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In-rush resistor failed

Discussion in 'Hobby Electronics' started by RMD, Sep 23, 2011.

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  1. RMD

    RMD Guest

    Hi All,

    A friend of mine has an LG DVD/VCR combination unit which suddenly
    went completely dead.

    These units are getting pretty hard to buy these days, so I lent her
    an old VCR of mine so she could use her VHS tapes.

    It seemed to me it was likely a power supply problem, and so it proved
    to be.

    A 2.2ohm 2W current in-rush resistor right at the mains input had gone
    open circuit. I went down to Jaycar and bought a 5W resistor for $0.40
    and once I installed the new resistor the unit sprang into life again.

    Checking voltages around the power supply then the voltages are
    generally what I would expect to find. Nothing looks damaged or
    overheated anywhere. No bulging capacitors either.

    However, it has been my experience that repairing these switching
    power supplies is never quite as straightforward as it might seem to
    be in the first instance. Most of the components are fairly highly
    stressed and one weak sister can fail other components and the weak
    sister survives itself.

    Anyway, while I guess the inrush resistor might fail open like any
    resistor might rarely fail this way, is there anything else I ought
    feel suspicious about if this in-rush resistor fails?

    And just by the way I don't really understand why this resistor needs
    to be 2W either. VCR's usually only use about 5-10 watts or so, so it
    is hard to see where 2 watts of heat comes from in the 2.2 ohm
    resistor. Or is this resistor really being sized for a voltage rating
    and the 2W rating just comes with the voltage rating territory?

  2. **The 2 Watt resistor is likely to be a fusible, flameproof resistor.
    Replacing it with a 5 Watt type invites potential disaster. The resistor is
    usually designed to fail under unexpected high load conditions. You should
    replace it with the same type. WES Components is a good source for these
  3. Phil Allison

    Phil Allison Guest

    ** It has to accept a 100 amp peak surge at switch on - when there is
    340V peak on one side and a few diodes and a uncharged electro on the other.

    Either a WW or fusible metal film type should be OK.

    ..... Phil
  4. RMD

    RMD Guest

    The replacement Jaycar resistor is wirewound and is finished with a
    cream ceramic glossy sort of finish just the same as the original

    The unit owner only turns the DVD/VCR on when she actually uses it. It
    is never used for timer recording. This means it is constantly being
    switched on from cold so it gets more of the switch-on surge current
    than units used mostly in standby mode which are swiched on from cold
    once in a blue moon.

  5. Sylvia Else

    Sylvia Else Guest

    Are you sure it says 2W and not 2J (being 2 Joules) ?

    The inrush current dissipates a certain amount of energy in the resistor
    in a short time. The resistor has to be able to hold that energy without
    overheating while it dissipates it.

  6. **Resistors are always (IME) rated in Watts, not Joules. A 2 Watt fusible
    will typically cope with very large, short term power dissipation. They will
    be typically rated at 16 times normal load for 30 seconds.
  7. Sylvia Else

    Sylvia Else Guest

    I thought I'd found one in a PS some years ago that had a clearly stated
    energy rating, but I can't find an example on line.

    The power dissipated during the inrush current would easily exceed even
    16 times normal load, albeit for a short time. On the face of it, it's
    just luck that the construction of high power resistors is such that
    they can withstand this short but very high pulse.

  8. Phil Allison

    Phil Allison Guest

    "Stupider Than Anyone Else "

    ** WW resistors often have "J" printed on them.

    J = 5% tolerance.

    .... Phil
  9. Sylvia Else

    Sylvia Else Guest

    Yes. I kind'a already knew that, thankyou.

  10. Dimmer

    Dimmer Guest

    He just can't resist it, can he...
  11. **No luck involved. The 16 times thing, for 30 seconds is a thermal
    limitation. For (say) 0.2 second, the dissipation might be (say) several kW.
    Long enough to charge the capacitor.
  12. **That would be 0.02 of a second of course.
  13. atec77

    atec77 Guest

    Finding a being like else less able to think than himself probably
    causes a burst of self stimulation and other philthy things
  14. Phil Allison

    Phil Allison Guest

    "Stupider Than Anyone Else"
    ** Bullshit you did.
  15. Phil Allison

    Phil Allison Guest

    "Trevor Wilson"

    ** With a 2.2 ohm resistor and a 47uF electro cap, the charging time
    constant is 100uS.

    Be fully charged in about 300uS if switched on near a voltage peak.

