power supply modification

Discussion in 'Embedded Systems and Microcontrollers' started by blind2_man, Dec 9, 2011.

  1. blind2_man

    Thread Starter Active Member

    May 12, 2008
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    I have tried to build polyphonic doorbell from elektor but after weeks of working I have several problems:
    1) resistor R30 is very hot
    2) the amplifier is entering into oscillation.
    3) I have poor result on how melody sound on the output.
    I have attached original design and second file are the schematic adapted and board routing.

    1)The voltage supply is 16V on the C12. Current consumption on R30(470ohm) is 20mA.
    I should also mention that signal that are not routed are connected via wires.
    Can you give me hits first on the hardware part ?
    2)I had a look on the datasheet of tda2006 and the advice is to have fixed resistors not a potentiometer on the feedback?!
    3) I will add later the code as soon I fix first two items.
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    With 16V on C12 there should be about 15V over R30. That is 480mW that needs to be dissipated. The original circuit has 9V input voltage.

    Increase the resistors wattage, lower the input voltage or increase the resistor value of R30 a bit.

    Please post a picture in .png or jpeg format of your layout/PCB. Not everybody here uses a PCB viewer, also different CAD software uses the same extension, so it's not clear what software u used.
    Or did you use the board from the "Poliphonic".pdf ?
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok got it, it's EAGLE. :) Quite some modifications...
    [​IMG]
    [​IMG][​IMG]
     
    Last edited: Dec 9, 2011
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok, a few questions:

    1. Why did you choose to change the value of C13 in your schematic (C15 in the polyphonic pdf) from 100p to 22n? Not that I ever worked with this IC, I'm just curious.
    2. What voltage do you measure from pin 5 to pin 3 of the TDA2006?

    Is it really 12V? And is your input voltage 16V? Meaning there is a voltage drop of 4V over the BD240??

    3. Not sure about the oscillation, but the first thing I would try is to put bypass capacitors near the TDA as indicated in the datasheet (figure 15).
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I think his main problem is heat buildup in the 12V supply in this schematic, that's where I found R30, anyway.

    [​IMG]

    What is the input voltage to that circuit?
     
  6. blind2_man

    Thread Starter Active Member

    May 12, 2008
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    Regarding the resistor R30=470 I will increase the wattage of the R30 but I am not convinced that the value 470 ohm is realy OK. I want to change the value to 1k or 1k5.
    But I not sure that the BD240 will enter in saturation.
    There is no 12V on but 16V.
    On the first run I have found this error and remove C13 and also the volume pot. The amplifier part is just like figure 15 I have add the decoupling capacitors.
    Please have a look on the picture to see the modifications made.
     
  7. blind2_man

    Thread Starter Active Member

    May 12, 2008
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    The input voltage is AC V=12V.
     
  8. blind2_man

    Thread Starter Active Member

    May 12, 2008
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    On R30= 470 Ir30 = 30mA
    If R30 =1K Ir30 =14mA.
    The amplifier stage is disconnected(pin 5 is not connected to 16V).
    The R30 is still very hot. I did not change R30 power, same is 1/4W.
    I think it is necessary to change the power, because the power to be dissipated on the R30 is the same 320mW.
     
    Last edited: Dec 10, 2011
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    1. Assuming 16V DC on C12 and the Q1 and T1 saturated the voltage on R30 will be 16V - Voltage drop on BE of Q1 - Vsat of T1 = about 15.2V, let's say 15V.

    15V x 15V / 470R = 478mW

    15V x 15V / 1k = 225mW , so even for 1k you are at the limit with a 1/4W resistor.

    Why is the power supply at the output of the BD labelled "12V" ? If the BD240 is saturated how can it be 12V?

    2. I'm not saying that there cannot be errors in an Elektor schematic but why not try it the way they built it first? Since there is even a PCB included in the article I'm almost sure that someone tested it and didn't encounter the oscillation problems.

    "On the first run I have found this error and remove C13 and also the volume pot. The amplifier part is just like figure 15 I have add the decoupling capacitors.
    Please have a look on the picture to see the modifications made."

    Well your schematic is NOT just like figure 15. The output RC in your circuit is after not before the series output capacitor, you used a potentiometer instead of the fixed gain and you added a 22n feedback capacitor. And I don't see the decoupling capacitors on your board. (I didn't verify the PCB layout, I'm referring to your schematic)
    So , how exactly does your schematic look like ? The way it's in your EAGLE file or as in figure 15?
     
  10. blind2_man

    Thread Starter Active Member

    May 12, 2008
    33
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    1. The label of the schematic "12V" should be 16V.
    2. First I have tried the original schematic, the layout come out smaller even if I printed 1:1. Anyway even after I succeeded to solder the parts, 2 good pic16f84 burned. Having no more pic16f84, I decided to redesign the project with 16f876A and learn something in the process.
    I will make an update of schematic on eagle and post again the schematic.
    The actual situation is as in foto picture.
     
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