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Metal Oxide Resistors (thin film)

Discussion in 'Electronic Basics' started by kfel, Sep 21, 2006.

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  1. kfel

    kfel Guest


    I need to know the pulse handling capability of 3K9/2W MOR(thin film)
    for a voltage pulse having a maximum voltage of 1600V and which decays
    exponentially. Also this pulse repeats every 10ms and is superimposed
    onto the line voltage of 230VAC/50Hz. I am told that MORs can withstand
    upto 2.5 times their rated wattage for upto 3minutes. Is this true?
  2. Eeyore

    Eeyore Guest

    It'll depend on construction. You ought to consult the manufacturer for pulse
    data too. You mean an average dissipation of 2.5x the rating for 3 mins. You
    *might* get away with it in that size - it'll depend on the quiescent
    dissipation of course. You'll have to get rid of the heat some way eventually

  3. Phil Allison

    Phil Allison Guest

    ** Groper alert !

    ** Expect very early failure.

    Such high voltage spikes are way to much for the fine clearances between the
    spiral cut into the metal oxide.

    ** 230 x 230 / 3900 = 13.5 watts !!

    FORGET IT !!!

    ** Who cares.

    What you just asked is insane.

    You will need a series string of such 2W resistors to handle that abuse.

    ........ Phil
  4. kfel

    kfel Guest

    The voltage spikes last only 10 micro seconds.
  5. Phil Allison

    Phil Allison Guest

    ** Groper Fuckwit Alert

    ** Lots little pricks can be very annoying.

    100 per second is way more than any Koala can Bear.

    Piss off - half wit.

    ....... Phil
  6. Eeyore

    Eeyore Guest

    It's the energy in them that counts. Doesn't matter how long or short they are.

    Are you familiar with integration ?

  7. redbelly

    redbelly Guest

    I think Phil meant you could get arcing inside the resistor. In that
    case, what counts is the voltage (between adjacent spirals) divided by
    the **small** gap between them. Divide by a small number, and you get
    a big number.

  8. Phil Allison

    Phil Allison Guest


    ** Precisely correct.

    That is why ordinary, spiral cut, tubular resistors have low continuous
    voltage ratings - 250 to 500 volts max.

    ** How VERY true.

    Divide by zero and the universe collapses..

    Right back to a singularity......

    ....... Phil
  9. Chris

    Chris Guest

    Hi, Kfel. From a hobbyist or even a one-off engineering prototype
    standpoint, it's usually better to just nuke problems like this. It's
    not worth the hassle to try to find the cheapest solution that probably
    won't smoke. Unless you're talking about making these in quantity,
    it's better to just overbuy and be done with it.

    For your steady state power requirement ( P = V^2 / R = 13.6W), as well
    as joule rating, I'd recommend a significantly higher wattage
    non-inductive wirewound resistor. Surplus Sales of Nebraska has a 4K,
    25 watt non-inductive Kool-ohm for $12 USD. It's their P/N (RNI)

    Wirewound resistors have great joule and overload ratings in comparison
    to other types. This will do the job. Get it and move on.

    And whoever gave you the story about 2.5X rated overload for 3 minutes
    for a film resistor was spinning a fabulous tale. It'll be toast for

    Good luck
  10. Eeyore

    Eeyore Guest

    Because of the high voltage. You'd have to check the resistor spec.

    The inter-spiral voltage will be ~ Vapplied / Nspirals

    Still doesn't help about the basic energy involved though.

  11. redbelly

    redbelly Guest

    Well, yeah, I didn't mean to imply that pulse energy was not a concern,
    though it probably seemed that way in that I posted as a reply to your
    post. Sorry about that.

    Energy per pulse, time-averaged power, and peak voltage are all factors
    of concern.


  12. redbelly

    redbelly Guest

    Well, I wouldn't get carried away now. When you say stuff like that,
    there are 1 or 2 people around here who might get the wrong idea.
    (Referring to recent thread about Ohm's law and division by zero.)

    Of course, even when you DON'T say stuff like that, there are still 1
    or 2 people who get the wrong idea!

  13. kfel

    kfel Guest

    Can thick film MORs be used instead of wire wounds??
  14. Chris

    Chris Guest

    Your 2 watt 3.9K resistor will curse you as it emits it's fragrant
    aroma and lets the smoke out.

    As Graham mentioned, this is an application-specific problem that the
    manufacturer's apps engineers can answer. Of course, if you place
    enough of your 3.9K 2W metal film resistors in series/parallel
    combination (5 X 5 will work, 3 X 3 is too close, 4 X 4 is probable but
    you should contact the manufacturer). A lot of this has to do with
    arcover at peak voltage, and a lot also with the joule rating of the
    resistor (which again brings up the decay rate question -- you can't
    calculate joules unless you have a decay rate).

    Look, life is short, and there are always more things to do. You're
    potentially looking at an earnest exchange of emails with a
    manufacturer apps engineer who may take days to respond. If this is a
    real problem, and there's no quantity involved, the very appropriate
    technical response is to shoot the engineer, git r done and ship the
    product. ;-)

    Get something you know will work reliably. If you want to save money,
    buy surplus -- it's only $12, even if your metal film resistors are
    free. Go home on time, have a beer, give the kids a hug, and pat the
    War Department on the fanny with a wink and a smile because you did a
    good job -- you got r done.


    (By the way, thin film is a metal alloy. You generally don't start
    calling them metal oxide film, though, until the resistance gets a lot
    higher than 3.9K.)
  15. Chris

    Chris Guest

    Sorry -- edit:

    There are other considerations besides steady-state power and joule
    raing which make a non-inductive (Ayrton-Perry winding) wirewound
    appropriate, and these are specific to your application, which you also
    haven't described. This is a good basic summary (wait for the intro

    This is a trade magazine article, "Specifying Resistors for Motor
    Control Applications". It has some additional information which may be
    of use. Wait for the splash ad.

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