Cap voltage without load

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  • notallbad
    Badcaps Legend
    • Oct 2012
    • 1190
    • UK

    #1

    Cap voltage without load

    Two Axial polarised electrolytic caps, 2700uf and 220uf 40v in parallel across a 12v DC supply (using a 12v car battery charger as supply voltage) with no load connected.

    What would be the expected P.D voltage across the terminals be?

    Trying to determine whether the caps are good or not.
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: Cap voltage without load

    Devices in parallel circuit will see the same applied Voltage, so the Voltage across the terminals of BOTH caps will be 12VDC. You should draw out the diagram so you will see how it works.

    http://www.bbc.co.uk/schools/gcsebit...uitsrev1.shtml
    https://learn.sparkfun.com/tutorials...allel-circuits
    Last edited by budm; 06-22-2016, 08:58 AM.
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
    http://s807.photobucket.com/user/budm/library/

    TV Factory reset codes listing:
    http://www.badcaps.net/forum/showthread.php?t=24809

    Comment

    • notallbad
      Badcaps Legend
      • Oct 2012
      • 1190
      • UK

      #3
      Re: Cap voltage without load

      Thanks, shall take a look at those links.

      DMM across terminals reads around 18v which does not make sense to me!

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: Cap voltage without load

        What kind of 12VDC power supply? Linear unregulated power supply (Transformer, rectifiers, caps)? The no load Voltage of those type of power supply will be higher until fully load and the input Voltage applied is the same as printed on the unit.
        For example, the label on the power supply shows 12VDC 1A, 120VAC input, it will put out 12V when the load is drawing 1A, but if the load is <1A then the Voltage will not be 12V but will be higher.
        Last edited by budm; 06-22-2016, 09:55 AM.
        Never stop learning
        Basic LCD TV and Monitor troubleshooting guides.
        http://www.badcaps.net/forum/showthr...956#post305956

        Voltage Regulator (LDO) testing:
        http://www.badcaps.net/forum/showthr...999#post300999

        Inverter testing using old CFL:
        http://www.badcaps.net/forum/showthr...er+testing+cfl

        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
        http://s807.photobucket.com/user/budm/library/

        TV Factory reset codes listing:
        http://www.badcaps.net/forum/showthread.php?t=24809

        Comment

        • R_J
          Badcaps Legend
          • Jun 2012
          • 9535
          • Canada

          #5
          Re: Cap voltage without load

          A car battery needs a min of 13.8 volts to start charging, and usually the charging voltage is around 14.2 volts. +18 unloaded sounds about right
          Last edited by R_J; 06-22-2016, 11:00 AM.

          Comment

          • notallbad
            Badcaps Legend
            • Oct 2012
            • 1190
            • UK

            #6
            Re: Cap voltage without load

            Thanks Budm, used an alternative 12v supply.

            Comment

            • notallbad
              Badcaps Legend
              • Oct 2012
              • 1190
              • UK

              #7
              Re: Cap voltage without load

              Thanks R_J, that also helps.

              Comment

              • Th3_uN1Qu3
                Believe in
                • Jul 2010
                • 6031
                • Romania

                #8
                Re: Cap voltage without load

                Originally posted by notallbad
                Thanks Budm, used an alternative 12v supply.
                I assume this was a mains-transformer powered supply, if you say alternative. Learn about the peak voltage of a sinewave, which is 1.414 times its RMS value (which is what's printed on your transformer). So 230v mains will have 325v peak, and a transformer outputting 12v RMS will have a peak voltage of 16.9v

                With no load, a rectifier will charge a capacitor to the peak voltage of the AC supply. Under load the voltage drops and some AC will make its way on the output, this is called ripple. Ripple depends on the current being drawn, the power capability of the transformer and the size of the capacitor bank. Bigger capacitor, less ripple, higher average voltage. With ideal transformer, rectifiers and capacitor bank you would always have a DC level of the peak AC input, this does not happen in real life, of course. But with zero load, the voltage is extremely close to the peak.

                12v times 1.414 is ~17v, but also, a transformer is rated for a certain voltage output at a certain load, with light or no load voltage will be higher. That explains your 18 volts.
                Originally posted by PeteS in CA
                Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                A working TV? How boring!

                Comment

                • notallbad
                  Badcaps Legend
                  • Oct 2012
                  • 1190
                  • UK

                  #9
                  Re: Cap voltage without load

                  Thankyou for taking the time to explain, it is appreciated.

                  I am trying to repair an electronic (sensorless) power steering pump.
                  It was continuously running after ignition switched off and drained the car battery. Battery good-around 11.8v(now recharged, alternator good- outputting around 13.8v.
                  Replaced 2 of the 4 MOSFETs BUK7608-55A (one showed signs of 'blowing', the other shorted between gate,drain and source.
                  Now it just keeps stopping and starting, 3-4 sec intervals.
                  The sensorless i.c which drives the motor (Elmos 10203C 28-pin) is presumably the culprit for shutting down the motor but to be honest I am not really sure what to check now. other than a few smd caps and resistors there isn't anything to test.
                  The pump takes battery feed and grounds to chassis and alternator output which acts as an 'on/off switch', pump only runs when the car is running.

                  If anyone can help please let me know though I know this site isn't really for car repairs so understand if no replies of advice.

                  Comment

                  • notallbad
                    Badcaps Legend
                    • Oct 2012
                    • 1190
                    • UK

                    #10
                    Re: Cap voltage without load

                    Photo of the pcb (taken from the web).
                    Attached Files

                    Comment

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