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Blown resistor in automotive latching relay

Discussion in 'Electronic Basics' started by AshMan40, Apr 25, 2007.

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  1. AshMan40

    AshMan40 Guest

    Hi all. This is my first post to this NG forum, so please be kind.

    I am trying to repair a 12v latching relay for my Volkswagen Beetle
    (~1975). It controls the dimming of the headlights (hi/lo).

    The trigger is a momentary ground signal (S) which actuates the coil.
    The arm that the coil pulls on has a plunger which toggles a rather
    complicated see-saw arm which moves a contact between posts 56a &
    56b. Also attached to the arm is a contact which connects to both
    outputs (56a & 56b) while the arm is being pulled (I can actually only
    see where it connects to terminal 56b on the actual relay). The
    diagram may be a bit off since only one of the outputs is "hot" during
    actual operation.

    The below diagram came from the relay casing. Here is a pic w/ the
    circuit diagram screened on the outside of the casing:
    http://www2.cip1.com/PhotoGallery.asp?ProductCode=VWC-111-941-583
    I've added all the terminal labels in parentheis.
    The BAT terminal has constant 12v(+). The IGN terminal has 12v(+)
    when both the ignition and the headlight switch are ON. The GRD is
    connected to the momentary switch on the turn signal (I've confirmed
    it grounds when pressed). The 56a & 56b circuits power the HI/LO beam
    circuits, respectively. These are fused after this relay which means
    this relay must flow all the current for the headlights.
    I believe the mark on the contact that swings between the two outputs
    indicates it is a latching contact which stays closed to one of the
    outputs until the next activation of the coil.
    The extra "swinging" contact to the very right of the diagram would
    seem to apply power to both outputs for the brief time that the coil
    is energized. The result is that you can dim the headlights w/o the
    IGN being on and the headlights will momentarily light up.

    The HI circuit is fused w/ two 16A fuses in parallel (total 32A), one
    for each headlight. The LO is fused w/ two 8A fuses in parallel
    (total 16A).

    +-----------------------+
    IGN | |
    +(56)------|-------------+ |
    | | |
    BAT | | |
    +(30)>-----+-------+ | |
    | | + +
    | | \ \
    R? COIL=====\====<====\
    | | \ \
    MOM | | / \
    GND(S)>----+-------+ + \+ + \
    | | |
    +-----+---+
    | |
    (56a) (56b)
    HI LO


    My problem appears to be the resistor located in parallel w/ the
    coil. It is obviously burnt out on my relay. It looks like it
    "popped". It is mostly black and I cannot make out the colored
    stripes to determine the rating.
    I'm assuming the resistor was protecting the coil. If it were to burn
    out, wouldn't the coil then take the full current/voltage? I haven't
    taken the relay apart enough to test the coil separately, but wired
    normally it wasn't activating (which is why I opened up the relay).

    While I could buy a new relay for around $20USD this one was brand new
    and lasted only a day (it replaced an old one that had lasted 30-
    yrs!) I'm not sure if that says the quality is bad, or my car's
    headlight circuit is at fault. The HI circuit was upped to 16A fuses
    by the previous owner to accomodate Halogen lights. I know the wiring
    can handle the current, but maybe the components can't?

    Any suggestions for replacing the resistor? The original was very
    small - about 5mm long and 2mm in dia. What should the rating be?
    Would it help if I clipped the resistor and measured the current
    through the coil? Could I replace it w/ a diode (1A, 50v)?



    AshMan40
     
  2. Tom Biasi

    Tom Biasi Guest



    Sounds like you have a bi-stable relay. One click one way another click the
    other way. Mechanical failure is common in those.
    Are you sure the device across the coil is a resistor? Some had a diode in
    reverse.

    If you added Halogen lights to a car that did not have them and kept the
    stock relay you are asking for trouble.

    Tom
     
  3. Rich Grise

    Rich Grise Guest

    If the one blew out after 30 years, and then a new one blew out in a day,
    either the new unit was defective, or there's another problem in the
    electrical system you'll have to diagnose, or have someone fix it for you.

    Good Luck!
    Rich
     
  4. AshMan40

    AshMan40 Guest

    I thought it might be a diode since I have seen diodes wired in
    parallel across other relay coils. I'm not sure. It is shaped like a
    resistor, but is burnt/chared so badly I can't tell. It has a few
    stripes around the body that are charred black, but I guess there are
    diodes that look like this too.

    The reason I said resistor was based on the below article I found
    while looking for a solution to my problem. It talks about using
    resistors, diodes and other things to prevent relay contacts from
    welding themselves together.
    http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

    The "cheapest" solution appears to be a resistor. And since this
    relay died in a day, I'd say it was REALLY cheap! :)

    I'm going to dig up my MM and test the continuity thur the coil. If
    it's open (fried) then there's no hope in fixing this.

