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Need competent help with power supplies, filter capacitors and safety

Discussion in 'General Electronics Discussion' started by MFDK, Jul 13, 2015.

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  1. MFDK


    Jul 5, 2015
    Thanks for helping me out in my first thread.

    Need your help again try pointing out a right component for specific task, this time in a bit different configuration, used to solve a problem with ground potential.

    noground potentialpsu.png

    Normally when making these series configurations, the ground potential at the DC output will turn out be -12V.
    Now with some chargers in the Radio Control world, they are also made with a USB port, so you can hook up a computer in the setup, and do things like data logging.

    Especially with desktop pc's, these are always grounded, with ground potential of 0V.
    If the example says this 0V ground potential have directly connection from the USB cable of the computer, to the RC charger and the DC GND output of the power supplies, you create an unexpected ground loop, a situation that could cause malfunction to the USB connection, probably do damage as well. The guidance to avoid this problem tells to remove DC ground internal from the metal case, this time in both power supplies. Then solder a filter capacitor from DC GND to the metal case and one from +12V to metal case. The capacitors then damp high frequencies coming from the AC, that would otherwise do harm to the USB connection and to my knowledge also recover some protection that you normally would lose by "floating" the DC GND completely, This should then make the units safer. If the DC GND is connected to the metal case, you face the issue with the ground potential difference, because the ground potential will be -12V.

    Again, the source lacks specifications on the filter capacitors. What should I look for?

    Another question for the guys who can answer:
    What would it likely change on safety, when adding these capacitors rather than having DC GND to ground? Anything to be concern about?
    Attached another png with the DC GND connected to ground as example.

    Feel free to ask questions.

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Have you tried without capacitors? If that works, you don't need them. If not, try 2.2nF ...4.7nF ceramic capacitors.

    These capacitors have nothing to do with safety. Safety is (or should be) ensured by the potential insulation between primary and secondary circuit within the power supply. Double or reinfiorced insulation is required. If the insulation within the power supply is not of the double or reinforced type, you should not remove the direct connection between the output and chassis. This connection then is the safety fallback in case the insulation fails. Capacitors will not help in this case.

    If you're in doubt about the safety of the power supply, I recommend you do not remove the DC connection between output and chassis. Get another power supply that is better suited to your circuit needs then.
    davenn likes this.
  3. MFDK


    Jul 5, 2015
    If I could ground everything up properly in the setup, without issues, it would work without any. But since two DC grounded power supplies connected in series can never work out together, it is not possible.
    In another example, If the computer has no ground connection and at least one device has enough ac filtering, in such case it might too work with pc and a USB connection connected. But it's not easy to tell when a device is enough protected or not. I've seen one guy damaged his laptop, because he didn't ensure any damping on the power supplies, so I suspect it is not safe leaving them out. I do not want to risk damage my pc.

    I see you're a German, and it happen the instructions I am following are in German as well.
    Maybe you can help me tell, with the quotes I am posting here below, what I read is rubbish or not.
    To me it's explaining well, in what configuration with ground connection, to expect a setup with a computer connected to work, and also what purposes the capacitors have.

    I do understand removing the DC completely from the chassis might be a bad idea. That's why I really need to know what I am doing here of maybe dangerous stuff. These server units definitely do not have double isolation, since they're normally mounted and further protected inside a server.
    In all circumstances whatever I end up with these power supplies, however, I do plan to reinforce them. Thanks for your post!

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Please don't expect me to translate the whole text, sorry,

    In my view the important parts are:
    To discharge dangerous voltages in case of a failure not to a human but to earth, the metallic housing has to be connected in all cases to protective earth.

    To keep uncontrolled erro voltages away from the DC output of the power supply, this output is best connected to protective earth, too. Typically the AC side is well enough isolated from the DC side (we're coming back here to the double or reinforced insulation, see post #2). At least there are safety standards that should be fulfilled in a good power supply. ...For voltages less than 50V the direct connection from output to Earth can therefore be omitted.

    The mounting of the power supply within server racks is no reason why the power supplies should not have double or reinforced insulation. The servers are accessible by humans, so are the interfaces to and from the servers. Therefore a failure within the power supply has the potential to damage human operators and the power supplies need to be save accordingly.

    I can only repeat: If you're not sure about the safety features of the power supplies in question, get newones, suitable ones, with plus/minus outputs.
    It is difficult to impossible to upgrade an existing power supply to higher safety standards than the ones it was designed for.
  5. MFDK


    Jul 5, 2015
    @Harald Kapp
    No please do not translate the whole text. it's not necessary. :)

    thanks for the clarification, appreciated!
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