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Resistor Current Handling Capability

Discussion in 'Electronic Design' started by Bob Penoyer, Feb 26, 2004.

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  1. Bob Penoyer

    Bob Penoyer Guest

    I have a design that uses a 1.5-ohm resistor and I'm not sure what
    physical size the resistor should be. According to the simulation that
    I've done, a 1.5-amp, 20-ns pulse passes through the resistor. But the
    average dissipation is only 15 milliwatts.

    I'm concerned that if I use a resistor as small as an SMR0402 (63 mW)
    it might pop like a kernal of popcorn. Does anyone have experience
    with this sort of problem or have a reference for maximum current in
  2. It is a legitimate worry. Some resistors are specifically designed ot
    work with large pulse over wattage for short times, but I doubt that
    your 402 is one of these. Then again, 20 ns is a short time compared
    to even this tiny devices thermal time constant. My advice would be
    to test it at a significantly larger current (perhaps 1.5 times for
    twice the peak dissipation) and if it survives that it will probably
    last quite a while.
  3. electricked

    electricked Guest

    So long as the resistor is rated at the correct power I don't think you
    should have any problems. That is, the power capability of the resistor
    should exceed the max current possible at a specific voltage.

    If the at the point where current peak is max, say 1.5amp and the voltage at
    that point is say 20V then the power capability of the resistor should be
    P=20*1.5=30W. At least 30Watts. Make it 40/50W for safety.

    But then again I'm a newbie so... you can just ignore this post.

  4. The full data sheets should have information (if not, I'd look for
    another supplier for this particular part), but that pulse length is
    rather short. You could extrapolate, which is probably safe in this

    I pulled one manufacturer's (paper) data sheet for their 0603 part and
    (extrapolating on the log-log scale from 1usec minimum pulse width) it
    looks like your application would be okay for 0.1% duty cycle, but not
    with a lot of margin.

    Best regards,
    Spehro Pefhany
  5. John Larkin

    John Larkin Guest

    No problem. 45 nJ won't raise any part of the resistor by any
    detectable temperature. It takes a lot of energy to pop a kernel of
    popcorn or to explode a resistor. Your safety factor here is probably
    in the range of 1000:1.

  6. There may be something else going on here... the curves show only a
    permissible 1.7 or 1.8:1 increase in peak power for a 10:1 decrease in
    duration (10usec to 1usec). It might not be an outright failure but
    some kind of slow shift in value or long-term reliability issue.

    Just ran into someone that had a consumer product failing from tiny
    SMT resistors being overloaded..

    Best regards,
    Spehro Pefhany
  7. Jim Thompson

    Jim Thompson Guest

    On Thu, 26 Feb 2004 19:43:02 GMT, Spehro Pefhany <Spehro Pefhany

    I've observed solder fatigue where SMT resistors came loose after
    significant repetitive surges. My guess is the part expands slightly
    during the surge.

    I've also had large microchips come unbonded from the substrate.
    Cured by using a molybdenum slice between substrate and chip to absorb
    the movement.

    ...Jim Thompson
  8. Well, if I'm capable of using the Internet properly, a 0402 resistor is
    0.65 mm x 1.15 mm x 0.8 mm, for a total volume of 0.6 mm^3. If we assume
    uniform bulk heating (bad assumption, but we'll hit that one later), no
    external cooling (pretty valid for a 20 ns pulse) and a specific heat equal
    to water (4.2 J/ml K), the 0402 resistor will have a temperature rise of
    397 K for every Joule dissipated. If we hit the resistor with a 1.5 A
    (3.375 W) pulse that lasts 20 nsec, we should increase the temperature of
    the resistor by 27uK for each pulse we pass through it. Since the average
    power is also under the power rating of he resistor, based on these rough
    calculations, your 0402 resistor has a high chance of survival.

    - Can you guarantee the maximum 20 nsec pulse?

    - If something causes a much longer pulse, do you want the resistor to fail
    first or is that "A really bad thing"?

    Finally, SMT resistors are made by laser etching a "trimming" pattern onto
    a substrate. Because of this trimming, the power dissipation in the
    resistive substrate is non-uniform. If the resistor is operated close to
    its power limits, or pulsed rapidly, cracks can appear in the substrate in
    areas of high field stress, which will increase the resistance of the
    resistor. There was a good article on this in a recent (last 6 months?)
    issue of Electronics World.
  9. Terry Given

    Terry Given Guest

    I have a design that uses a 1.5-ohm resistor and I'm not sure what
    When designing with resistors, you must take into account both the average
    and peak power dissipation.