    Does anyone have any suggestions for building a solid state
    replacement for this type of relay? I was thinking I could use a
    cheap 40A SPDT relay to switch between the 56a & 56b terminals (that
    way it can handle the current flow of the Halogens), but have a simple
    solid state circuit handle the "bi-stable" portion of the circuit.

    Or better yet, a completely solid state circuit w/ no moving parts! I
    could always add relays at each headlight circuit to reduce the
    current flow thru this circuit.



    AshMan40
     
  5. Unkown

    Unkown Guest

    Normally what you think is a resistor should be a 24V 1/2 watt zener diode,
    which is used to absorb the transient voltages when the relay coil is
    energized and de-energized. It is connected with the cathode(the bar)
    towards the +ve supply voltage. Sometimes an inverse 1N4004 is used instead
    of a zener diode, but a zener diode is better. The zener can absorb both the
    positive and negative transients.
    In recent years, a resistor is used instead of a zener diode in order to
    save money. They all serve the same purpose.
     
  6. AshMan40

    AshMan40 Guest

    Tore my relay apart today. De-soldered the thing I thought to be a
    resistor and it was. It measures 98ohms on my MM, in both
    directions. Does that mean it is a resistor, or would a zener diode
    show resistance? If it is a resistor, since it is much smaller than
    my 100ohm/1W resistor I'm guessing the power rating is much lower (I
    still can't make out the colored stripes). Check out the pics below
    and maybe you can decide if it is a resistor or a diode.

    With the resistor off I could measure the resistance thru the coil....
    infinite. :-( My coil is shot. Time to dump the relay and go to
    Plan B. I've ordered a German-made dimmer relay which I hope to be
    more reliable.

    In case anyone is interested, here are some pics of the inner workings
    of the relay. Quite interesting for a "bi-stable" latching relay.
    http://images.thesamba.com/vw/gallery/pix/347211.jpg
    http://images.thesamba.com/vw/gallery/pix/347212.jpg
    http://images.thesamba.com/vw/gallery/pix/347215.jpg
    http://images.thesamba.com/vw/gallery/pix/347217.jpg
    http://images.thesamba.com/vw/gallery/pix/347218.jpg
    http://images.thesamba.com/vw/gallery/pix/347219.jpg


    AshMan40
     
  7. You are having way too much fun with this problem. ;-)
     
  8. ehsjr

    ehsjr Guest

    Nice pictures! It is definitely a resistor.

    Ed
     
  9. GPG

    GPG Guest

    Possibly some fault is holding the relay on for extended periods,
    burning the coil and the resistor out.
     
  10. AshMan40

    AshMan40 Guest

    I've tried replacing my dimmer relay w/ three cheaper 30A (Bosch-
    style) relays. The first relay below on the left is the relay for the
    headlight switch. The relay to the bottom right is for the "dimmer"
    which switches between hi/low beams. The third relay at the top right
    allows the dimmer switch to act as a "daylight flash" when used while
    the headlight switch is off. The single switch to the very right
    controls both right-hand relays via their coil ground.

    BAT
    +(30)>--------+ +--(30)>--------+
    | | |
    (85)>--+ | +--(85)>--+ |
    | | + | | +
    GND | \ | | \
    COIL==<==\= | COIL==<==\=
    | \ | | \
    | \ | | \
    (86)>--+ + \+ | (86)>--+ + \+
    | | | | | | |
    | | | | | (87) (87a)
    HL SW (87) (87a) | +-|<----+ |
    | | | 1Ax2 | | /
    | | | +-|<----+ | /
    | +---+ | +---|-----+ +--GND
    | | | | HI LIGHT
    +---------+---|----(30)>--+ | SW
    | | | |
    | (85)>--+ | |
    | | + |
    | | \ |
    | COIL==<==\= |
    | | \ |
    | | \ |
    +--(86)>--+ + \+
    | |
    | |
    (87) (87a)


    Problem, when wired as above the circuit works, but there is a loud
    buzzing from one of the right two relays. Above you can see I've
    added diodes between the two terminals (#85, #86) that are grounded by
    the HI LIGHT switch (one each) to limit this part of the circuit to
    negative flow only (cathodes towards terminals). This helped to
    prevent the relays from buzzing all the time. Still, the relays
    "buzz" when the HI LIGHT switch is turned on.
    If I remove one of the right two relays the remaining circuit works
    fine. I believe it has something to do w/ using a common ground
    switch between the two right relays.

    Any idea how I can get this circuit to work? I was thinking of
    switching to a DPST switch at the right.



    AshMan40
     
  11. AshMan40

    AshMan40 Guest

    Sorry there is an error in the above diagram. Here is the correct
    one.

