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Rectifier output?

Discussion in 'Electronic Basics' started by Gloria West, Sep 24, 2009.

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  1. Gloria West

    Gloria West Guest

    120vac input to full-wave rectifier bridge. Without any filtering components
    what is the output RMS voltage?

    This does not use a center-tapped transformer or such.

    Thanks.
     
  2. Phil Allison

    Phil Allison Guest

    "Gloria West"

    ** 120 volts, of course.

    If you connect a 120 volt lamp to the DC output, it will be the same
    brightness as usual.

    If you add a filter capacitor, the rms voltage rises up to a maximum of 170
    volts DC.



    ...... Phil
     
  3. Gloria West

    Gloria West Guest

    120vac input to full-wave rectifier bridge. Without any filtering components
    Relays run with this rectified supply are buzzing. These are in a 45-year-old
    printing press.

    I want to replace them with something more modern (the contacts are also
    intermittent).

    I found some that are rated for 110 dc:

    <http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-
    1&storeId=10001&catalogId=10001&productId=301209&>

    Will these buzz also? Is there a simple way to keep them quiet?
     
  4. Phil Allison

    Phil Allison Guest

    "Gloria West"

    ** Yep.

    Add a 4.7uF, 250 volt electro across the output of the bridge and put a 4700
    ohm, 1 watt resistor in series with the 110 volt relay.

    This should give you 160 volts DC average on the cap and 110 volts on the
    relay.

    I used ohms law and the formula: I = Cdv/dt to get the values.


    ....... Phil
     
  5. John G.

    John G. Guest

    And who is to say the contact rating will be correct?
    Why don't you fix the problem (whatever it is ) rather than trying to
    redesign the machine from a position of not enough knowledge?

    John G.
     
  6. Gloria West

    Gloria West Guest

    Add a 4.7uF, 250 volt electro across the output of the bridge and put a 4700
    Thank you Phil.
     
  7. Why don't you just fucking answer her question instead of trying to spout
    your superiority? If she wants to **** it up then it's her choice. It isn't
    going to cost you anything. You assume she's not intelligent enough simply
    because no one could have even close the intelligence you have? Or is it
    that you really care about her safety? Yeah.. I bet...

    Get over it... you are not the smartest nor ever will be even close. Trying
    to keep other people down does not make you more intelligent.

    Of course the real reason is you simply don't want any competition.
     
  8. John G.

    John G. Guest

    Actually there is no way to answer her question because she never gave
    enough data.
    And it has long been proven that the best way to repair a device is to
    return it to its design standard rather than redsign it without enough data
    or design knowkedge.

    John G.
     
  9. So even if it was designed improperly it's still the best to fix it exactly
    the way it was? That sounds very intelligent. In any case, that is not the
    problem. The problem is that you are assuming she is a moron. That may or
    may not be the case but why do you do so? She asked a question and you gave
    her a statement. Not just any statement of course but a condescending and
    arrogant statement. A common trait among engineers and liberals.

    Just because someone does not give enough information does not mean you get
    to be arrogant. Either assume what you can or ask for more information but
    personal attacks have nothing to do with engineering or helping someone.
     
  10. ~0.7*Peak. 120V is RMS. The rectified RMS is exactly the same. Think of it
    as power. Rectifying it will heat up a resistor jut as much cause the same
    amount of power is dumped into it. Mathematically, squaring a the abs of a
    function is the same as squaring it.

    So |f|^2 = f^2 (assuming f is real)

    So it does not effect the result.

    (Note that 0.7*Peak is only valid for pure sinusoidal waveforms)
    There are two main types of rectifiers, AC and DC. AC rectifiers work
    directly from AC without the need for rectification(They sorta have it built
    in). DC needs dc or you will get "chatter". The chatter is from the contact
    in the relay getting turned off and on repeatedly. This is due to the AC
    reversing the magnetic field that normally would pull(or push) the contact
    closed. What you get then is the ac pushing(or pulling) and then pulling(or
    pushing) the contact.