    ..... Phil
  16. RMD

    RMD Guest

    Hi Trevor,

    I've been busy elsewhere, so away from the computer.

    Thanks for the heads up re the fusible nature of any replacement
    resistor for the in-rush current resistor.

    The original resistor was 2W wirewound, and the replacement is 5W
    wirewound. WES doesn't actually have a branch in Adelaide, but the WES
    closest replacement in wirewound is 3W.

    My thoughts are that if I put a 2.2 ohm 5W wirewound resistor across
    the mains (as in assuming all other series components in the power
    supply failed short circuited) then it would be momentarily
    dissipating 24KW and it wouldn't last long before it vapourised. I
    concede it might last a tiny bit longer than a 2W wirewound resistor,
    but my thoughts are the difference would not be significant
    practically speaking.

    The remote control for the LG DVD/VCR combo was literally removed from
    the council wheelie bin, and the combo unit itself was already on the
    council hard waste pile, so just to have it working again for however
    long it might work is a big plus for the owner in any case.

    If (say) the electrolytic capacitor fails catastrphically short
    circuit and the 5w wirewound resistor fails too slowly then I guess
    the unit will just go back on the hard waste pile, so no great loss

    Those are my thoughts anyway. I don't see it as being a fire hazard in
    catastrophic failure mode with the 5W wirewound resistor, since the
    case is metal anyway.

    As a more general comment I last worked as an "on the ground"
    electronic design engineer back in the middle 70's. PC boards back
    then had pretty wide track widths and multilayer cards were the big
    thing of the time. An 8080 processor was the latest LSI of the time.

    Looking at the build quality and line quality of the PC cards etc
    inside this LG unit I have to say I think it is just a miracle of cost
    engineering to sell something of this quality so cheaply.

    If by some miracle of time travel we could have seen this unit 30
    years ago we would have been in awe at both the achievement and the
    reliability of current electronics.

    We've come a very long way in a such short time.

  17. RMD

    RMD Guest

    On Sat, 24 Sep 2011 03:16:15 GMT, (RMD) wrote:
    Oh, and just by the way, I remember when metal film resistors first
    appeared on the market. They replaced carbon resistors which weren't
    very reliable at all and quite given to going higher in value.

    Anyway, some co-workers of the time were quite interested in seeing
    how good these new metal film resistors were.

    So they overloaded these resistors so much they were eventually just a
    smoking black ruin. However these darn resistors still measured
    exactly the same as when they were undamaged.

    Metal film resistors are pretty tough guys!

  18. Phil Allison

    Phil Allison Guest

    ** Resistor wattage ratings and their ability to withstand large but brief
    overloads are NOT related. Even WW resistors vary enormously in design and
    things like the gauge of wire used for an given value and wattage rating.

    Reason being, there are a large number of different resistance metals that
    could be employed inside.

    ** It will blow the AC fuse in the item - or trip the AC supply breaker.

    ** Far more common is failure of the switching device or diodes in the

    ** It will be quick.

    ** WW resistors are the most common used for inrush surge limiting - they
    usually fail instantly and with no visible sign.

    So called " fusible " resistors are really just flame proof types, when
    overloaded they glow red and emit some smoke but do not catch fire

    See pic here ( taken by me).

    .... Phil
  19. RMD

    RMD Guest

    Hi Phil,

    <<<<** WW resistors are the most common used for inrush surge limiting
    - they usually fail instantly and with no visible sign.>>>>

    I'd have to say that I've had various electrical things failing on me
    for nearly 50 odd years.

    Usually the worst I've seen is a dark brown smell, or maybe a burn
    mark, but mostly things just fail silently and with no obvious damage.

    While catastrophe can happen, it is rare. Luckily for all of us! :)

  20. Phil Allison

    Phil Allison Guest


    ** Wish I could say the same.

    Burning resistors are a real hazard, they can easily set PCB material alight
    and that means black soot all over the inside of things. Carbon soot is
    highly corrosive and takes the plating of the chassis and any metal parts it

    Another real nasty is when a big electro decides to bleed its juice all over
    the PCB. That stuff is corrosive and conductive enough to start a fire if
    enough energy is available.

    Then there are items that have had drinks ( beer, OJ or Coke etc) spilt over
    them - I even saw one case where someone had puked on the front of a Disco
    mixer. It stank.

    .... Phil
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