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NTC 5D-9 Thermistor?

Discussion in 'Electronic Repair' started by Bob Shuman, Dec 16, 2005.

  1. Bob Shuman

    Bob Shuman Guest

    My son's 3-month old desktop computer failed and upon troubleshooting it, I
    found that the thermistor (TH1) that is in series with a 6.3A glass fuse and
    located in the 120VAC input power feed inside the power supply module had
    self-destructed. I tested the soldered in fuse and it was still good, so
    assume that it did not fail due to an over current situation. To determine
    conclusively that the failure did not result from a PSU secondary problem, I
    temporarily bypassed (shorted) the blown device with a jumper. When I did
    this the PSU and computer came up normally so I conclude that this is an
    isolated failure, possibly due to an under-designed component in a cheap
    imported PSU that came in the tower unit he built. (I believe it is a 600W
    PSU and the computer is pretty lightly loaded without many peripherals, so
    that is why I suspect it just being infant mortality, or possibly an
    under-designed part in this design).

    I know that the purpose of the device is to limit the start up inrush
    current to the PSU and protect down stream devices, so I definitely ant to
    replace it. Unfortunately, I'm not sure yet what the local Radio Shack will
    stock and wanted to know in advance what generic substitutes I might be able
    to use for this purpose.

    Basically, what I am asking is how do I read the 5D-9 value that was stamped
    on the old part? Is there a current rating and if so was the old part
    consistent with the 6.3A fuse or was it undersized and that is why it failed
    after just 3 months of use? Does the "5" in the first part of the rating
    indicate the resistance in ohms and if so, is this the steady state value or
    at the start up/inrush?

    Knowledge on the part nomenclature, reliability, and use in a SMPS is
    desired. Suggestions on suitable replacement parts that will prevent this
    from occurring again are welcome. Thanks in advance.

    Bob S.
  3. Bob Shuman

    Bob Shuman Guest

    Thanks for the spec sheet, but I'm not sure I can find everything I was
    looking for here on that sheet or another I also found via google search.
    The sheet clearly answers one question: the "5" in the 5D-9 part identifier
    is the resistance in ohms at 25 degrees Centigrade (I assume this is the
    startup resistance and that it decreases as the temperature increases due to
    current passing through it). If I read the table at the end, it appears
    that the steady state "hot" resistance drops to 0.18 ohms, but at what
    current, steady state temperature, and how quickly?

    More importantly, if I wanted to beef up this part to prevent this from
    happening again, what generic parameters should I be looking for in a
    thermistor at the local RS store? The 5 ohms and .18 ohm are a good place
    to start, but I did not see any type of current limit or power rating
    (assuming the .18 ohms) for the device.

    Additional answers/advice would be appreciated.

  4. If it is a 600W non-PFC power supply we might assume at 80% efficiency
    the input power to be somewhere around 750W.

    750/120 = 6.25A (sound familiar) - full power operation.

    Per Ohm's Law: 5 ohm starting resistance = 31W if starting inrush is
    100% of full power load (not likely).
    ..18 ohm ending resistance = 1.125W at full power.

    Note that the inrush current limit for the 5D-9 is only 3A, so either
    you don't have a 600W supply or (most probably) the max starting surge
    is calculated at 50% of the full power load or perhaps as you stated the
    device is underrated. You decide.
  5. Bob Shuman

    Bob Shuman Guest


    Thanks again. Yes, I can do the math and understand ohm's law... and your
    75% efficiency number seems about right (I assumed 80%) and consistent with
    the fuse sizing, but the piece I missed in the spec sheet is the maximum 3A
    in rush current. Where was this specified?

    I think you've hit upon why the part exploded and burned up. It appears it
    was indeed undersized for the application. So from their listing, which
    part has the higher in rush current limit? Again, I did not see this
    specified in the part list table. Thanks in advance.

    Bob S
    Column labeled 'at 25ºC (A)' - 3A
    25ºC is the starting temperature and thus the 3A must be the inrush
    current, or at least this is the way I read it.
    I would replace it with a NTC 5D-18 which has a 7A current rating.
  7. Oops - my error - you want the same terminal resistance so use a NTC
    8D-18 (8A).
  8. Bob Shuman

    Bob Shuman Guest

    Thanks again for taking the time to help educate me. From your explanation
    of the table (which makes perfect sense), I think the 5D-15 (6A in rush)
    seems to double the original current limit, but retains the 5 ohm startup
    resistance and 0.18 ohm steady state resistance. The 5D-18 would probably
    also do the trick ... it just has a slightly lower resistance, but I do not
    think this is an issue at all since ideally it would be zero anyway.

