120 VAC to 5 VDC Bridge

Discussion in 'The Projects Forum' started by zpriddy1, Aug 20, 2009.

  1. zpriddy1

    Thread Starter New Member

    Aug 20, 2009
    3
    0
    Hello

    I am working on a project and i need to go from 120 VAC to 5 VDC, i have been looking at building a rectifier instead of using a transformer because of my space restrictions. I understand the concept of how to build one, but after i convert it to DC i will be at ≈86 VDC and most of the vregs that i have look at can not handle that voltage, I am looking for any other suggestions on how to do this,


    Thanks!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The suggestion is that you not do that, as it is inherently unsafe. By not including isolation from the line, you risk a lethal shock anytime you come in contact with any part of the circuit. There is no way to make such a circuit safe to the user.
     
  3. zpriddy1

    Thread Starter New Member

    Aug 20, 2009
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    0
    The user would not be in contact with the circuit.. There is a 1 button interface which would communicate with a micro
     
  4. zpriddy1

    Thread Starter New Member

    Aug 20, 2009
    3
    0
    The size constraints that i am under are about the size of a light switch, but i need to have room for more then just the 120 VAC to 5 VDC, i also need to put the other components for the unit in there..
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    You do not quite grasp the problem - the circuit can deliver a lethal shock to the user. It cannot be made to operate safely or to fail safe. There is a lethal shock hazard in any mode of operation.

    You say that this is a simple circuit -
    Surely the microprocessor's DC supply can also be used for the switch?
     
  6. sirknobjockey

    New Member

    Aug 20, 2009
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    Why would isolation from the line be necessary? Using some simple diodes with some caps one can easily transform AC into DC. After that it is a simple matter of using a voltage divider to drop the voltage down, this can even be done in multiple steps, the first step going from 80 V to about 30 V, to 10 V, and then into a standard linear voltage regulator, if each step has another cap to help smooth out the spikes the voltage going into the vreg is going to be pretty clean.

    What would be so dangerous about that? I have seen it implemented on plenty of automatic timer switches that are being sold at HomeDepot and other places. Are you suggesting that they are doing it wrong?
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Without galvanic isolation provided by a step down transformer, a circuit that is attached to the line is at line potential relative to ground. Any contact with part of that circuit results in coming in contact with a lethal voltage.

    A manufacturer may choose to dispense with isolation and use a plastic case and legal notification about "no user serviceable parts inside" as a liability limiter. This site can't guarantee that an inexperience experimenter would not cause harm to himself or another person is attempting to rectify and regulate line voltage.

    Our rules have it that
    We see creating lethal shock hazards as falling within that area. You cannot use a circuit connected to the AC line safely. It always presents a lethal hazard.
     
  8. sirknobjockey

    New Member

    Aug 20, 2009
    2
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    And DC is not harmful at all? ... Seems kinda short-sighted and without merit. DC can be just as damaging as AC. Did you get your education from Edison?
     
  9. peranders

    Well-Known Member

    May 21, 2007
    87
    0
    You can use a "hot" power supply ONLY if the whole circuit is fully insulated and every part regarded as "live".

    You can read more about here
     
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