Unbalanced Power supply

Discussion in 'The Projects Forum' started by oneilracgni, Nov 4, 2015.

  1. oneilracgni

    Thread Starter New Member

    Nov 4, 2015
    2
    0
    Hi people having a problem with my current project at the moment. My circuit is attached and I'm using a circuit from tangetsoft for my +- supply. Now once I connect the power supply circuit to the PWM circuit it becomes unbalanced, eg. -16V and 2V (using two 9V in series). I feel like I'm missing something simple, but yeah struggling right now haha. It works perfectly using a separate power supply. Thanks in advance!

    Ps. blank space in my circuit is a regulator to drive my laser which is now run separately.






    [​IMG]
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    Your virtual ground circuit works only if the two load currents are equal. If there is any net ground current, it flows through one of the 4.7 K resistors. 1 mA of unbalanced current moves the ground 2.35 V. To visualize what is happening. reduce R1 and R2 to 1 ohm to simulate a very low ground impedance, and then put a 2.35 K resistor between the junction of the two capacitors and VGND going off to your circuit.

    To handle unbalanced loads you need an active circuit. This can be done with a spare opamp section if the max unbalanced ground current is within the opamps output current spec.

    ak
     
  3. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    Here is my solution to the problem:

    VirtualGnd.png
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    If the current imbalance is small; you can buffer the voltage divider with an op-amp. For something a bit more heavy duty, there are various power op-amps. A few audio power chips behave in the same way, but some have different input structure and don't!

    Decouple at the resistors, not the op-amp output - or it'll become unstable.
     
  5. oneilracgni

    Thread Starter New Member

    Nov 4, 2015
    2
    0
    OK thanks for the help guys! The above makes perfect sense thanks! Don't know why i didn't think myself that it was going to be unbalanced. So I need an active circuit but, I'm hoping to minimise components so after reading a few other topics on this forum I've come across the ICL7660. Would this work ok in my situation and how would I set it up if so? (To get +/- 9v?) Thanks!!
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,535
    1,251
    The easiest way to get+/- 9 V is to add a second 9 V battery. Also, doubling the system power supply rails rather than splitting one will cut your battery life, maybe in half depending on how much current is static in the devices vs dynamic in the loads. The 7660 might work depending on what the center ground current is now.

    ak
     
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