    Even if you rectify the AC so it is "DC" the DC relay still will chatter
    because the rectified voltage is too low at some points in time. You can
    look up on the net exactly how relays work for it to be clear.

    So, if they are DC relays, which most likely they are because of the
    rectification, then you have to use a filter cap, a cap that "smooths" the
    ac to dc like. Basically when the AC goes low, the cap will act like a
    battery and supply current to the relay until the AC can get back up. The
    cap has to be sized so that it is large enough to handle the current needed.


    Generally electrolytic caps are used. It does not hurt too much to use too
    large of a cap but using too small will still create chatter. Electrolytic
    caps are polar which means you gott hook the + to the + side of the
    rectification.
     
  11. It's not 'fix' that's been described, it's 'redesign'. Changing a
    branch
    circuit from AC to DC might be OK, or (depending on the situation)
    might
    be disastrous. Switch contacts and fuses that work fine on AC are
    unsafe
    with DC, for instance.

    An acquaintance once had to dismount a 1/4" steel plate to access a
    switching SCR, and wasn't keen to lift the plate back into position
    when
    he was done. He hunted up the original designer, who recalled that
    the circuit that SCR was on had a quarter-million joules of stored
    energy in a magnetic field. The steel plate went back up that
    afternoon.
    ----

    I haven't seen anything mentioned about a redesign. She wants to replace
    components that have failed. Replacing components is not considered a
    redesign in my book. There have been many things I've "fixed" by replacing
    components that have failed yet it would not even have come close to what I
    would call being redesigned.

    Obviously one has to have some idea about the circuit involved along with
    it's intentions. Replacing relays cannot ever cause any serious harm unless
    the design was flawed from the begining. Obviously there is the slight
    chance of a catastrophic failure but that can happen even with the original
    design. No design is perfect and hence has a redesign that can make it more
    perfect(theoretically).

    Almost all designs, in the universe of designs, are implemented in a cost
    effective way. As you know, this directly competes with a perfect design.
    While the best option is usually to replace broken components it is not the
    only option. We've all experienced poorly designed systems that could be
    redesigned. It's neither her nor there because the origina question had
    nothing to do with a redesign. Simply replacing components is not
    redesigning the system.

    When you replace a battery in your remote did you redesign it? Even if it's
    not the same battery? (Maybe if your a pedant)

    In any case, it's not up to me, you, or anyone else to decide if anyone has
    the right to redesign something unless someone's safety is at issue. But we
    shouldn't take this too far since every person has the right to put himself
    at danger and even someone other people. Most devices that are in wide use
    have the potential to kill people but we don't not complain about such
    things and ban them. This is the cost of progress and convienence.

    i.e., don't get overboard on such a simple thing. Replacing relays isn't
    going to cause a nuclear meldown and kill millions of people. Probably not
    even 1. If the wrong relays are used either:

    1. They will burn up and need to be replaced again.
    2. They will burn up something else.
    3. They will not work as intented.
    4. The device could work even better. (specially if the device no longer
    works)
    5. Much progress can be made if the device did not work before or cause
    major problems because of the relays.

    The probability of Gloria killing herself is pretty low. Probably lower than
    the probablity of you dieing from a car wreck tomorrow. (yet you don't
    hesitate to drive?) The probability of her killing or injuring someone else
    is even much lower. The relays could potentially malfunction at the most
    inoppurtune time but the likelyhood of this happening is probably rather
    low(mainly because relays don't generally fail in such a way).

    This is a printing press and not a nuclear reactor. We see many people
    doing stupid things on a local scale and don't complain about it so no need
    to do it here. If we woried about what *might* happen then we wouldn't get
    much done.

    If Gloria wants to redesign it then I'm all for it. Who knows... maybe she
    will make the thing work much better. If she kills herself in the process
    then at least she tried to do something instead of being paralyzed by fear.
    Progress has never been easy and never will be. (well, except if Obama gets
    his way)

    Of course what you guys are really saying is "All you stupid people gotta
    leave it up to the "experts"! Just give me a call and we'll discuss how much
    it will cost you for me to redesign it for you."
     
