Does My AC to DC Schematic Have A Problem?

Discussion in 'General Electronics Chat' started by ajm113, Apr 28, 2011.

  1. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
    5
    Hello!

    I'm currently starting a new project and I wanted to experiment with AC alarm system power supplies. (You know those big fat ugly wall-warts you screw on a outlet that have two screw terminals?) Well I currently have a DSC PTC1640U I want to use for my experiment.

    The DSC Power Supply Specs:
    Primary Voltage: 120VAC 60Hz 0.48A
    Secondary Voltage: 16.5V 40VA


    My problem is I run my test schematic through a simulator before I build it to avoid problems or components to catch on fire/smoke. It appears my voltage keeps dropping and it looks the only way to fix this, is too add a ground on where the capacitor connects on the bottom (On the schematic I uploaded).

    If I do have to add a ground, where does it come from? If my "power supply" wall wart act as one and it's connected through the diodes, where do I connect the ground to exactly? Or ... am I going about that all wrong and have to change something else?

    Reference Links:
    Where I got the schematic: http://www.youtube.com/watch?v=cyhzpFqXwdA
    My Simulator (Use With MySchamatic.txt): http://www.falstad.com/circuit/
     
    Last edited: Apr 28, 2011
  2. PackratKing

    Well-Known Member

    Jul 13, 2008
    850
    215
    At least you are using a transformer off your mains, your thread would be closed in short order otherwise.

    In the schematic, your "load resistor" should be in parallel, downstream right next to the filter capacitor.

    it is normal for open circuit voltage to be a tad higher than specs indicate, tho' as soon as you place a working load on it that draws ~ a couple amps, your voltage will drop, which is also normal.......
    It's why using a mismatched wart, that puts out more voltage than a given device needs, will be allowable if the device loading the wart pulls the overvoltage down to specs, with whatever amperage the device wants to draw.............


    Confused yet ?? :p
     
    Last edited: Apr 28, 2011
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    I have tried this applet and it seems to suffer from mis-convergence issues with an LED inserted as in your diagram. Note that with only 10 ohms in series, that LED would have to be a mighty big one: the current would be over an Amp!

    Adding a ground does seem to help the simulation maintain convergence - but this is a property of the simulation process and does not reflect the real world. Connecting the (negative?) side of the output to ground may be desirable for safety reasons, but that is another matter

    What do you mean by a "compactor"; do you really mean "capacitor"? A compactor generally means a device for squeezing loose material into a denser form, like this: http://en.wikipedia.org/wiki/Compactor
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    SPICE simulations need to have a ground reference somewhere in the schematic, and every part of the schematic needs to have a path to the ground reference. Otherwise, voltages will be arbitrarily large; large enough to overflow the variables used for such data storage.

    In the real world, you connect the negative side of your supply output to the earth ground from the wall. You also fuse the "hot" side of the transformer with a 1A to 2A fuse in an approved fuse holder. That way, in case a fault develops in the transformers' windings, the fuse blows instead of you being electrocuted.
     
  5. ajm113

    Thread Starter Member

    Feb 19, 2011
    176
    5
    Okay I see now! I was a bit afraid, because of what the simulation is giving me of the voltage. I didn't want my IC's to turn on and off constantly messing it up.

    @Adjuster, haha sorry about the misleading typos. I kinda just woke up and I noticed I was still making grammar errors when I was on Skype. :p

    Anyways that 10 ohm, was actually a 1K ohm (I was playing around with the variables) and yes I did mean capacitor.

    And that makes much more sense to me now! I was wondering if that simulation was accurate or not, because from what I saw on that video I got differently in the simulation, so I wasn't sure if I was doing something wrong to make it go negative 900 mV.

    Thanks for the help guys! I'm going to put this thing together and see what I get on the multimeter.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    I think your diode bridge is rotated 90°.

    [​IMG]

    I'm surprised no one else caught it.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Bill,
    While I generally prefer the schematic convention that you have shown for a bridge, our OP's schematic is electrically correct, and it still follows the "inputs from the left, outputs to the right" and "positive towards the top, negative towards the bottom" conventions.
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Right you are. Never mind.
     
  9. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Thought I'd mention my preference in recent years. Once I started working more with multi-phase systems, I began using the convention that scales easily with the number of phases, as shown in the attached drawing. It meets the requirement for inputs on left and outputs on right, relieves the need for diagonal components and connections and allows all diodes to be in the same direction. Also, it's easy to keep adding phases and keep the same structure.

    Another benefit for me (as someone with bad memory who gets easily confused), is that I have no trouble remembering how to draw the usual one-phase bridge configuration. Previously, I always had to stop and think about the current flow direction to get it right.
     
    Last edited: Apr 28, 2011
  10. rapidcoder

    Member

    Jan 16, 2011
    37
    3
    Falstad's applet is not a SPICE simulator. It uses some home-made algorithms which are of a very poor quality - sufficient for educational purposes, but not for any real work.
     
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