    BAT
    +(30)>--------+ +--(30)>--------+
    | | |
    (85)>--+ | +--(85)>--+ |
    | | + | | +
    GND | \ | | \
    COIL==<==\= | COIL==<==\=
    | \ | | \
    | \ | | \
    (86)>--+ + \+ | (86)>--+ + \+
    | | | | | | |
    | | | | | (87) (87a)
    HL SW (87) (87a) | +-|<----+ | /
    | | | 1Ax2 | | /
    | | | +-|<----+---|----------+ +--GND
    | +---+ | +-----+ HI LIGHT
    | | | SW
    +---------+---|----(30)>--+ |
    | | | |
    | (85)>--+ | |
    | | + |
    | | \ |
    | COIL==<==\= |
    | | \ |
    | | \ |
    +--(86)>--+ + \+
    | |
    | |
    (87) (87a)





    AshMan40
     
  12. ehsjr

    ehsjr Guest

    Your diodes are drawn backwards, and the connection
    from 87 upper right to 87A lower right is wrong. You
    need a connection from 87 of the daylight flash relay
    (upper right) to 86 of the headlight relay (upper left)
    and 4700uF from 85 of the flash relay (upper right) to
    gnd. See modified diagram below.

    I suspect that is not the end of the story. (For one
    thing, I left out transorb protection for the cap)
    As you drew it, it appears the HI LIGHT switch is
    not a momentary switch. If that's true, I think
    you're home free with the addition of a transorb
    across the cap and a series resistor. But first,
    try the circuit. My bet is that the HI LIGHT switch
    is momentary, and a different circuit will be needed
    for hi/low to work properly. I think that the high
    beam will come on when you activate the switch, and
    return to low beam when you release it, regardless
    of the daylight flash circuit.

    Ed

    BAT
    +(30)>--------+ +----+--(30)>--------+
    | | | |
    (85)>--+ | 4700uF +--(85)>--+ |
    | | + | | | +
    GND | \ Gnd | | \
    COIL==<==\= | COIL==<==\=
    | \ | | \
    | \ | | \
    (86)>--+ + \+ | (86)>--+ + \+
    | | | | | | | |
    | | | | | | (87) (87a)
    HL SW | (87) (87a) | +->|----+ | /
    | | | | 1Ax2 | | /
    | | | | +->|----+---|----------+ +--GND
    | | +---+ | | HI LIGHT
    | | | | SW
    +---- | ----------- | ----------+
    | |
    +---------+---|----(30)>--+
    | | |
    | (85)>--+ |
    | | +
    | | \
    | COIL==<==\=
    | | \
    | | \
    +--(86)>--+ + \+
    | |
    | |
    (87) (87a)
     
  13. AshMan40

    AshMan40 Guest

    Thanks Ed. Actually the HI LIGHT switch is a SPDT non-momentary
    switch.
    What is the function of the 4700uF cap? It would appear to store a
    charge to keep the flash relay on even after the headlight relay is on
    and power is cut to the flash relay.

    After posting my last diagram and staring at it for 10-min I realized
    that the power going to the lower right relay's 87a connection would
    also continue on and power the HI/LO RELAY (lower right). As soon as
    teh HI/LO RELAY went on it would cut off it's own power and switch
    off. It would then toggle back and forth on/off really quickly. This
    is the buzz I was hearing. I've come up with this new design that
    disconnects the HI/LO RELAY while the FLASH RELAY is on. I've also
    corrected the diodes (thanks Ed).

    BAT
    +(30)>--------+------------(30)>--------+
    | |
    (85)>--+ | +--(85)>--+ |
    | | + | | +
    GND | \ | | \
    COIL==<==\= | COIL==<==\=
    | \ | | \
    | \ | | \
    (86)>--+ + \+ | (86)>--+ + \+---------------+
    | | | | | | | |
    | | | | | (87) (87a) |
    HL SW(+) (87) (87a) | +->|----+ | / |
    | | | 1Ax2 | | / |
    | +---+ +->|----+---|----------+ +--GND |
    | | +-----+ HI LIGHT |
    | | | SW |
    +-------------|----(30)>--+ | |
    | | | |
    (85)>--+ | | |
    | + | |
    | \ | |
    COIL==<==\= | |
    | \ | |
    | \ | |
    +--(86)>--+ + \+ |
    | | | |
    | HI BEAM (87) (87a) LO BEAM |
    | |
    | |
    +-------------------------------------+

    It's raining now, so I won't be able to try this until later this
    week. Doesn anyone see a problem w/ it?


    AshMan40
     
  14. jasen

    jasen Guest

    the buzzing is probably caused buy currents flowing throgh the relay coil
    to the headlamp filament which you didn't show.

    more info on the intended function and outputs of this cicuit would help.

    Bye.
    Jasen
     
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