  12. Yes ;/ Guess I'm starting to get senile.
     
  13. Ross Herbert

    Ross Herbert Guest

    :120vac input to full-wave rectifier bridge. Without any filtering components
    :what is the output RMS voltage?
    :
    :This does not use a center-tapped transformer or such.
    :
    :Thanks.

    The relays, presumably, did not "buzz" when the system was first designed.
    Unless the spring loading on the armature has been increased then the relay
    should not buzz when driven from a non-filtered bridge rectifier output. Since
    the mains input period is 16.7ms there is only a very brief period (somewhere in
    the region of 3-5ms) between the 8.35ms half cycles when the voltage will be
    reduced below the holding voltage of the relay and the normal magnetic inertia
    of the magnet system should be sufficient to keep the armature from chattering.

    The new relays in your link have a nominal release time of 10ms and the 110Vdc
    coil has a resistance of 10Kohms - which indicates a large number of turns with
    significant inductance. When you replace the relays, and if buzzing does occur,
    try connecting a 10Kohm 3 to 5W resistor directly across the relay coil. This
    will allow the back emf to "slug" the release time and stop the buzzing.
     
  14. Eeyore

    Eeyore Guest

    Surely very slightly less, e.g. 118.5 due to 2 x Vf of the diodes in the
    bridge but without If being specified and the specific diodes, hard to
    be exact ?

    Graham
     
  15. Phil Allison

    Phil Allison Guest

    "Ross Herbert"

    ** Just tried a couple of 48 volt relays: a Finder octal base type (DPDT
    10A)
    and an Omron PCB mount ( SPDT 16A) miniature.

    Both worked just fine with full wave rectified, 50Hz AC.

    Both closed with just 30 volts rms supplied via a bridge rectifier = 26
    volts DC average on the coil.

    NO TRACE OF BUZZING NOISE WHATSOEVER !!

    However:

    With half wave rectification, both relays buzzed loudly and would not close
    at any voltage

    Conclusion:

    The OP simply has a faulty bridge rectifier.


    BTW:

    110 volt DC relays are apparently designed to work from rectified US mains
    with no filtering.



    ...... Phil
     
  16. Baron

    Baron Guest

    He can't speel either !
     
  17. Ross Herbert

    Ross Herbert Guest

    :
    :"Ross Herbert"
    :>
    :> The relays, presumably, did not "buzz" when the system was first designed.
    :> Unless the spring loading on the armature has been increased then the
    :> relay
    :> should not buzz when driven from a non-filtered bridge rectifier output.
    :> Since
    :> the mains input period is 16.7ms there is only a very brief period
    :> (somewhere in
    :> the region of 3-5ms) between the 8.35ms half cycles when the voltage will
    :> be
    :> reduced below the holding voltage of the relay and the normal magnetic
    :> inertia
    :> of the magnet system should be sufficient to keep the armature from
    :> chattering.
    :
    :
    :** Just tried a couple of 48 volt relays: a Finder octal base type (DPDT
    :10A)
    :and an Omron PCB mount ( SPDT 16A) miniature.
    :
    :Both worked just fine with full wave rectified, 50Hz AC.
    :
    :Both closed with just 30 volts rms supplied via a bridge rectifier = 26
    :volts DC average on the coil.
    :
    :NO TRACE OF BUZZING NOISE WHATSOEVER !!
    :
    :However:
    :
    :With half wave rectification, both relays buzzed loudly and would not close
    :at any voltage
    :
    :Conclusion:
    :
    :The OP simply has a faulty bridge rectifier.
    :
    :
    :BTW:
    :
    :110 volt DC relays are apparently designed to work from rectified US mains
    :with no filtering.
    :
    :
    :
    :..... Phil
    :
    :
    :

    Thanks for confirming my hypothesis Phil. I concur the most likely reason for
    the OP's buzzing relay may be a faulty diode in the bridge.
     
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