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About DC Supply Voltage

By on May 31, 2011

If you repair any electronic circuit board and found that the supply voltage is lower than usual then there are two possible causes. If the supply line is 12 volt but you only got 8 volt at the vcc pin of an IC, you will know that it is either the IC itself have problem or the supply line can’t sustain itself when there is a current draw. To confirm which one is the cause, just touch on the IC to check if it too hot to be touched or not. If it is very hot you know that it is indeed the IC itself that have drained the extra current causing the supply voltage to drop. If the IC remain cold then the problem is in the components along the supply line. It could be a bad filter cap (that have high ESR) ,leaky diode and etc.

If the supply is coming from a voltage regulator IC like 7812/7805 then there is also a possibility this IC could breakdown when under load.



  1. ghofin

    June 1, 2011 at 12:48 am

    Hi Mr Jestine, I want to ask how to cope if the power supplay pc 12v down to 11v, 5v and 3.3v but that does not go down. How do I fix this? Thanks.

  2. Jimmy Lane

    June 1, 2011 at 10:09 am

    Hey Jestine, this is good, practacle, troubleshooting. If you do have a low voltage at an ic like 8 volts and it comes from a 12 to say 25 volt source you probabley have a dropping resistor too that is going to get hot and could change value. I used to see this a lot in Zenith tv vertical circuits. Also if you are not sure if the power source or load is the problem, disconnect the b+ line and substitute an variable dc power supply with current limiting to test with. This way you wont burn up parts if you set the current to a low amps and see if the circuit works. I learned this several years ago from a tech at the sencore factory. Hope this helps someone. Jimmy Lane

  3. admin

    June 1, 2011 at 10:47 am

    HI Jimmy,

    That was a good sharing and hope readers will not have any more problem in troubleshooting this kind of fault.


  4. admin

    June 1, 2011 at 10:50 am

    HI Ghofin,

    First you need to check the filter cap along the 12 volt line. If all the components in the 12 volt line check to be good then suspect a bad transformer.


  5. Kasim

    June 2, 2011 at 12:46 am

    please clarify my doubts
    1. Can we measure various voltages on crt neck board while tv is working
    2. I am worried about high voltage in tv(lot not the filter capacitor voltage) which keep me away from tv repairing. Can u tell me about the issue. I know how to discharge anod cap and main capacitor. I am only concerened with checking voltage while tv is powered on. Please point out the voltages that i can measure with my multimeter
    that is not dangerous

  6. admin

    June 2, 2011 at 4:51 am

    HI Kasim,

    Yes you can check the voltages on the crt board. Before you start to test on the circuit you must know how the circuit work and if you are note sure you can select your meter to highest range so that it will not spoil your meter. You also must know if the circuit is running on dc or ac. For example, the secondary output diode is dc voltage thus you set your meter to dc instead of ac.


  7. dorin dodita

    June 3, 2011 at 4:30 pm

    felicitari pentru noul site,deasemenea multumesc pentru informatiile si sugestiile tale in depanarea echipamentelor electronice.

    best regards,ddmosu

  8. admin

    June 3, 2011 at 7:56 pm

    Hi Ddmosu,

    Sunteti bineveniti!


  9. Amir Mukhtar

    June 4, 2011 at 1:19 am

    Sir Jestine

    Thanks you for share an important tips about voltages problem fault.
    In pakistan same faults occurred frequently because of UN-stable Ac voltage, surges, and jerks by power supplier companies etc.
    Therefore, i face same type fault while repairing couple of time.

    Also, thanks you for your valuable tips and information.

    A Student
    Amir Mukhtar

  10. admin

    June 4, 2011 at 9:05 pm

    HI Amir,

    Will using UPS helps in your country situation about power instability?



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