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Can a 1W resistor handle 50W for 7msec?

Discussion in 'Electronic Basics' started by Jason Hsu, Nov 25, 2003.

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  1. Jason Hsu

    Jason Hsu Guest

    It's part of a design for a T/R sense circuit for a noise cancellation
    device.

    I won't bore you with too many details. One thing I noticed in a
    design I'm looking at is that 1W resistors could be subject to as much
    as 50W of power apiece during the time it takes for a relay to
    respond. This response time is 7msec. The overall duty cycle will be
    low (well under 1%).

    Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
    PEP) for .007 sec? 50W over .007 seconds is .35 Joules. .35W for 1
    second is also .35 Joules, which a 1W resistor should have no trouble
    handling. Can the resistors be damaged during that .007 seconds?

    Also, how much time does it take to damage a toroid? If it can handle
    X units of flux density continuously, how much flux density can it
    handle for .007 seconds with a low overall duty cycle (like well under
    1%)?

    Jason Hsu, AG4DG
    usenet AAAAATTTTT jasonhsu.com
     
  2. Bill Turner

    Bill Turner Guest

    _________________________________________________________

    You really need to ask the manufacturer of the resistor. They are well
    aware of the problem - if you get to the right person.

    Having said that, here is a generalization: If the resistor's element
    is a solid block of material, such as in a carbon composition type, it
    will have very good pulse power ratings. On the other hand, if the
    element is a film, it may develop tiny hot spots during pulsing and
    eventually fail.
     
  3. GPG

    GPG Guest

    Make a parallell/ series combination to equal 51 ohm. Immerse in oil,
     
  4. N. Thornton

    N. Thornton Guest

    Hi. I've always read upto 10x the rated diss is workable. 50x is
    really pushing it I would think. My experience with this is limited
    though - but I can tell you from experience that 1500x rated power
    doesnt work so well :)


    Regards, NT
     
  5. But what happens if the relay fails to close in 7 mSec? OOPS!

    Or fails to close at all? BIG OOPS!

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  6. Al

    Al Guest

    Many years ago I analyzed a problem with resistors which were
    discoloring in the field. This involved two identical circuits with
    identical singal inputs. In one circuit, a resistor kept turning brown
    over time, in the other it did not. We were getting field returns as a
    result of troubleshooting that focused on the brown resistors.

    What was the difference? The resistor which did not discolor was resting
    on two metal traces which ran under it, the one which discolored, was
    resting on the PCB. That tiny amount of heatsinking provided by the
    traces was the difference. Obviously, the circuit needed to be
    redesigned, but it worked great on the breadboard when it went into
    production.

    Al
     
  7. Ban

    Ban Guest

    Jason Hsu wrote:
    || It's part of a design for a T/R sense circuit for a noise
    || cancellation device.
    ||
    || I won't bore you with too many details. One thing I noticed in a
    || design I'm looking at is that 1W resistors could be subject to as
    || much as 50W of power apiece during the time it takes for a relay to
    || respond. This response time is 7msec. The overall duty cycle will
    || be low (well under 1%).
    ||
    || Can the 1W-51 ohm resistors handle this 50 RF volts 0-peak (about 50W
    || PEP) for .007 sec? 50W over .007 seconds is .35 Joules. .35W for 1
    || second is also .35 Joules, which a 1W resistor should have no trouble
    || handling. Can the resistors be damaged during that .007 seconds?
    ||
    || Also, how much time does it take to damage a toroid? If it can
    || handle X units of flux density continuously, how much flux density
    || can it handle for .007 seconds with a low overall duty cycle (like
    || well under 1%)?
    ||
    || Jason Hsu, AG4DG
    || usenet AAAAATTTTT jasonhsu.com

    I have here some datasheets of Beyschlag MELF-resistors (CMA0204). They take
    up to 40W continuous pulses if the pulse length is 200us or shorter. So 2-3
    of those should be able to absorb your pulse. They also have non-inductive
    types for RF-apps.
     
  8. Fred Bloggs

    Fred Bloggs Guest

    Your question is improper- no mention of resistor type - almost any
    resistor type can take the limited overload if you up the steady state
    power rating. When in doubt go with bulk ceramic- these products are
    well characterized for pulsed overload operation and used in RF apps.
    http://www.globar.com/ec/resistor.php.html
    This will be easier because the heating time constant is longer and RF
    is reflected away when it saturates- you will have to measure this
    yourself- it is unlikely any manufacturer has done it.
     
  9. R.Legg

    R.Legg Guest

    Depends on the resistor's construction. It is a rating that will be
    published in the mfr's data sheets. Cement-coated resistors have a low
    peak stress rating. Enamelled and welded wirewound, composition and
    ceramic resistors can have high pulse power handling capabilities in a
    single surge event that is characterised as being less than a half
    cycle of the AC line frequency ( a common surge stress rating).

