AC main and power supply common reference

Discussion in 'General Electronics Chat' started by olvine, Jul 7, 2014.

  1. olvine

    Thread Starter Member

    Mar 10, 2014
    99
    0
    Hey all..

    I am working on a Digital AC voltmeter.
    There are two parts in circuit one with microcontroller and having a constant 5V and 0V as VDD and VSS Power supply.
    The other part contains resistors and diodes which inputs AC voltage....
    The Question is how can I connect the whole circuit to one reference point ?
    what happens if I connect the AC main's neutral with Power supply -ve terminal ?
    What should be done to achieve the common reference..?
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,504
    380
    hi,
    I would strongly advise you NOT to do that.

    Are you using an isolation transformer in your AC meter circuits.?
    Posting a diagram of your proposed circuit would be helpful.
    E
     
  3. olvine

    Thread Starter Member

    Mar 10, 2014
    99
    0
    No, I am using only a Resistor divider network to attenuate the voltage n then rectifying it using single diode....after this I am feeding the signal to analog input of a microcontroller...
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,504
    380
    hi,
    I would recommend the use of a step down isolation transformer in order to ensure a higher level of personal safety...non isolated mains projects can be a serious safety hazard.

    E
     
  5. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    @ olvine

    This is an exceptionally bad design for a power supply. Especially in your first steps in electronics you should always use an enclosed and fused isolation / step down transformer to obtain a safe voltage to work with. The transformer should be sized (in Watts or VoltAmperes) appropriately for the needs of your project.

    It is not clear what you are trying to do. Do you want to measure the line voltage with a microcontroller which is powered by the same line?
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Circuits directly connected to the mains are not allowed over here.
    Isolation is a must.
    See the ToS for more info:
    Terms of Service

    If you still proceed with the direct mains connection, I will close the thread.

    Bertus
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,568
    2,380
    I am not sure of the electrical system in Islamabad, but it is sometimes normal to common the power supply common of circuitry and earth ground if needed, but never the neutral itself, in spite of the earth ground and neutral being common at some point.
    This is the customary way that practically all P.C. power supplies use.
    IOW your mother board common is at earth potential also.
    Max.
     
  8. olvine

    Thread Starter Member

    Mar 10, 2014
    99
    0
    @ericgibbs

    I am just trying to make an AC digital voltmeter and I can't use isolated transformer because of the cost. AC mains is just one voltage source to measure. I can use transformer to step down the voltage and then measure that voltage to check whether voltmeter is measuring correctly or not.
     
  9. olvine

    Thread Starter Member

    Mar 10, 2014
    99
    0
    @Georacer

    I want to design a Ac digital voltmeter. Line voltage could also be measured. power supply could be same or could be different.
     
  10. olvine

    Thread Starter Member

    Mar 10, 2014
    99
    0
    @bertus

    Ok I will use a step down transformer and measure that voltage for testing.
     
  11. olvine

    Thread Starter Member

    Mar 10, 2014
    99
    0
    This is the simple design.
    In the schematics You'll see a ground connected to the AC source....without this ground I cant get the correct values on the microcontroller and also on the oscilloscope. But the measurements on the voltmeters remains same.
    I want to know how can I achieve this practically ?
     
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