltage- but only momentarily. Then it went to over 8 volts. When i get time I am going to redo this pedal again. I think I'll also move to strip board. There were a lot of wires when i used the PCB and the chances for error and shorts is greater.
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    11,015
    2,506
    Nov 17, 2011
    Inspect the connection of R32 to ground and the connection of R32 to R33. If any of these is broken, it will have the observed effect.
     
    ratstar likes this.
  12. SparkyCal

    SparkyCal

    362
    14
    Mar 11, 2020
    Thank-you folks. What I have concluded is that any of a number of things could have caused this. The major learning from this round, is to use strip board. I used PCB which necessitated a lot more wiring and soldering than had I used strip board. So, i just built the voltage divider on strip board and it works fine. I am going to continue on strip board and see if the end result is a lot tidier and less chaotic.
     
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