reference point for single supply

Discussion in 'General Electronics Chat' started by OliG, Apr 11, 2010.

  1. OliG

    Thread Starter New Member

    Apr 11, 2010
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    Hello,

    I'm looking to make an all in one solution to get a momentary switch working as an on-off switch. I have found the folowing schematic who does exactly what I need with a hex inverter and a relay :
    http://www.electronic2002.8k.com/alt1.htm

    I want to power this with a standard 0-12V regulated supply and put all components on a single pcb. I have made a layout but I cant figure out what is the best option (I know this is a newbee question):
    - use a ground plane and connect it to the chassis.
    - use the AC return and do not connect the ground.

    Here is the schematic (sorry it is a bit ugly, didnt wanted to export at first) :

    [​IMG]

    Here the 2 layouts :

    with ground plane and ground output :

    [​IMG]


    And the one without the ground connection :

    [​IMG]


    I also have a question concerning the led relay (G5LE) wich is a SPDT, is it ok to ground the unused second pin as I did in the first layout? I know it would be clever to use a dpdt relay, instead of two relays but I try to use what I already have.

    I know this is a lot to ask in a first post but I would really appreciate some advices.

    Thanks a lot !

    Oli
     
    Last edited: Apr 11, 2010
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The relays is shorting the powersupply as drawn.
    Isolate pin-O from relays K2.

    Bertus
     
  3. OliG

    Thread Starter New Member

    Apr 11, 2010
    6
    0
    Thanks Bertus, that answer my second question.

    I have isolated pin O, the layout now look like this :

    [​IMG]


    What about the first question? is it better to ground to chassis with a 3 pins connector, or to use only the AC return?

    Thanks again !

    Oli
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Are you going to be using a 3 prong plug? If so, you can connect the ground plane to the AC ground.


    If no, use the AC return. That will avoid having a possible "hot chassis"
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I do not see a fuse. You must use a fuse on the primary side of the transformer.

    We do not know where in the world you are, so don't know what kind of mains power you have.

    In the States (USA), a typical 120VAC 60Hz wall outlet accepts a 3-prong plug:
    1) Hot - usually a black wire; 120VAC relative to the Neutral wire.
    2) Neutral - usually a white wire; this is the "power return" wire, nominally within a few volts of earth ground.
    3) Ground - this is earth ground, connected to long copper-clad spikes driven into the earth, for safety purposes.

    It is most preferable to use earth ground for chassis ground, where available. The earth ground should never be used for anything but a safety ground; there should be no current on the ground wire.

    If you do not have 3-wire outlets, then you need to determine which of the wires is "hot" and which is "neutral". If you cannot determine that, then you will need to have your circuit and chassis completely isolated from the mains.

    You need to use at least one fuse on the primary side of the transformer, on the "hot" wire. If you wish to include a power switch, it should be between the fuse and the transformer.

    Your AC trace going to pin 5 of the transformer is much too close to the mounting pad.
    I suggest running it straight down for a bit, then making a 45°bend until it lines up with the transformer's pin 5, and then straight to the pad.

    You need more isolation between the ground plane and the AC traces.

    You might wish to make connector AC_IN pin 1 as HOT, pin 2 as NEUT, and pin 3 as GND.
    You should label these in some manner on the board itself to help avoid confusion/mistakes when assembling, or repairing some years down the road.
     
  6. OliG

    Thread Starter New Member

    Apr 11, 2010
    6
    0
    Thank you retched and SgtWookie for your answers.

    I live in Canada, so yes this circuit will be plugged to a 3 prong plug. Sorry for the confusion about the AC_IN connector, it does not represent the AC from the wall outlet, but the input for the transformer. I also forgot to upload the new schematic who show a 3 pin connector as in the last layout.

    There will be an all in one IEC module in between that include a switch and a fuse. The ground of the IEC will be connected to the metal case and all ground planes will be connected to this place via a single heavy gauge wire.

    I will rename the AC_IN pins as you suggested SgtWookie, and so will I redo the AC traces to isolate from the mounting pad. I will also isolate more the AC from the ground plan. I guess I should do the same for the TO_PSU connector, as it will be carrying AC to an other power supply.

    What I still dont understand, is concerning the AC neutral vs Ground. if you look at the schematic, the return path to neutral is also tied to ground. So the ground plane goes to neutral and ground. I know this is a common practice for safety, but how can I ensure that there is no current in the ground wire? Is it because the voltage drop of the neutral is within the ground voltage drop so current will take this path?

    This is probably an evidence for you guys, but I'm still looking for the answer...

    Thanks again !

    Oli
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    This is confusing. Are you saying that you have a transformer external to this circuit, and there is another transformer on the board?

    My brain is on half-speed today. IEC?

    Yes.

    The AC hot and neutral are isolated by the transformer.

    The ground simply establishes the 0v reference point for the chassis and board ground layer. Once the 0v reference has been established, there should be no current flow in the ground wire; the current will come from the transformer secondary, and be returned to it.

    Back to your schematic; it is terrifically hard to read due to the black background. I've tried inverting the colors and changing it to black/white, but it's still hard to read.

    What is U2? A 556 dual timer?

    Transistor T1 has no part number. You are using it as a voltage follower rather than as a saturated switch. It has a 10k resistor in the base.