    Toroids are basically thermally limited to their material construction
    class. If wire doesn't fuse, the em force applied doesn't cause
    mechanical damage and you don't care if volt-seconds are exceeded
    (saturation), a 7mSec overwattage isn't going to hurt the part
    permanently.

    RL
     
  10. The resistor should never have been run at that close to its maximum
    dissipation to begin with. That's a lack of what they call a
    conservative design.

    --
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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
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  11. Fred Bloggs

    Fred Bloggs Guest

    That's exactly right-and this story about "it worked right during
    production testing" points up the fact that the price you pay for
    misapplying a component is extended in-house testing designed at a
    higher skill level than the usual application engineer possesses.
     
  12. Paul Mathews

    Paul Mathews Guest

    Resistor pulse ratings can be difficult to obtain from manufacturers.
    As stated by another poster, peak power ratings on datasheets are
    often about 10x continuous, sometimes even lower. For high value
    resistors, the voltage across the resistor may be the limiting factor.
    Anyhow, I've destructively tested lots of low value resistors. For
    some intermediate range of pulse durations, their failure point tends
    to follow the famous 'I-squared-t equals K' curve, which describes the
    ideal tradeoff between time and power level. At some short pulse
    duration, the failure threshold becomes roughly constant. The
    resistor reaches a power density that causes rapid destruction of the
    conductor. For wirewound and metal oxide resistors, this power level
    can be 100s of times the average power rating. For carbon composition
    resistors, 50x may or may not be destructive. Carbon film and thick
    film resistors have failure points that vary all over the map. This
    is partly due to the way that they are trimmed: scribing or laser
    cutting produces a narrow area in the resistive element, which
    concentrates power in a small zone. Failure is usually due to
    cracking of the element as it expands more rapidly than the substrate.
    This is why pulse ratings for these type resistors are usually very
    low multiples of their continuous rating. So, it depends.
    Paul Mathews
     
  13. Bill Turner

    Bill Turner Guest

    _________________________________________________________

    The only problem with do-it-yourself pulse testing, as opposed to using
    the manufacturer's specs, is the manufacturer might change their
    processes over time, resulting in a resistor which no longer behaves the
    same way. If the mfr doesn't spec pulse ratings, they will be justified
    in changing things with no notice to the users, at least justified in
    their own minds. It happens.

    I'll say it again: Use the manufacturer's pulse rating specs. If they
    don't provide such specs, you'd be better off finding one who does.
     
  14. I worked for a small company that was owned by a conglomerate. The
    conglomerate said that they'd never use any of our equipment that were
    in a blue case. Which, since almost everything was in a blue case...

    Anyway, we had problems with the 7805 regulators shutting down from
    overtemp. They were mounted on the PCB with a small heatsink. So we
    had to ECO them by adding a few inches of wire and bolting the 7805 to
    the aluminum case, which soaked up plenty of heat. After that, no
    more weird behavior.

    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  15. Kamaya Ohm Resistors. I love those guys. Check it out, 1W SMD 500W @ 7ms.

    http://www.kamaya.co.jp/us/image/catalog-46.pdf

    enjoy
    harry
     
  16. Ken Smith

    Ken Smith Guest

    Also:

    If the maker's specification says "2.5 times overload for 3 seconds",
    don't figure that 3 times overload for 2.5 seconds is ok too.

    Watch out for the voltage limits as well as power limits. With high value
    resistors it is sometimes the voltage that limits you.

    With SMT parts, all limits only apply if the part in mounted the way the
    maker expected.
     
  17. Brian

    Brian Guest

    I think the resistor becomes a fuse.
     
  18. Not an expert on instantaneous surges in resistors (you could ask my
    daughter - she is in third year physics ;-) but I would suggest that
    you consider using Flame-Proof resistors in this application. these
    are resistors that open internally and do not burn up your board when
    they fail...sometimes known as a fuse resistor....

    John :-#)#

    (Please post followups or tech enquires to the newsgroup)
    John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
    Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
    www.flippers.com
    "Old pinballers never die, they just flip out."
     
  19. Roger Gt

    Roger Gt Guest

    Check for wire wound resistors, they are most likely to be capable of a high
    pulse of current. But check for the manufacturers specification on what
    they have been tested to tolerate.
    There used to be several sources of these, but I haven't used any for years.
    Of course a insulated bobbin with copper or iron wire could be used, if size
    isn't critical. Wind it as a single or double layer to get the heat out.
     
  20. http://www.globar.com/ec/resistor.php.html shows some bulk ceramic
    resistors. One of their data sheets shows their 1.5 W resistor having a
    peak energy rating of 75 Joules. The key here is having a lot of physical
    mass directly in the current path (something that metal film resistors
    don't do well).

    PS: What happens when your relay fails?
     
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