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WR overlay designation for a fusible resistor?

Discussion in 'Electronic Repair' started by N_Cook, Apr 23, 2008.

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  1. N_Cook

    N_Cook Guest

    W for Wood's metal for the fusible part ?
    Googling "wood's metal" AND "fusible resistor" produces nothing
     
  2. Arfa Daily

    Arfa Daily Guest

    Who can tell ? Why "Q" for a transistor ? Why "U" for an IC ? Why "VDD" ?
    Why "VSS" ? I bet if you think for a while, there's loads of these that
    don't 'appear' to make any sense, but must have to the o.e.d. I don't think
    that there is anything particularly special about fusible resistors in terms
    of the materials used to make the resistive element. As far as I know, they
    are still carbon or metal film, with a completely non-flammable coating. I
    think it is probably the internal connections that are 'necked' or something
    to make them deliberately vulnerable to excess current, or heat in the
    substrate, generated by excess dissipation.

    Arfa
     
  3. Eeyore

    Eeyore Guest

    Wimpy resistor ?

    Graham
     
  4. Eeyore

    Eeyore Guest

    By whom ?

    In the UK we legend ICs as 'IC3' for example ! Too obvious ?

    Now tell me a transistor is a 'Q' in the USA ! We use 'TR'.


    Graham
     
  5. Eeyore

    Eeyore Guest

    Yes, modern 'fusible resistors' are typically made using metal film over a
    ceramic former and covered with a 'cement' coating. All of which is
    non-combustible and flameproof..

    In comparison, carbon film covered with laquer is very combustible.

    Graham
     
  6. N_Cook

    N_Cook Guest

    Fusible resistors are what it says on the tin - a resistor and a fuse.
    The MO or metal bit and then the fusible bit, so avoiding the possibility of
    seriously high temperatures building up in a fault condition, rather than
    just non-combustibility.
    I've only ever scraped one apart, long ago , out of curiosity, and there was
    a definite fusible part that I remember as a metalic? blob rather than a
    necking.
    Unfortunately I did not know of Wood's metal, then, and didn't try heating
    it up.
    I'll try scraping back a 2.2 ohm one, tomorrow.
     
  7. Ross Herbert

    Ross Herbert Guest

    :W for Wood's metal for the fusible part ?
    :Googling "wood's metal" AND "fusible resistor" produces nothing


    Can't say I have ever heard of Wood's Metal being used for fusible resistors in
    electrical or electronic equipment. I seem to remember that a major use for
    Wood's Metal was as the fusible element in thermal break-circuit fire detectors
    used back in the 30's - 50's. They were superseded by much better detectors.
     
  8. Arfa Daily

    Arfa Daily Guest

    There certainly used to be a pellet of Wood's in the 'daisy' fire
    sprinklers. I don't know of that is still the case. It's a simple, reliable,
    and more to the point 'on-the-spot' and unpowered system. The lid of my
    pressure cooker still has a Wood's metal pressure pressure relief valve,
    which coincidentally, I replaced just last week.

    Arfa
     
  9. Arfa Daily

    Arfa Daily Guest

    Weak resistor ?

    How about "CR" for a diode ? Copper (oxide) Rectifier, maybe ?

    Arfa
     
  10. N_Cook

    N_Cook Guest

    I scraped off one of a batch of 2.2 ohm 1/3W , called fusible resistors and
    heated with hot air gun to 180 deg C with no fusing anywhere.
    MO over ceramic construction with spiral cut into the oxide starting and
    ending 1/10 way in, so highest risistance in the axial central area between
    the 2 cuts in that area , so any excess current failure would be in that
    section.
    So little more than a standard MO resistor , but not having the continuous
    spiral. The scraped off coating seemed no more than slightly olive coloured
    varnish
     
  11. Ross Herbert

    Ross Herbert Guest

    :
    ::>
    :> :W for Wood's metal for the fusible part ?
    :> :Googling "wood's metal" AND "fusible resistor" produces nothing
    :>
    :>
    :> Can't say I have ever heard of Wood's Metal being used for fusible
    :> resistors in
    :> electrical or electronic equipment. I seem to remember that a major use
    :> for
    :> Wood's Metal was as the fusible element in thermal break-circuit fire
    :> detectors
    :> used back in the 30's - 50's. They were superseded by much better
    :> detectors.
    :
    :There certainly used to be a pellet of Wood's in the 'daisy' fire
    :sprinklers. I don't know of that is still the case. It's a simple, reliable,
    :and more to the point 'on-the-spot' and unpowered system. The lid of my
    :pressure cooker still has a Wood's metal pressure pressure relief valve,
    :which coincidentally, I replaced just last week.
    :
    :Arfa
    :


    All of the sprinkler systems I have looked at in the past 50 years or so use the
    glass bulb which breaks due to expansion thus allowing the plug sealing the
    outlet orifice to pop out under water pressure.

    You are correct regarding the pressure safety valve on a pressure cooker. It
    does have a slug of low melting point alloy but not sure if this is classified
    as Wood's Metal, although it would probably be very similar. Wood's Metal
    appears to have set proportions of bismuth (50%), lead (25%), tin (12.5%) and
    cadmium (12.5%) designed to melt at approx 150 deg F.

    http://evans.mse.berkeley.edu/~dan/msds-evansgroup/msds-Woods Metal.pdf
     
  12. Geo

    Geo Guest

    Crystal Rectifier (as in your original crystal set). Followed by point contact
    diodes.

    Geo
     
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