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resistors wanted

Discussion in 'Electronic Components' started by [email protected], Mar 22, 2007.

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  1. Guest

    Can anyone help?
    I am looking for carbon resistors - in Johannesburg.
    spec: 2.2 mega ohm, 2 watt - 450 needed.
  2. Salmon Egg

    Salmon Egg Guest

    That used to be a staple product. I suggest that when searching for them,
    use the term "composition" rather than "carbon" to describe them.

    -- Fermez le Bush--about two years to go.
  3. Ross Herbert

    Ross Herbert Guest

    Carbon film on ceramic substrate would be the way to go. A number of
    asian manfs make them but the OP may have difficulty in locating a
    distributor locally who carries 2W types.
  4. Might not be... Metal or carbon films break down catastrophically with high
    voltage, but solid carbon resistors don't, until much more is forced on
    them. I found that out when making a limit resistor for a HeNe laser tube,
    I ended up with 2W solid carbon resistors, 4 in series, wrapped in adhesive
    lined heatshrink. It lasted for years, but previous experiments with metal
    film AND carbon film types ended up like little fireworks in seconds, even
    when selected for an apparent voltage rating that could stand the supply
    voltage. My guess is the supply went high to ignite the tube, and having
    started a burn, continued it even though tube ignition brought the voltage
    within safe limits. Solid carbon doesn't care about that, it just works.

    The OP didn't say why he wanted them, but my guess is similar high voltage
    systems where the current path must be safely distrubuted through the whole
    volume of the resistor.
  5. I ought to add that it never failed. :) I dropped the tube, but the
    resistor assembly and PSU survived all.
  6. Al

    Al Guest

    Plus the construction is different. The composition resistor material
    fills a hollow tube; the composition varies according to resistance. The
    film resistors are spiral cut to get high values and can be inductive.
    You can also get arcing across the cuts if the voltage is high enough
    (despite the coating).

  7. Ross Herbert

    Ross Herbert Guest

    I agree that the generally available metal film resistors don't tend
    to stand up well to high pulse voltages but I have found carbon film
    to be much better in this regard. Carbon composition may be preferable
    in this regard depending upon the application.

    The problem is that fewer (none?) manufacturesrs these days make
    carbon composition resistors, and even then they will usually be max

    For example, check out the product lines of two well known
    manufacturers and see if you can locate a 2W, 2.2Mohm carbon
    composition - or carbon film even.

    Ohmite do make a ceramic composition resistor as a replacement for the
    carbon composition and the OY series is 2W. I would go for this option
  8. I pass. >:)
    The emphasis on high voltage stuff being commonplace is vanishing steadily
    along with CRT's, as LCD's and such take over.

    I think there will always be specialised high-volt parts manufacturing
    though. You won't find (m)any oil filled multi-KV rated caps or solid
    carbon resistors at Digikey or RS, but people do still make them. I guess
    the problem is that most aren't made to general preferred values anymore,
    but to specific bulk customer order only.

    Nice find on the ceramic composites. I like the look of those. I bet they
    cost a bit, though..
  9. Prepair Ltd

    Prepair Ltd Guest

    Since the resistor market has been cornered by a couple of companies,
    conventional carbon and metal film product is vanishing fast, as the demand from
    high-volume producers is almost over.

    I can remember some years ago picking up a bulk load of resistors, probably a
    couple of million for less than $10 the lot. It took 2 van loads to clear and
    was all good quality product.

    That was then. Now I find that demand is rising as manufacturers wake up to the
    fact that unless they start buying this stuff from the main manufacturers, their
    product lines are going to stop through lack of parts.

    The ranges that RS Components (for example) carry these days in conventional
    leaded resistors is very small compared with 10 years ago.

    Even specialist parts like flame-proof resistors are getting expensive and hard
    to find from regular distribution, and the makers don't want to know for
    relatively small quantities.

  10. Franc Zabkar

    Franc Zabkar Guest

    This has always bothered me. In TV sets there are special high voltage
    "focus" resistors. My catalogue lists a 1W, 10KVDC, 7KVAC RMS type
    with values in the range 100K to 10M. I can understand a pulse rating
    of 10KVDC, but I can't understand the RMS rating. Even at 10M, the
    maximum sustainable voltage for a 1W resistor is only 3kV.

    Furthermore, my experience with switchmode PSUs in all manner of mains
    powered equipment suggests that there is an unusually high failure
    rate in the 100K-500K "startup" resistors that are used to kickstart
    the chopper transistor, even though these resistors are operating well
    within their specified wattages.

    - Franc Zabkar
  11. My guess (not a very good one) is that although their RMS rating would
    force the resistor to dissipate more than 2W, it would at least not arc
    over until that voltage was reached. It might be designed as a short-term
    max rating.

    I don't know enough about switchmode PSU's to comment other than suggesting
    that pulses that reach those resistors them might have a high enough peak
    current to erode their ability to withstand what should be a safe average
    current. The term 'kick start' suggests to me that some heavy peak currents
    might hit them.
  12. Franc Zabkar

    Franc Zabkar Guest

    These resistors only ever see the peak rectified AC mains voltage
    which in 240VAC locales is 340VDC. A 330K resistor would therefore see
    a max current of around 1mA, and a continuous wattage of around 350mW.

    - Franc Zabkar
  13. I had a look at and a couple of diagrams, and I can't see why
    either. The only thing I can think of is that they run hot, and might not
    be as good at running hot permanently as they're supposed to be. Power
    cycling would aggravate that. Also, the fams in many PSU's might draw gunk
    onto them that reacts with them. Heat would accelerate that, and if the
    resistors rely on a film, it won't take much to damage it if there's a
    heat-assisted reaction. I doubt the makers test for that much, but it would
    be interesting to know if those nicely glazed and shiny looking ceramic
    types have a lower failure rate in the same situations, compared to the
    more porous coatings.
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