    Assuming a rectangular pulse, you have a 3.4W power loss for 20ns, i.e.
    68nJ, so if average power is 15mW then the repetition rate must be 15mW/68nJ
    = 222kHz

    Average power handling capacity is governed by a resistors surface area as
    heat can only be only lost through surfaces. Many power resistors are
    specified for a 200 - 300 degree C temperature rise (ie put 5W into a 5W
    resistor and watch it get HOT) so in any "real" application you will size
    the resistor based on an allowable temperature rise (people who dont realise
    this are the cause of burnt PCBs). Clearly any resistor has a "thermal
    resistance Rth" which can be used, along with Pavg to find out the
    temperature rise.

    Because resistors have mass, they have a "thermal capacitance Cth" also - it
    takes a finite amount of time for heat to spread through the body of the
    resistor. Therefore a resistor which has a power step (say from 0 to rated
    power) applied to it will have a temperature-vs-time curve just like an RC
    charging curve - the time constant Tc = Rth * Cth, and final dT = Rth * P.
    Increasing mass means larger Cth hence larger Tc, but of course wont change
    the final temperature rise. Likewise decreasing mass means smaller Cth hence
    shorter Tc.

    Peak power rating is more complex. If the pulse is short compared to the
    thermal time constant of the resistor, no heat is dissipated externally, so
    all of the heat energy is absorbed by the resistor, raising its temperature
    (adiabatic heating). If you know the mass m of the resistive element, and
    its specific heat capacity cp, dT = E/(m*cp) is the local temperature rise.
    When that gets too high, the resistor fails.

    A good resistor manufacturer will provide curves of peak pulse power rating
    vs pulse duration, for various duty cycles - BC components certainly do.
    Looking at the data for an RC31 0402 resistor, a single 1us pulse (ever!) of
    5W is permissible; repetitive pulses will have a much lower rating. An RC21
    0603 resistor has a single 1us pulse rating of 6W, and an repetitive 1us
    pulse 0.1% duty cycle (ie 1kHz rep rate) rating of 1.8W. We can therefore
    guess that the 0.1% pulse rating of the 0402 resistor is about (1.8W/6W)*5W
    = 1.5W

    Your duty cycle is on the order of 222kHz*20ns = 0.4%, so to be safe I would
    use at least three 0402 resistors in parallel, probably about four.

    Dont forget the 0402 resistor is rated at 63mW average, so a single resistor
    would be taking about 3.4W/63mW = 54 times its rated power!

    This not-so-good pulse rating is due to the resistor construction - smt
    resistors have very little mass as the resistive element is only a thin
    layer on top of the ceramic substrate, so they have very short thermal time
    constants, and also high temperature rise during adiabatic heating. A
    physically larger resistor would be better - an RC01 1206 resistor has a
    pulse rating as above of 3W, so a single 1206 would almost do.

    Alternatively you can use metal glaze resistors, which have a much larger
    amount of active material, hence longer thermal time constants and lower
    adiabatic temperature rise. Some of the 1206 MG parts from IRC can take 30W
    pulses (these guys have a lot of good data).

    This is a common reliability problem with power electronics - "designers"
    often neglect peak pulse power in MOSFET gate resistors - I saw one design
    with 4.7Ohm Rg, +12V Vg i.e. 31W peak, using 0805 resistors that failed
    after a short time, causing catastrophic failure; so much damage was done
    each time that the root cause was totally obscured, and the "designer" could
    not stop it happening. oops.

    A similar argument applies with power supply inrush limiting resistors - I
    have made 10W W/W resistors emit flashes of light by charging large caps. A
    common trick in cheap, nasty SMPS is to have a 1R 5W resistor in series with
    the bridge rectifier to "limit" the peak inrush current. At turn-on, this
    resistor can see Vpeak^2/R = 325V^2/1R = 106kW. And the peak input current
    is "limited" to 325A. This type of PSU often fails as the 5W resistor goes
    open (106kW/5W = 21,160 * rated power) of the bridge rectifier blows up as
    325A >> peak current rating of rectifier
  10. Ken Smith

    Ken Smith Guest

    No problem. 45 nJ won't raise any part of the resistor by any
    detectable temperature. It takes a lot of energy to pop a kernel of
    popcorn or to explode a resistor. Your safety factor here is probably
    in the range of 1000:1.[/QUOTE]

    I disagree. Surface mount resistors of the 0402 size only have a few
    nanoslugs of mass in the actual resistive element. With pulses this
    short, there is no time for the heat to get out of the element. Some SMD
    makers give a rating for their resistors in pulsed applications. You
    should find one of those makers and use the data they provide.
  11. Ken Smith

    Ken Smith Guest

    I've looked at one that did this. Under the microscope it looked like the
    nickel plating had come off the ceramic. I unsoldered the unbroken end
    and looked at what was still attached to the board. The surface looked
    sort of like a smooth metal surface with a bit of grit on it.
  12. Fred Bartoli

    Fred Bartoli Guest

    I disagree. Surface mount resistors of the 0402 size only have a few
    nanoslugs of mass in the actual resistive element. With pulses this
    short, there is no time for the heat to get out of the element. Some SMD
    makers give a rating for their resistors in pulsed applications. You
    should find one of those makers and use the data they provide.

