commone ground problem

Discussion in 'The Projects Forum' started by mah, Mar 31, 2014.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
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    2
    I have a problem with my circuit , i control buck converter using atmega8 and data acquisition card . my circuit has also LM35 temperature sensor .in the beginning i used one power supply to give +12 v and +5 v using lm7805 and that makes the regulator got hot , the problem is coming up when i connected all the ground to just one point . the control and power circuit have the same ground and that increased the output of lm35 however it should be lower . when i disconnect the power circuit the output of lm35 is gonna to be alright . then i tried to use power supply for the lm35 and another power supply for the power circuit but in a vain . lm35 is just connected to capacitor and resistor that is existed in its datasheet for filtration then a voltage follower op amp. i display PWM using oscilloscope (terminal and its ground)i attached the power and control circuit .so please help. ​
     
    Last edited: Mar 31, 2014
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi mah,
    I dont see any capacitive de-coupling on your circuit for the power lines +5V.??

    E

    EDIT:

    You should have a 100nF and 100uF on the inputs side of the 7805 and 100nF and 22uF on the output side
     
    Last edited: Mar 31, 2014
  3. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    i have already used 1uF on the output side of lm7805 , isn't it enough? could that be the reason ?
     
  4. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    I think that the problem is with my power circuit , because i removed it and i measured temperature it was ok
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    If you current load on the output of the 7805 is less than ~5mA it will not give a regulated voltage.

    Looking at your circuit is shows the 7805 is only being used to power a OPA and the LM35.?? The I quiescent of an LM35 is only ~150uA.

    Try adding a resistor between the +5V and 0V, say a 270R, draw ~18mA

    E

    EDIT: I see the LM35 is on +12V, so there even less load on the 7805

    It helps if you post a complete circuit.:)
     
  6. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    7805 is the source for every +5v in the circuit - op amp ,IR2112 and the micro controller so it drives 5mA almost , 7805 should work well without minimum current load as a proof for that it can drive just the op amp however op amp current in uA
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi mah,
    If your posted circuit image had been complete, we would both not have wasted our time in suggesting solutions.?

    I often use LM35's on PIC based projects and I never have a problem with the LM35.
    Perhaps there is a problem with you actual circuit layout, could you post a photo.?

    E
     
  8. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    here is a complete however i posted it in #1
     
  9. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    You have added the 1uF on the +5V line since #1.:), no problem

    The way you describe the problem when the power circuit is not connected the LM35 works OK, suggests a ground loop problem.

    Do you have a photo you could post of your layout.

    BTW: The 7805 regulator if not correctly decoupled on the input and output close to the 7805, say with a100nF capacitor, can allow the 7805 to oscillate and run hot.

    The point I made about a minimum 5mA load on a 7805 in order to ensure regulation is valid, its stated in the datasheet.
     
  10. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    this is the power circuit ,the microcontroller and sensors interface on another single board +5v and ground on the power circuit are supplied from the sensor circuit
     
  11. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    If I am reading your description correctly, you say the 0V of the Power PCB is connected to the Sensor PCB 0V, then to the Power Supply.?

    Does this mean the the Power Supply is connected to the Sensor PCB then to to the Power PCB.?? If yes, this is not the way to interconnect the 0V lines.

    Ideally the the Power Supply 0V should be connected to the Power PCB 0V, and also to the Sensor PCB 0V.
    Sometimes called a 'Star' connection, this minimises any ground/0V loop offset current problems, which can shift the level of the 0V lines to sensitive circuits.
     
    mah likes this.
  12. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    I tried Star for ground connection and it works
    thanks:)
     
  13. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Pleased to hear that, 0V/Common lines can be a problem in mixed power and low signal projects.;)

    E
     
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