Finding 0V reference point without a schematic

Discussion in 'General Electronics Chat' started by bypass666, Jun 11, 2014.

  1. bypass666

    Thread Starter New Member

    Jun 11, 2014
    Hi all

    I am generally new to electronics but have had experience using an oscilloscope and multimeter using a schematic as a guide.

    I was hoping to get into repairing run of the mill electrical products (amplifiers included).

    My question is in regards to 0V ground reference points both for AC and DC in circuitry. The equipment i have experience with has schematics so i know where to reference and what to look for.

    My knowledge has expanded greatly over the past year however i would like to know if there is any clear cut way to identify a reference point for measuring voltages (in any place) in equipment that has both DC and AC voltages (without a schematic)?

    I am studying hard at the moment so i am sure i will learn but if anyone could shed some light that would be great.

  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Best place to start is the power supply, ground or 0V terminals.
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    If there are large electrolytic capacitors, especially near a transformer or inductor, these might be power supply filter caps. Good place to start. Remember that many audio amps have bipolar supplies, so some of the filter caps will be connected with the + terminal to GND. But in this case there will be others with the - terminal to GND, and that is the clue.

  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    just watch out for switching suplies, the (usually two) large electrolytics are on the line side of the supply, and not a good ground to use to test with. usually 200 volts or so. if there isnt any other caps rated that high a voltage, they should be on the secondary side, and most filter caps have one lead connected to other caps, that is probably the ground.
  5. #12


    Nov 30, 2010
    Watch for large areas of copper on the circuit boards...large areas that have connections that seem to go all over the board. It isn't fool proof, but it's a likely candidate.

    Connecting to metal chassis with one lead and seeing if several voltage measurements make sense would indicate that the "case" is grounded. Like, finding a couple of electrolytic capacitors rated at 50 volts, and both of them have zero volts on one side and 30 to 40 volts of DC on the other connection.

    A connector that is obviously connected to the chassis on one side.
    Two input or output connectors that are wired together on one leg of each.
  6. bypass666

    Thread Starter New Member

    Jun 11, 2014
    Hi all

    Some good replies there and definitely a good place to start looking. My worry always was - am I in any danger of causing damage if I use what I think is a reference point and testing for voltage, only to realise that it wasn't a good/0V reference point?

    Such as - trying the chassis as a reference point however it actually isn't, or any of the other points suggested?

    Also can using a reference point in an High voltage circuit and then testing in a low voltage area by mistake cause an issue? I've read about hot and cold ground however I've not read about it in any UK sites. I'm assuming accidentally testing for DC voltage using an AC reference could be bad?

    It probably sounds quite bad here but this is mainly to aid what I am studying right now. :) funnily enough the info about the caps is in the book I am reading but the info here has helped me understand a bit better.

    Thanks again
  7. MrChips


    Oct 2, 2009
    Use an ohmmeter to test the resistance between what you think is 0V reference and earth ground. You should read less than 1Ω. Do this with the power disconnected.

    Oscilloscopes and their grounding clips on the probes are referenced to earth ground. Don't connect the grounding clips to any place but 0V ground.

    To be on the safe side, you could choose not to use the grounding clips if you are not making critical measurements at high frequencies.