    Are you sure ?

    Suppose the film has a 5J/g.K specific heat (about the one of water. The
    real one won't be off by a tenfold and has chances to be above)

    Suppose a metal film with a density of 10 (worst case).

    The 0402 is about 1x0.5 mm. Assume a film size of 0.5x0.25 mm

    For a 10K rise, the 45nJ pulse will require 45/5 = 9 ng of film.

    This will be under 1um thick resistor worst case.

    I don't know the details of resistor manuèfacturing but I'm ready to bet
    that thickness is higher than that.

  13. John Larkin

    John Larkin Guest

    I disagree. Surface mount resistors of the 0402 size only have a few
    nanoslugs of mass in the actual resistive element. With pulses this
    short, there is no time for the heat to get out of the element. Some SMD
    makers give a rating for their resistors in pulsed applications. You
    should find one of those makers and use the data they provide.


    OK, I tried it. I'm in the pulse business, so I should really know
    stuff like this (I rationalize.)

    A standard Panasonic 0402 thickfilm, 10 ohms (the lowest I have
    handy.) Charged a 1 uF cap to various volts, and discharged into the
    resistor 20 times at each voltage step. Measured resistance with a
    Kiethley 2000 DVM, 4-lead; initially 9.7955 ohms maybe.

    After 10 volts, 50 uJ, 20 zots, nothing happens. Resistance is 9.7955,
    exactly the original value.

    After zotting with 20 volts, 200 uJ, 20x, the resistance went down to
    9.7943, -120 PPM or so, but it's really hard to measure down here,
    with thermals and whatnot. This is 40 watts peak, 10 usec time

    At 50 volts, 1.25 millijoules, 250 watts peak, I zapped it. Got sparks
    when I made contact, and the resistance increased to 11.4104 ohms.

    So, I'd say the safety factor is at least my 1000:1 guess, maybe

  14. Do you think Philips is being unnecessarily (wildly) conservative?
    From the 0603 5% thick film curves (smallest they had back then):

    Pulse type 1usec 10usec 100usec
    single pulse 6.3W 4W 2.6W
    rep 0.1% duty cycle 1.8W 1W 0.7W

    They are pretty "linear" (in a log-log sense) over that
    range (below about 1msec). If I extrapolate to 20ns (which looks
    reasonable from the curves) I get about 10W maximum for repetitive
    pulses, and maybe 30W for one-time.

    His peak power is over 3W and he's dealing with a resistor that
    is less than 40% of the area...

    Maybe there's reliability problems associated with current doing funny
    things on the edges of the laser trimming on the RuO layer or
    something like that.

    Best regards,
    Spehro Pefhany
  15. John Larkin

    John Larkin Guest

    It is interesting that my blown resistor is very stable at 11.4104
    ohms. Maybe I blasted one of the trim regions or something.

    I'd guess (knowing nothing actually about the physics) that, once you
    get below some inherent thermal time constant, the only thing that
    matters would be energy per zot, so allowable power should go as 1/t
    until the voltages get too high and other failure modes kick in. Power
    mosfets behave this way. Further, I'd guess that the operative time
    constants would be in the ~~ 1 millisecond range for a thickfilm
    resistor, give or take a decade or so. There's a lot of ceramic below
    the resistive film, and a bunch of glass on top, adding mass.

    But I'm making all this up.

    I *have* tested high-meg 0603 resistors to over 1KV.

    "One experiment is worth a thousand expert opinions."

    - Werner Von Braun

  16. I bet they drift like crazy over 100 hours or so with DC..
    It's not rocket science?

    Here is a version of the printed data I'm looking at. America/Web Data/5% Chip Resistor.pdf

    See for example, page 9 (0603).

    Best regards,
    Spehro Pefhany
  17. John Larkin

    John Larkin Guest

    The figs don't make sense. Why is the 1206 good for only about 3 watts
    repetitive pulsed at 1 usec (fig 6), but the 0402 is rated for 5 watts
    (fig 12)?

    Wimps, anyhow.


  18. Thanks for posting the info Mr. Pefhany. Here is another datasheet by
    Vishay that might be useful for comparison purposes:

    On the last page they have a couple of graphs for pulse applications. In
    their figures the curves seem to level off after awhile at which point the
    power rating doesn't go up for shorter duration pulses.

