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Cold Ground Or Hot Ground?

By on September 2, 2007

Many of us sometime confused about the difference between the cold ground and the hot ground. It is easy to differentiate between the grounding. If you repair the switch mode power supply you will definetely see the biggest electrolytic capacitor in the primary side. It is usually rated at 150uf -330 uf with 400 to 450 volt. Look at the negative sign on the sleeve of the filter capacitor. That negative sign point to the circuit track and that track is the “Hot ground”. The cold ground is the ground that comes from the 3 pin plug. If you still don’t understand, then look at the filter capacitors negative sign at secondary side of the power supply. The negative sign is actually pointing to cold ground!

If you want to measure the voltage at the primary side of switch mode power supply, place the meter ‘s black probe to hot ground and if you want to measure voltages at secondary side then place the black probe to cold ground. If you don’t place it accordingly, you may get inaccurate result or may even burn the meter. So be sure to which ground you should put before measuring the voltages of electronic equipment.

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28 Comments

  1. aman

    October 7, 2010 at 8:25 am

    it is a great technical approach.i appreciate.
    regards:
    Aman

    Likes(3)Dislikes(0)
  2. John Doe

    April 13, 2011 at 8:18 pm

    What if the plug only has 2 prongs,hot and hot return,I know that ground is relative to its potential, but how would the transformer CREATE a ground on the secondary side.Great article by the way.

    Likes(1)Dislikes(0)
  3. admin

    April 14, 2011 at 2:56 pm

    Hi John,

    There would be no ground in the secondary side because of isolated by the transformer. The secondary side produce only positive and negative and we treat the negative as ground.

    Jestine

    Likes(4)Dislikes(0)
    • Chirodip Das

      July 14, 2016 at 7:44 pm

      sir,
      i'm having a problem with my inverter 'air conditioner' pcb.
      the cold ground is become hot ground. i mean in the secondary output, like 5v, 12v section, gives 90v ac in respect to the secondary ground and input neutral. it is also visible with normal electrical tester.

      pls help me.

      thank you,

      Likes(0)Dislikes(0)
      • Jestine Yong

        July 15, 2016 at 4:21 pm

        Hi Chirodip,

        Sorry i don't get your meaning. The power supply produce 5 and 12 volt and where do you get the 90 Vac? Which point of the inverter you measured that got this 90 vac?

        Jestine

        Likes(1)Dislikes(1)
        • Chirodip Das

          July 15, 2016 at 11:38 pm

          sir,
          thanks for your reply. let me explane with more details.

          it's a dc inverter type 'air conditioner' outdoor power supply pcb. the pcb consist of 2 igbt(toshiba -gt30j126) as inverter switch, an ipm(intelligent power supply module) to control and monitor the compressor voltage.

          every thing works fine, but......
          when i touch the electrical tester to 7805 heatsink, it glows & shows a voltage, even i cnt touch it with my 'bear hand', and this voltage is all over the circuit also in the control section of the igbt and ipm. it only shows after rectify the voltage.

          one more thing, one of the secondary winding of smps transformer is not show this problem.

          i have checked all mov's in the primary section.

          hope;

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          • Jestine Yong

            July 18, 2016 at 6:17 pm

            Hi Chirodip,

            Ok noted. Have you checked if there is any ground connected to the power supply board? Or just the live and neutral wire only?

            Jestine

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            • Chirodip Das

              July 18, 2016 at 7:30 pm

              no sir,
              rectifier diode's are okk. i've checked them.
              there are no connection between ground and live or neutral.

              Note: total primary capacitor value is 1500uf/450v.
              mr4020 is the power fet.

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              • Jestine Yong

                July 19, 2016 at 9:32 am

                Hi Chirodip,

                Ok noted. I meant does this power supply have a earth ground going to the power supply beside the neutral and the live?

                Jestine

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                • Chirodip Das

                  July 19, 2016 at 8:58 pm

                  yes sir.

                  Likes(0)Dislikes(0)
                  • Jestine Yong

                    July 20, 2016 at 7:24 am

                    Then is this ground is also going to the secondary side of the inverter?

