In-rush resistor failed

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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?

Ross

Re: In-rush resistor failed
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**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
resistors.


--
Trevor Wilson
www.rageaudio.com.au



Re: In-rush resistor failed

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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
there.

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.

Ross

Re: In-rush resistor failed
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!

Ross



Re: In-rush resistor failed

"RMD"

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** 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.


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


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


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**  It will be quick.


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** 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
THEMSELVES.

See pic here ( taken by me).

http://sound.westhost.com/articles/incandescent.htm#norm


...  Phil



Re: In-rush resistor failed

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There's a wealth of what look like good quality articles & info tucked away
there as well. cheers.

http://www.sound.westhost.com/articles.htm



Re: In-rush resistor failed
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! :)

Ross









Re: In-rush resistor failed
"RMD"
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 ** 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





Re: In-rush resistor failed

"RMD"

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**  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






Re: In-rush resistor failed

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

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.

Ross



Re: In-rush resistor failed
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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.

Sylvia.

Re: In-rush resistor failed
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**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.


--
Trevor Wilson
www.rageaudio.com.au



Re: In-rush resistor failed
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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.

Sylvia.


Re: In-rush resistor failed

"Stupider Than Anyone Else "


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

     J = 5% tolerance.



...  Phil



Re: In-rush resistor failed
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Yes. I kind'a already knew that, thankyou.

Sylvia.

Re: In-rush resistor failed
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He just can't resist it, can he...



Re: In-rush resistor failed
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Finding a being like else less able to think than himself probably
causes a burst of self stimulation and other philthy things
--
X-No-Archive: Yes


Re: In-rush resistor failed
 "Stupider Than Anyone Else"
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** Bullshit you did.




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Re: In-rush resistor failed
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**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.


--
Trevor Wilson
www.rageaudio.com.au



Re: In-rush resistor failed
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**That would be 0.02 of a second of course.


--
Trevor Wilson
www.rageaudio.com.au



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