PSU and IC power questions

Discussion in 'General Electronics Chat' started by diddy02, Aug 17, 2010.

  1. diddy02

    Thread Starter Member

    Sep 26, 2008
    10
    0
    Hey all,

    I have a dual output PSU... each output has three connectors: Positive, Negative, and Gnd. I'm not sure what the Gnd connector is... if I measure the potential across either positive and ground or negative and ground it's just a few mV. Could anyone explain why there's a ground connector?

    I wanted to use the supply as a bipolar source to power some op-amps. To do this I thought I'd use one channel for the +ve polarity, and the second channel for the negative source (by just reversing the leads). I want a common ground on my circuit though since there are more inputs. Is what I'm doing right?
     
  2. zxsa

    Member

    Jun 11, 2010
    31
    2
    Let me guess: you already tried connecting the two output channels as you suggested by reversing the leads and it made a bit of a bang/smoke?

    Normally a PSU would have the grounds of ALL its output channels internally connected together. Often connected to earth (mains electricity earth) as well. In your case, the 'ground' lead is probably earth and the DC voltage is between the positive and negative wires.

    If you try to reverse the leads connecting the positive output of a second channel to ground of the first channel, you'll be causing a short circuit on the second channel driver. A cheap PSU would not allow such punishment wihtout permanent damage.
     
  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Not in any of the dual or multiple channel power supplies I have seen through the years.

    The PSU output(s) connection, both positive and negative, are floating with respect to earth and independent from other channels.

    Therefore it is perfectly OK for user to use the power supply as a +/- supply by connecting the positive terminal of one channel to the negative terminal of another channel as no current will flow. Calls this the 0V or common for the external circuit that needs to be powered.
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    When in doubt, (and assume to be) test or take apart to verify. I've seen them built both ways and more expensive doesn't always equate to separate secondaries.
     
  5. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    zxsa,

    You might be right about the Ground connections. But the original poster was talking about connecting the positive of one supply to the NEGATIVE of the other supply, NOT to the GROUND of the other supply. Or at least I hope he was. In that case, the pos-neg connection would be the new virtual ground of the resulting dual-polarity supply. All floating DC supplies should work fine when connected that way. And it sounds like he proved it was a floating supply by measuring a very small DC voltage between both ground and positive and ground and negative.

    diddy02,

    If the AC plug has three prongs, the Ground connectors might be there just to give you access to the AC Mains Earth/Ground connection. If each of your DC supplies is truly a floating type, then you should also, if you ever have a need, be able to connect any one of the positive or negative outputs from each supply to the Ground connector. In that case I think you should also be able to connect the dual supply's pos-neg virtual ground to the Ground connector. You might never need to do that, except in cases where the circuit you are powering has to connect to another circuit or piece of equipment that does NOT have a floating DC supply, and also requires the earth ground reference to be the same, for some reason (for which I can't think of an example, at the moment).

    However, an Earth Ground connection is also usually REQUIRED, in "permanent" DC circuitry that runs from AC Mains power, as a safety ground. (And a safety ground is actually just as important in "non-permanent" DC circuits, since your life is at much higher risk otherwise.)

    Imagine that there is a fault inside your DC power supply that causes AC Mains voltage to appear on your DC power rails, perhaps even on one of the ones that connects to your dual supply's new virtual ground. That could easily put Mains voltage where it could kill you, or someone else. Typically, you "must" have your virtual ground or circuit ground connected to the safety earth ground. And it must be connected in such a way that the connection will last long enough to allow the current to trip a circuit breaker or blow a fuse, even if the maximum available AC short-circuit current is running through it. That usually means a bolted or welded connection is required between your circuit's chassis and the safetly earth ground. Solder will not do, since it might melt or vaporize and break the connection before a breaker or fuse could blow.

    Obviously, not everyone uses a safety earth ground for experimental circuits. But anyone working that way is expected to understand the risks and take proper precautions. But as soon as you box up a project in a case or chassis that has an AC mains cord, always consider the safety earth ground to be a strict requirement.

    One final note: For audio circuits, many DIY builders use a "ground lift" type of connection between their "star" ground point and their chassis [which then connects to the safety earth ground] (which in some cases can reduce noise), which includes a low value resistor, typically 10 Ohms, often in parallel with a small capacitor. In that case, they should also always be paralleled by two very-high-current anti-parallel diodes, or a large high-current bridge rectifier, to enable the fault current to last long enough to blow a breaker or fuse.

    Cheers,

    Tom
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Somehow I got to thinking about computer power supplies, now that I read it again the GND is likely indeed just the earth connection to the case and ground prong of the AC plug.

    In other words it's a single supply, you'd need two of them or a voltage divider creating a virtual ground to get the + & - some op amp and comparator ICs prefer.
     
  7. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    But he said, "I have a dual output PSU... each output has three connectors: Positive, Negative, and Gnd. ".

    So it's actually two single supplies and should be perfect for making a dual + and - supply with a virtual ground.
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    If thats the case he's in, just use an ohmmeter to verify the sources are isolated from each other.
     
  9. soda

    Active Member

    Dec 7, 2008
    174
    13
    On a dual supply, you will need a center tap transformer like a 9-0-9v or 12-0-12v .The ground will then connect to the 0v of the transformer.
    The 2 filter capacitors will be in series with each other where the fist one's +v will be connected to the + side of the rectifier. Then this capacitor -v will be connected to the + side of the next filter capacitor and the next filter cap negative will be connected to negative of the rectifier. So, the positive supply will be between the rectifier +v and 0v and the negative supply will be between the negative of the rectifier and 0v

    I hope you understand what i mean
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    It's the connection to the earth terminal or body which is earthed via the mains

    Supply Negative rail is not connected or 0V for that matter.
    If it were 0V, then ur meter should get a reading between the green to red or black

    Ur's is not a dual supply but a single out put.
    A dual is PSU with ± Outputs which has a 0V reference, not an earth
     
  11. soda

    Active Member

    Dec 7, 2008
    174
    13
    Ok I add a diagram of a dual supply
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    My reply was to the OP

    But for you, according to diagram 0V is the center of ± Values, not earth but GND
     
  13. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    Soda,

    The OP (Original Poster) already has a commercial/purchased power supply, which is what he was asking about.

    Rifaa,

    The OP's power supply is not a single supply. It contains TWO single-voltage floating DC supplies. He can make a symmetric dual-voltage supply with them by making one connection (+ of one to - of the other) and then setting the two supplies' voltages to be the same.
     
    Last edited: Aug 20, 2010
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    If this the case he can make a dual PSU
     
  15. soda

    Active Member

    Dec 7, 2008
    174
    13
    Rifaa

    I except it if i'm wrong because i can see you know what you're talking.Thanks for putting me on the right track.
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    Any time buddy.
    That's why we are here for
     
Loading...