                    Jestine

                    Likes(0)Dislikes(0)
  4. Manikandan.A

    June 30, 2012 at 1:40 am

    Sir
    I got very good msg from your comment thank you
    Manikandan.A

    Likes(0)Dislikes(0)
  5. daniel

    September 11, 2012 at 5:27 am

    sir,
     when i measure voltages on a hot grounds which is the primary side it is ok. when i measure voltages on a secondary side which is a cold side it is non but when i placed neg. prong on the primary side there is a voltg.  on the secondary side.....please help!!

    Likes(0)Dislikes(0)
    • Jestine Yong

      September 11, 2012 at 10:23 pm

      Hi DANIEL,

      Could you elaborate more about the type of power supply you are dealing? What type of equipment and what was the complaint? Is there any output voltages from the power supply?

      Jestine

      Likes(0)Dislikes(0)
  6. daniel

    September 15, 2012 at 3:03 am

    yes sir there is an output voltage of the power supply or the primary side when i measure the voltage with respect to the hot ground or  to the neg. leg of  big capacitor but when i measure the secondary side by placing the neg. prong to the chassis or the neg. leg of secondary capacitors there is no voltages. so i try to place back the neg. prong to the primary side in neg. leg of big capacitor there is reading that i measure on the secondary capacitors. Sir ........please help again
                                                                                                                                                 practicing to repair tv,

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    • Jestine Yong

      September 18, 2012 at 8:38 am

      Hi Daniel,

      This is not the right way to test. If you want to check the secondary side then place the black probe to secondary side. If you want to check the primary side then place the black probe to primary side. In your case I suggest the you check all components in the primary side and if all tested good then direct replace the power ic and retest.

      Jestine

      Likes(1)Dislikes(1)
  7. daniel sabado

    October 3, 2012 at 8:43 pm

    hi sir,
    thank you very much for the help.....i replce the power ic and yes its now working........i want to buy some books to you sir but my problem  i don't have any account and how i going pay sir.....help?
     

    Likes(0)Dislikes(0)
    • Jestine Yong

      October 3, 2012 at 9:31 pm

      Hi Daniel,

      I have sent a personal reply to you. You can check your email.

      Thanks.

      Jestine

      Likes(0)Dislikes(0)
  8. Maurizio

    February 21, 2014 at 1:16 am

    Sir,
    how is the meaning of the small ceramic capacitor between hot and cold grounds in some SMPSs? Is that the responsible of the slight current dispersion of the PSU as detected by a simple test light?

    Likes(0)Dislikes(0)
    • Jestine Yong

      February 22, 2014 at 9:20 pm

      HI Maurizio,

      That capacitor has nothing to do with the light bulb test. The capacitor is use to reduce the mains pollution.

      jestine

      Likes(0)Dislikes(0)
  9. dalganjan

    November 3, 2016 at 12:04 pm

    my dth receiver cold ground touching cause electrical souck. can above sad capacitor sort or leak can couse this problem?

    Likes(0)Dislikes(0)
    • Jestine Yong

      November 3, 2016 at 4:05 pm

      HI Dalganjan,

      Have you checked if the earth ground of your receiver is properly grounded? Yes if the capacitor is leaked it can also cause electrical shock.

      Jestine

      Likes(1)Dislikes(0)
  10. dalganjan

    November 6, 2016 at 10:20 pm

    thank you sir you right it was small ceramic capacitor connected to hot ground and cold ground i replaced it and now not getting electricle sock touching cold ground.

    Likes(1)Dislikes(0)
    • Jestine Yong

      November 6, 2016 at 10:29 pm

      Hi Dalganjan,

      Congratulation. What happened to the capacitor? Open circuit or capacitance value out? What is the value and rating of that capacitor?

      Jestine

      Likes(1)Dislikes(0)
  11. dalganjan

    November 8, 2016 at 12:59 pm

    it's ceramic capacitor number printed 102m 1kv when i use my analog multimeter it measure 20k oms

    Likes(1)Dislikes(0)
    • Jestine Yong

      November 8, 2016 at 1:11 pm

      Hi Dalganjan,

      Thanks a lot.

      Jestine

      Likes(1)Dislikes(0)
  12. AMSTRONG

    January 7, 2019 at 7:36 pm

    Hello Sir please I will like to know where the cold ground is located on the circuit board

    Likes(0)Dislikes(0)
    • Jestine Yong

      January 8, 2019 at 2:28 pm

      Hi Armstrong,

      The cold ground is the power supply secondary side ground which is also your house earth ground.

      Jestine

      Likes(0)Dislikes(0)

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