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    Noob ohm low question

    Hi,

    I have a very basic question...regarding ohm low on motherboards. So, lets say i have laptop and 19,2v power adapter and on AC main net on the board i have 143 Kohm resistence to the ground. When i plug adapter i can check with multimeter and i have 19,2 V on ac main net. If i am not calculating something wrong i should have 0,13 mAmps ? But that doesnt seem right? Or?
    Thx
    Last edited by prow; 07-15-2020, 03:03 AM.

    #2
    Re: Noob ohm low question

    Many ICs draw little current unless powered with the proper voltage. Your ohm meter probably sources ~2V, which is way too low to cause active operation of a circuit that designed to be powered with 18V-20V. Further, many ICs are a mixed resistive and constant-current load. That is too complex for an ohm meter to measure accurately.

    If you really need to know how much input current your MB draws, either put a .01-.1 ohm resistor in the "+" lead to the MB, measure the voltage drop (be sure the resistor power rating is high enough), and do the calculation or get a clamp-on current probe (meter or oscilloscope).
    PeteS in CA

    Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
    ****************************
    To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
    ****************************

    Comment


      #3
      Re: Noob ohm low question

      thx for answer...but probably i was not clear...What i was saying is if i have 19.2 V and 143 Kohm resistence i should have somewhere near 0,13 mAmps according to the ohm low? A=V/R = 19,2 V/143 kOhm ? But that doesnt seem right?
      Or i am figured something wrong?

      Comment


        #4
        Re: Noob ohm low question

        The 143K Ohms you read is not pure resistive load, the circuits you are measuring the resistance contain many active devices, since your meter is only putting out very low Voltage as explained by Pete so not all the devices may be turned on, but when you applied 19V to the circuits, then lots of devices will be turned on and draw more current.
        Last edited by budm; 07-16-2020, 01:21 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


          #5
          Re: Noob ohm low question

          So, you are saying when checking with multimeter (no power adapter) the resistence is 143 kOhm, but when i apply 19V adapter and because there are many active devices that resistence will not be 143 kOhm but much lower?

          Comment


            #6
            Re: Noob ohm low question

            Originally posted by prow View Post
            So, you are saying when checking with multimeter (no power adapter) the resistence is 143 kOhm, but when i apply 19V adapter and because there are many active devices that resistence will not be 143 kOhm but much lower?
            Yes.
            If you want to learn you can do the experiment by building simple transistor circuit such as one in figure 4.37, then use variable power supply that can show low current display and Voltage. You will apply about 3V to the circuit and observe the current flow, then you turn up the Voltage to 18V you will observe that the current draw will go up, so if you use Ohms law you will see that the load resistance at 3V is higher than load resistance at 18V.
            https://cdn.badcaps-static.com/pdfs/...83431ecd0b.pdf
            Attached Files
            Last edited by budm; 07-16-2020, 02:04 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


              #7
              Re: Noob ohm low question

              Thank you...one more question...is it safe to measure resistence with multimeter while power adapter connected?

              Comment


                #8
                Re: Noob ohm low question

                No, you will risk destroying the multi meter.
                "The one who says it cannot be done should never interrupt the one who is doing it."

                Comment


                  #9
                  Re: Noob ohm low question

                  I understand. Is there any way i can measure resistance while laptop is on? without risking destroying equipment...

                  Comment


                    #10
                    Re: Noob ohm low question

                    Not directly, but you can do it like Pete described above:
                    Originally posted by PeteS in CA View Post
                    If you really need to know how much input current your MB draws, either put a .01-.1 ohm resistor in the "+" lead to the MB, measure the voltage drop (be sure the resistor power rating is high enough), and do the calculation or get a clamp-on current probe (meter or oscilloscope).
                    "The one who says it cannot be done should never interrupt the one who is doing it."

                    Comment


                      #11
                      Re: Noob ohm low question

                      I think i understand...if i check voltage on "current sense resistor" of lets say 0.05 ohm before and after...i can calculate the current?
                      hmm but how? lets say the difference is 0.2 V?

                      Comment


                        #12
                        Re: Noob ohm low question

                        I = V/R
                        0.2V / 0.05R = 4A

                        http://www.measurementest.com/2010/0...ing-shunt.html
                        "The one who says it cannot be done should never interrupt the one who is doing it."

                        Comment


                          #13
                          Re: Noob ohm low question

                          All is clear now thank you all very much!

                          Comment


                            #14
                            Re: Noob ohm low question

                            Update.... so i checked on the motherboard and i have 0.01 ohm (10m Ohm) current sense resistor and i have drop in voltage across ressistor by 0.01 V. So if i use ohm calculator the current is 1 A. Is this correct?

                            Comment


                              #15
                              Re: Noob ohm low question

                              Originally posted by prow View Post
                              Update.... so i checked on the motherboard and i have 0.01 ohm (10m Ohm) current sense resistor and i have drop in voltage across ressistor by 0.01 V. So if i use ohm calculator the current is 1 A. Is this correct?
                              That is correct, so that 19V power supply is seeing about 19 Ohms load.
                              Last edited by budm; 07-17-2020, 01:43 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


                                #16
                                Re: Noob ohm low question

                                Originally posted by prow View Post
                                Update.... so i checked on the motherboard and i have 0.01 ohm (10m Ohm) current sense resistor and i have drop in voltage across ressistor by 0.01 V. So if i use ohm calculator the current is 1 A. Is this correct?
                                Yes, but do keep in mind the accuracy of your multi meter.
                                If the drop is just 10mV but the multi meter does not have a dedicated millivolt range then the measurement error can be very significant.
                                "The one who says it cannot be done should never interrupt the one who is doing it."

                                Comment

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