In-rush resistor failed

RMD · 2011-09-23T04:21:20+00:00

Hi All,A friend of mine has an LG DVD/VCR combination unit which suddenlywent completely dead.These units are getting pretty hard to buy these days, so I lent heran 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 provedto be. A 2.2ohm 2W current in-rush resistor right at the mains input had goneopen circuit. I went down to Jaycar and bought a 5W resistor for $0.40and once I installed the new resistor the unit sprang into life again.Checking voltages around the power supply then the voltages aregenerally what I would expect to find. Nothing looks damaged oroverheated anywhere. No bulging capacitors either.However, it has been my experience that repairing these switchingpower supplies is never quite as straightforward as it might seem tobe in the first instance. Most of the components are fairly highlystressed and one weak sister can fail other components and the weaksister survives itself.Anyway, while I guess the inrush resistor might fail open like anyresistor might rarely fail this way, is there anything else I oughtfeel suspicious about if this in-rush resistor fails?And just by the way I don't really understand why this resistor needsto be 2W either. VCR's usually only use about 5-10 watts or so, so itis hard to see where 2 watts of heat comes from in the 2.2 ohmresistor. Or is this resistor really being sized for a voltage ratingand the 2W rating just comes with the voltage rating territory?Ross

Sep 23, 2011 20 Replies

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"RMD"

** 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 touches.

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