Project: CMV2A virtual ground circuit

Discussion in 'The Completed Projects Collection' started by cumesoftware, Jun 21, 2013.

  1. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    1,330
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    This simple circuit works as a substitute for a more expensive dual tracking power supply. It is meant for lab or experimental use as is practically fool-proof: it can withstand shorts from ground to one of the rails, polarity reversals and any kind of difficult load, inductive or capacitive. However, it should be used with a single regulated power supply and the current should be limited to no more than 5A, since it has no protection fuses built in.

    The circuit is pretty straightforward. It uses an OPA548 as a voltage follower (in a particular configuration to avoid instability if the output is shorted, as voltages at the inputs should always be within the CMVR ). Resistors R2 and R3 work as a voltage divider, which in turn yields the reference voltage for the circuit (C4 and C5 are for noise suppression). R1 defines the current limit value to around 2A (again, C3 is for noise suppression). C6 plays an important role, as it cancels any noise present in the feedback network (without C6, R4 and R5 offer a noise gain greater than unity, even if, in practical terms, the overall gain is still unity). R6 and C7 constitute a Zobel network to add more stability. Diode D1 protects against reversed polarity. On the other side, D2 and D3 protects the output against high voltage spikes that might be generated by inductive loads and also helps dealing with highly capacitive loads, by providing a discharge path.

    List of components:
    C1 – 100µF electrolytic capacitor (50V);
    C2/6/7 – 100nF polyester capacitor (63V);
    C3-5 – 10nF polyester capacitor (63V);
    D1 – SB550 Schottky rectifier diode;
    D2/3 – SB350 Schottky rectifier diode;
    HS – 1,2°C/W passive heatsink;
    IC – OPA548 power operational amplifier;
    J1/3 – Red banana jack (4mm);
    J2/5 – Blue banana jack (4mm);
    J4 – Black banana jack (4mm);
    R1 – 21,5KR metal film resistor (±1% tol., 1/8W);
    R2/3 – 10KR metal film resistor (±1% tol., 1/8W);
    R4 – 10KR carbon resistor (5% tol., 1/8W);
    R5 – 56KR carbon resistor (5% tol., 1/8W);
    R6 – 10R carbon resistor (5% tol., 1/2W).

    Electrical parameters:
    - Vd. mín.= 8V
    - Vd. máx.= 32V
    - IL máx.= 5A
    - Id.(Vd. = 8V, IL= 0A) = 17,4mA
    - Id.(Vd. = 32V, IL= 0A) = 18,6mA

    Operating characteristics:
    - Current limiting (to ground): ±2,021A
    - Noise: < 0,2mVrms
     
    Last edited: Jun 22, 2013
  2. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    1,330
    10
    Meanwhile, some pictures:

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    The project is not yet complete. More photos will be posted soon!
     
  3. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    1,330
    10
    Here are the pictures of the finished project:

    [​IMG]

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    I will do some tests soon. Stay tuned!
     
  4. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    1,330
    10
    I did some tests, and the circuit behaved very well, even exceeding my expectations. Here are some videos:

    Voltage readings: http://vimeo.com/70407751

    Test under load: http://vimeo.com/70649329

    While doing the test under load, the deviation was only 0.2% at 62% load capacity. The virtual ground noise remained minimal.
     
    Last edited: Jul 21, 2013
  5. Shagas

    Active Member

    May 13, 2013
    802
    74
    not bad , quite a big board and heatsink :)
     
  6. Wendy

    Moderator

    Mar 24, 2008
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  7. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    1,330
    10
    Adding to Bill's answer, the heatsink has to dissipate 32W in the worst case. This circuits acts as a voltage regulator with a twist: the OPA548 is able to either source or sink current due to its push-pull output configuration. It uses a configuration similar to the one suggested in the LM675 datasheet, with different resistors and with additional capacitors to suppress noise.

    Thanks, Bill!
     
  8. radiohead

    Active Member

    May 28, 2009
    474
    31
    Nice, clean and professional looking project. Good job.
     
  9. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    1,330
    10
    Thanks for your feedback, radiohead!
     
  10. cumesoftware

    Thread Starter Senior Member

    Apr 27, 2007
    1,330
    10
    Although this project was mainly designed to power op-amp circuits, it can also power difficult and demanding loads, such as power amplifiers. In the following video, you can see it powering an LM3876 based power amplifier. The amplifier was producing around 15W (given the ±15V supply and the 4Ω load).

    See the video here: http://vimeo.com/74863188
     
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