    If I am interpreting them right they seem to show the 0402 capable of
    handling about 4W peak for what appears to be single pulse type application
    (nearly zero continuous dissipation), but only around 2W if the average
    power dissipation approaches the continuous rated power.

    Based on this information it would appear the OP's example case might be
    somewhat marginal but perhaps not outside (too far?) of the device's
    ability. I guess if the OP has a philosophy of being brave and living life
    dangerously to increase his sex appeal, then a single 0402 is definitely the
    way to go. On the other hand an 0603 device would offer peace of mind and
    isn't really more expensive or that much larger.

    My suspicion is manufacturers are rating their parts rather conservatively,
    especially for very short pulse durations.
  19. I read in that John Larkin <[email protected]> wrote (in <[email protected]>) about 'Resistor Current Handling Capability', on Fri, 27 Feb
    Different materials, perhaps. If the 1206 is nichrome but the 0402 is
    platinum, for example.
  20. Paul Mathews

    Paul Mathews Guest

    Figures 9 to 13 show the classic K=I^2*t on the right side, and the
    classic flattening (rather than approaching infinity) on the left
    side. The physical mechanisms for this are interesting and account
    for otherwise perplexing differences in resistor series and even
    ranges of values. For example, failure testing reveals that a 3R3
    resistor might tolerate much higher pulse power than a 4R7 in the same
    product series, while the reverse might be true in another product
    series. For another example, some 0603s might be more tolerant of
    high pulse power than some 0805s.

    The 'element' of film resistors, both thin and thick film types, are
    made by depositing a layer of material on a substrate and connecting
    the film to pads or leads. To control resistance, these are the
    available variables:
    1. Element formulation (what its made of)
    2. Element thickness
    3. Element width
    4. Trim, i.e., adjustments to element geometry, usually width, that
    are used to reach the desired Ohms value

    It's important to understand how big a role Trim plays in resistor
    production: Most manufacturers produce a very limited range of
    Thickness, and Width product, and Trim is used to produce the
    incredible range of resistance values. For example, suppose that a
    particular formulation of carbon-based thickfilm resistor 'ink' is
    screened onto ceramic substrates with a particular geometry, including
    a fairly uniform thickness. Suppose that this produces 1 Ohm
    resistors plus or minus 20%. A trimming operation can be used to
    adjust resistances to any value greater than 1R2 Ohms. In practice,
    these same raw resistors might be trimmed for the range of 1R2 to 4R7,
    a ratio of 4:1. Other formulations and/or geometries would be used
    for higher value resistors.

    Now consider how most film resistors are trimmed: Some of the element
    is 'burned' away by a laser or other ablative tool under feedback
    control. This is usually done in the 'J' pattern, as shown in this
    diagram (fixed width font here):

    | || |
    | || |
    | || |
    | \\ // | ____
    | == | ^
    | | w current crowding region.
    ----------------------- ----

    That's supposed to show a typical planar element, with the 'J' shaped
    feature the laser cut. Electrode connections would be on the left and
    the right. They use the J-shape, because it provides more precise
    control of resistance than a straight '|' shape: resistance variation
    slows down as you enter the curve. (The reasons are left as an
    exercise for the reader. ;-) Anyhow, the important thing to realize
    is that current flow ends up crowding into the region below the J,
    which has width w. The region immediately below the J is where film
    resistors fail, because it is also the region of max power density.

    For very high pulse amplitudes and short pulses, the power density in
    this narrow region reaches a failure point, even if the average power
    is low. The amplitude of this failure tends to be fairly constant,
    regardless of pulse duration, hence the flattening of the I-squared-t
    curve. (Basically, this is because the failure occurs virtually as
    soon as the critical amplitude is reached.) And, the critical
    amplitude is strongly influenced by the value of w, the remaining
    width after trim. Following one of the examples above, suppose that
    we compare a 4R7 resistor and a 5R1 resistor. The 4R7, having been
    trimmed from 1R0 (nominal), will have a very small w, approx 25% of
    the untrimmed width. The 5R1, having been trimmed from a nominal 4R6
    (or something like that), will have a large w. Consequently, it's not
    unusual to see test results having strange steps and discontinuities
    when you plot failure point versus resistance. Similar strange things
    sometimes happen when you compare product lines and package sizes.
    These results can generally be explained in terms of how the resistors
    are designed and processed.

    The actual failure mechanism varies with resistor technology.
    Thickfilm resistors fail when the element cracks due to differences in
    expansion rates of the element, substrate, and passivation layers.
    Thinfilm resistors may fail when the element literally vaporizes.
    Perhaps the oddest result of all is this: Many low value thickfilm
    resistors will tolerate very high levels of steady state power without
    failing. You can literally melt their solder connections without
    destroying the resistor itself or much affecting its value.

    Paul Mathews
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