:: Newbie - Ground Question ::

Discussion in 'General Electronics Chat' started by suby786, Oct 27, 2006.

  1. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    Hey all,

    i have a grounding question, all the circuits i have seen to do with 240V AC have confused me a bit..

    the three wires coming in are L/N/and E... L and N go to your transformer (in my application)... on the other side you have the 4 diodes and capacitors, the symbol for ground is the TRIANGLE, and the symbol for the EARTH WIRE goes to CHASSIS GROUND...

    Now my project will ONLY have the plug connection, i.e 3 wires going into it, so where do i connect all my SIGNAL GROUNDS too? (triangle symbol). Is it safe to ground EVERYTHING to the E wire?

    With NO ground connected, i get 5v from my 5v regulator, with a floating ground... is this normal as that seems odd 2 me...and will this work ok?

    When i tie all my signal grounds to the E wire, the wire gets VERY HOT (its 16G wire, like normal cable... the hot wire indicates it doesnt like me connecting this up... any ideas? any help would be WKD... Heres part of the ckt...
     
  2. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    Heres part of the circuit
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    With the transformer secondaries paralleled, you really can't use a bridge rectifier. When you tie the D1 & 3 anodes to the earth lead, you short the transformer to ground every half cycle.

    Try a rearrangement of your circuit. Drop the connections between the transformer leads 8 & 6 and 7 & 5. Connect 6 & 7 together as a center tap, and use it as your ground reference. Run lead #5 to the D2 anode and #8 to the D4 anode. Eliminate D1 & 3 altogether. This will give you a full-wave rectified output from the transformer, plus the center tap of leads 6 & 7 may be tied to the earth lead so circuit ground no longer floats. Connections to the filter cap and regulator need no changes.
     
  4. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    hey mate

    thank u so much, you've been a great help,,, so i only need 2 diodes to have a full rectified signal? i thought 2 diodes = HALF wave and 4 diodes = FULL WAVE?

    So it is ok to tie the centre tap to the EARTH lead (E) from the plug? so that mean ALL SIGNAL grounds are connected to the EARTH?

    Ok at the moment im not usin a transformer as 240v in my bedroom is a bit unsafe for me to play with so im using a variable AC power supply, so ive set it to 12v AC... so at the moment im pluggin the L and N of the powersupply to D2 anode and D4 anode respectively...i also have an E wire, from the variable ac supply, can i connect this STRAIGHT to all my grounds?
     
  5. suby786

    Thread Starter Member

    Oct 27, 2006
    10
    0
    also i havent chosen a transformer to use, so its not necessarily going to be the 1 in the scematic.. so if you could suggest the BEST way of doing it, ill get the components to do that... my main issue is the ground connection..

    3 wires coming in... i assumed the only TRUE EARTH would be the wire i connect all my signal grounds too...
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    I'd like to say yes about the earth lead, but I'm a bit out of my depth with Australian house wiring conventions. If your E lead makes a good electrical connection to the metal frame of your variac, then it should go to earth. Likewise the electrical common of your power supply. I hope you have a meter to check for the frame connection.
     
  7. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    I hope your variable AC supply is not a Variac, as they are NOT isolated from the mains (look up "auto-transformer"). You would need an Isolating transformer in the mains line to make a Variac safe (and these normally do not have the earths on the isolated outputs connected..... isolated).
    Both NZ and Aus regs specify less than 1 ohm from the case or any exposed metal on any Non double insulated appliance to the earth pin on your mains plug.
    On an Earthed appliance it is fine to use mains earth as a signal earth. Watch out for earth loops/hum loops. Sometimes isolating the the mains earth from signal earth with a 0.1 to 0.47 uF 500v capacitor can help.
     
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