LDO Voltage Regulator Behaviour

Discussion in 'General Electronics Chat' started by qwertyuiop23, Sep 24, 2013.

  1. qwertyuiop23

    Thread Starter New Member

    Aug 21, 2013
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    Hi there,

    I have a straing gauge circuit containing an INA126 amplifier. The design is to use 5V however, I only have a 12V power supply available (from the wall), and was going to use an LDO to control the voltage to 5V. In theory I though this would work, in reality the voltage that the LDO outputs changes with the input voltage.

    At Vin equal to 5V it outputs 5V and the linearly decreases as the input voltage increases. I have checked the current draw and it is at 5mA well below what the LDO can draw.

    However, the output and ground terminals of the LDO are connected to the E+ and E- terminals of the strain bridge. If I unsolder the strain bridge the voltage goes back to outputting at the correct voltage. This leads me to believe that due to the voltage divider created by the full strain bridge the internal voltage divider inside the LDO has had its values change and therefore is no longer accurate.

    I have not attempted to fix this problem yet but I believe I can buffer the output from the LDO using a simple voltage follower op amp, but I would rather not do that if there is another way to fix this.

    I have attached a picture of the circuit. (Apologies about the paint drawing no Altium here to draw a proper one)

    Regards,
    Lance Henderson
    Mechatronics Engineer
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
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    675
    What regulator are you using, and are you sure of its pin out?
     
  3. qwertyuiop23

    Thread Starter New Member

    Aug 21, 2013
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    It is a LM7805CT. Yes the pin outs are correct. I am assuming that it is because of the low resistance the strain gauge has.
     
  4. ScottWang

    Moderator

    Aug 23, 2012
    4,858
    767
    If the pins connection is correct, then 7805 could be damaged, usually the Vin-Vout>=2.5V, unless have the internal shorted or wiring shorted.
     
  5. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Looking at what you've indicated is your pin out and that given in the datasheet, you haven't connected it properly.
     
  6. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
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    ...I probably should have seen the part number indicated in your drawing, whoops...:eek:
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
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    The MC7805CT (or LM7805CT) is not a LDO (low drop-out) regulator; it needs at least 7V input to regulate properly.
    What is the bridge resistance and required current? The regulator short circuit current is only 230mA.
     
  8. #12

    Expert

    Nov 30, 2010
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    The chip needs capacitors, too.
     
  9. qwertyuiop23

    Thread Starter New Member

    Aug 21, 2013
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    Are they not only input/output bypass capacitors? They simply help with the stability and transient response of the controller, nothing to do with the output voltage. I could be wrong though.

    Yea I am powering with 12V so it shouldn't have that problem. Unsure of the resistance of the strain gauge. Not high I would suspect. I would have thought that it could supply 2Amp even with a small resistance

    EDIT:: The bridge resistance is 1.005kOhm so nowhere near enough to draw the current max of the circuit. I canonly thnik that the fact the internal resistor is now in parallel it struggles to meet the demand

    I know, I can assure you that it is correct on the board.
     
    Last edited: Sep 24, 2013
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If the part is oscillating, you will read the wrong output voltage using a meter.
     
  11. qwertyuiop23

    Thread Starter New Member

    Aug 21, 2013
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    Would my multimeter not show the fluctuating voltage on it though?
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Because it is reading an AC voltage riding on a DC level.
     
  13. #12

    Expert

    Nov 30, 2010
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    Because the oscillation is too fast for the meter to interpret.
     
  14. qwertyuiop23

    Thread Starter New Member

    Aug 21, 2013
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    Okay, I will place the capacitor across the terminals and hook up my oscilloscope and get a look at the plot of the waveforms.
     
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