    Now to find this beast.... The local retail RS had nothing. They also did
    not have anything listed in their catalog either. in fact,. the only
    thermistor they carry was a 10K ohm initial value. I'll have to try some of
    the local electrical supply houses. Thanks also for the alternative CL40.
    I'll have to see what I can find.

  9. Unless you have a distributor nearby that will sell one to you you'll
    probably spend more time and effort trying to find it than it is worth -
    I'd simply order it online.

    Try Allied:

    or Mouser:
  10. Bob Shuman

    Bob Shuman Guest

    Yes, exactly. We did have one local electronics house here, Grainger, but
    they did not stock this item. I took your advice and ordered the CL40 from
    Mouser. My son is home from school for a couple weeks so hopefully we can
    get this in before he leaves to return to school.

    Thank you again for the help here in understanding the specifications.

  11. No problem - and I hope you'll get his PC back up and running.

    I just saw that CompUSA had an 500W Antec supply for $20 after rebates,
    sale good through tomorrow. You might want to get one of these for a
    spare or in case the thermistor fails again due to some undetermined
    problem. Order it online and you won't even have to drive to a store.
    Cheap insurance (but not as much fun as fixing a broken supply!).

    OTOH if you need a good source for a replacement PS in the future I'd
    recommend these guys.

    Oops, just noticed that this is a CompUSA branded supply, not an Antec
    supply! So buyer beware on that one (although I've installed a few 350W
    CompUSA supplies and they worked fine and are still in service after a
    few years).
  13. Bob Shuman

    Bob Shuman Guest


    I'll go for the replacement part thanks since I am 99+% certain that this
    will fix the supply given my previous experiment jumping around the blown
    part coupled with the fact the protection fuse was not damaged. Hopefully
    by putting in a replacement part that has twice the previous' current limit
    this should not happen again.

    I am a firm believer in minimizing waste and really hate to see anything at
    all go to a landfill that can be repaired. And besides the ecological
    aspect, it is so much more rewarding to fix something that is considered
    dead by most people. By the way, the part (1 piece) from Mouser was $1.39,
    and although I'm sure that the shipping and handling will increase that
    figure, it still should be much less than a replacement supply.

  14. I agree, but unfortunately more and more often now I am finding that it
    simply is too hard (time consuming and frustrating) to repair some of
    today's electronics.

    One of my Enermax high-end switching PC power supplies crapped out last
    week and while I quickly found the problem (a shorted Mosfet switching
    transistor and a couple of associated passive components) it was simply
    impractical to repair the supply without hours and hours of labor. The
    Mosfet was riveted to a heat sink that was in turn buried under several
    layers of other components. Removing all the layers would have taken a
    lot of time and extensive desoldering. While I would have preferred to
    save the supply I decided instead to replace it at a cost of $90. But
    now I've got some nice quiet 8mm and 9mm fans that I can use elsewhere.

    OTOH I recently repaired a 10 year old 27" Sony TV that had a failed
    power supply IC - it was easy to get to and while I was at it I improved
    the heat sink so it won't likely fail again. So you win some and you
    lose some.

    I hope you win the PS battle and that it only costs you $1.39 plus S&H.
    Post back and let us know if you succeed.
  15. CompUSA PSUs are made by several different companies, some great, like
    Win-tact, some awful, like Powmax/Leadman, and their supplier told me
    that the brand can change from week to week.
    Raidmax is now supposedly OK and is made by Topower, the same company
    that makes Aspire, OCZ, and Tagen. Topower's quality varies, but I
    believe that the Raidmaxes are made from the better models. Still, I'd
    probably rather buy a Fortron-Source, which can be as little as $40-50
    for a 400-450W model.

    I have a very well-built OEM PC PSU that has two thermistors, one for
    each AC line. One thermistor is fairly large, the other only half its
    diameter. What's the purpose of using two different sizes like this?
  16. I use Raidmax for the low end and Seasonic for high end applications.
    Enermax used to be my preferred high end PS but I've had a few problems
    recently with their supplies produced in 2002/03 so I'm waiting to see
    if they have addressed whatever supplier or design issue caused that.
    Good question. Are you sure that they both are thermistors and that one
    isn't a fuse link or RF filter?
  17. Here's a photo:

    On the circuit board they're labelled NTC1 and NTC2, indicating that
    they're thermistors. The RF filter is behind them in the photo.
  18. Here's a photo:

    I'm pretty sure that the two black things are thermistors because
    they're labelled NTC1 and NTC2 The RF filter is in back of them in the
  19. I'm pretty sure you are right. It is likely they are to provide two
    different NTC inrush curves - I suppose we'd have to ask the designer
    what s/he had in mind. Sounds like overkill to moi, or maybe they got a
    'two for one' deal on the thermistors.
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