    If U2 is a 556 timer, it's output will not go higher than Vcc-1.3v. Since you are using T1 as a voltage follower, it's emitter will not go higher than Vb-0.7v. So, you are losing at least 2v from your supply, assuming T1 has infinite gain.

    However, T1 will not have infinite gain, and you will have more of a voltage drop across T1, resulting in excessive power dissipation. This will likely result in neither of your relays engaging, and the transistor burning up.

    It would help a great deal if you would fix the background colors in your schematic capture program. What program are you using?
     
  8. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    For info, IEC connectors are the common power input connectors on computers etc.
    There are various standards, the typical desktop computer ones are IEC C13/C14, notebook 'brick' PSUs often C5/C6 or 2 pin C7/C8 etc..
     
  9. OliG

    Thread Starter New Member

    Apr 11, 2010
    6
    0
    Hello,
    Thanks for the time you take to help me out, I appreciate.

    Again, sorry for the confusion, I try to be as clear as possible but english isnt my primary language.

    As rjenkins explained, the IEC module is a type of power inlet. The one I have for this project has a built in switch and a fuse, so there will be a fuse on the primary side of the transformer. The Earth ground from the 3 prong plug will be tied to chassis.

    IC2 is a hex inverter (MC14069), this is a combination of 6 NOT gates in one IC. Datasheet :
    http://www.onsemi.com/pub_link/Collateral/MC14069UB-D.PDF

    T1 is a NPN General Purpose Amplifier and switch (2N4124). Datasheet :
    http://media.digikey.com/pdf/Data Sheets/Fairchild PDFs/2N4124.pdf

    As I already said, I did not design this schematic. I found it while looking for a solution to my problem, to get an on-off switch from a momentary switch. The original design can be found here :
    http://www.electronic2002.8k.com/alt1.htm

    What I did was to simply add a second relay to power a led integrated in the switch, and add a single rail 12VDC power supply. I know the original schema show 9V but I changed to 12V because 12V coil relays are easier to find. Here is the original psu schema :
    http://electrossama.ifrance.com/electro/IMAGES/schema01_7.gif

    Here is the schematic, I did export it on white background but it is still hard to read with grey writings... I am using Eagle Cadsoft. I did export it to 250 dpi so you can zoom it without losing clarity.

    [​IMG]

    And here is the reworked layout, I did isolate the AC more from ground plane. I will Add the "neutral" and "ground" to AC_IN connector at the end.

    [​IMG]

    So again, what I dont understand clearly is why on both schematics I have linked the return path is tied to earth and to neutral. If ground simply establishes the 0v reference point for chassis and board ground layer, and no current should flow in the ground wire, why is the ground connected to the return path?

    If you look at the layout, the return path and ground plane are the same thing... Connected to both the return of the transformer and to earth. I am not sure at all if this is ok. but if I follow the 2 reference schematics, I do not see the error.

    Again, I know this is a lot of reading and a lot of questions... But I will be very glad to get answers.

    Thanks a lot !

    Oli
     
  10. OliG

    Thread Starter New Member

    Apr 11, 2010
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    0
    Ok, I think I understand now... The neutral is almost at the same potential as ground. Neutral is connected to ground but serve as the return path for current.

    The ground symbol on schematics is only there to illustrate the 0V potential of the neutral, not a physical connection to ground via the earth of the 3 prong plug.

    Here is where I found really usefull infos :
    http://www.allaboutcircuits.com/vol_1/chpt_3/8.html

    Thanks SgtWookie, you already had told me the correct way to make this circuit safe, I simply didnt understood it at first.

    I really would have like to find the excellents volumes provided on this site before... I will now rework my layout...
     
    Last edited: Apr 13, 2010
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The only place that neutral and ground should be connected together is at the electrical service panel; ie: breaker panel, fuse box. Inside the panel, the neutral bus is connected to the ground bus.

    On a circuit that is carrying power, the further you get from the panel, the greater the voltage drop across both the hot and neutral lines, depending upon the current flowing through them. The ground wire should ALWAYS be at a 0v potential, as no current flows through it.

    No, ground is ground, a 0v potential.
    One lead of the primary side of your transformer is connected to Neutral, and the other lead is connected to Hot via a fuse.

    Use the earth ground for the secondary side of the transformer and the board/chassis to keep it at a potential of 0v. There will not be any current flowing through the earth ground unless there is a fault in the transformer or primary side winding.

    Every circuit needs some point to call a 0v reference, even if earth ground is not available. Without this 0v reference point, it would be difficult to communicate what the voltages in the circuit should measure.
     
  12. OliG

    Thread Starter New Member

    Apr 11, 2010
    6
    0
    Hello,

    Can you please elaborate a bit on this propriety? Does it mean that the transformer will pull as much current as it push to the rectifier diodes, even if the earth ground is also connected to the return path? In this case, that would be why there is no current flowing in the ground wire because the rectifier diodes pull all the circuit current?

    Thank you,

    Oli
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Any current flowing out of one wire of the transformer secondary must go back in the other wire of the transformer secondary.

    Without that, you would not have a complete circuit.

    There won't be any current flow in the ground wire, unless there is a fault on the primary side, or the transformer becomes shorted internally.
     
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