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voltage drop at power source -what is happening?

Discussion in 'Electronic Basics' started by catfarm, Apr 14, 2005.

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  1. catfarm

    catfarm Guest

    I have a circuit that is supposed to run on 9V. When I connect my 9V (well
    regulated) power supply to the plug Im only reading ~2.2 V. What would cause
    this? Is the implication that there is a short to ground somewhere?
  2. Lord Garth

    Lord Garth Guest

    You're either not 'well regulated' or you're attempting to pull more current
    the regulator can supply.

    How about telling us what the device is that you are powering and the specs
    the power supply.
  3. Ban

    Ban Guest

    If the short is to gnd, you would be reading 0V. So probably an IC got its
    supply pins reversed or a resistor has a wrong value. When you reversed the
    supply before you might have killed your circuit.
    Try to find out if the circuit works with another supply. When you have a
    multimeter with a DC current range of 2A or 20A you can also measure the
    current consumption and choose a wall-wart accordingly.
  4. catfarm

    catfarm Guest

    Its an audio modulator. It uses 2x quad opamps (TL074) and some LED/CDs
    pairs to modify the sound.

    Interestingly enough when I pull the chips I read a nice 9V at the plug.
    Putting in one chip drops the voltage to 3.7V. Bad chip? They aren't
    getting warm when the circuit is running.

    Im using a 9V source that comes from a 7809 and should be able to put out
  5. Chris

    Chris Guest

    One thing you might try is removing the op amp and putting a resistor
    across the 7809 output which simulates the load. Start with a 100 ohm,
    1 watt or greater resistor and see what happens. Also, thake a look at
    what's happening at the input voltage of the 7809.

    I'm assuming you've got the pinout of the 7809 right. That's another
    common newbie problem. If you've got a TO-220 package (square black
    plastic with metal tab), holding the plastic side forward and pins
    down, the pinout from left to right is


    Good luck

    Good luck
  6. Lord Garth

    Lord Garth Guest

    The LM7809 is good to 1 amp but you will need at least 11 volt input to the
    You can post a schematic on alt.binaries.schematics.electronic, that would
    helpful. Off hand, it sounds like a power supply problem but you have no

    You might try connecting your power supply to a resistive load to see if it
    in there. 9v/.5A= 18 ohms would load your supply half way and let you test
    whether the voltage stays correct. The power dissipated in the load would
    9v * .5 A = 4.5 Watts so use an appropriately rated resistor.
  7. catfarm

    catfarm Guest

    Im not worried about the power supply. I use it for a variety of other
    applications on a regular basis and it performs well. Its the circuit thats
    problematic. Somewhere there is lots of power being dumped. I can't seem to
    find out where though. It seems odd that the problem occurs when the opamp
    chip(s) are inplace. I might try substituting in another batch of chips to
    see what happens.
  8. Jamie

    Jamie Guest

    did you scope it for oscillation?
  9. Im not worried about the power supply. I use it for a variety of
    otherseem to

    If that amount of power were dumped something must get hot. It's very
    unlikely that 7809 deliveres more current than possible and therefore
    output voltage is reduced.
    Maybe it is your specific load that makes the power supply oscillating.
    Other loads could be more harmless. Do you have correct capacitors
    across input and output if 7809 regulator?

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