In-rush resistor failed

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

Reply to
RMD
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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
Reply to
Trevor Wilson

"RMD"

** 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

Reply to
Phil Allison

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

Reply to
RMD

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.

Reply to
Sylvia Else

**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
Reply to
Trevor Wilson

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.

Reply to
Sylvia Else

"Stupider Than Anyone Else "

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

J = 5% tolerance.

... Phil

Reply to
Phil Allison

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

Sylvia.

Reply to
Sylvia Else

He just can't resist it, can he...

Reply to
Dimmer

**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
Reply to
Trevor Wilson

**That would be 0.02 of a second of course.
--
Trevor Wilson
www.rageaudio.com.au
Reply to
Trevor Wilson

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
Reply to
atec77

"Stupider Than Anyone Else"

** Bullshit you did.

Reply to
Phil Allison

"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

Reply to
Phil Allison

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

Reply to
RMD

On Sat, 24 Sep 2011 03:16:15 GMT, snipped-for-privacy@invalid.invalid (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!

Ross

Reply to
RMD

"RMD"

** 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 bridge.
** 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 THEMSELVES.

See pic here ( taken by me).

formatting link

... Phil

Reply to
Phil Allison

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

formatting link

Reply to
Dennis

Hi Phil,

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

Reply to
RMD

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