In-rush resistor failed

[RMD]

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

 

[Trevor Wilson]

RMD wrote:

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?

**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

 

[Phil Allison]

"RMD"

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.

** 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

 

[RMD]

On Fri, 23 Sep 2011 15:01:28 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"RMD"


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.

** 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.

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

 

[Sylvia Else]

On 23/09/2011 2:21 PM, RMD wrote:

And just by the way I don't really understand why this resistor needs
to be 2W either.

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.

 

[Trevor Wilson]

Sylvia Else wrote:

On 23/09/2011 2:21 PM, RMD wrote:
And just by the way I don't really understand why this resistor needs
to be 2W either.

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.

**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

 

[Sylvia Else]

On 23/09/2011 6:25 PM, Trevor Wilson wrote:

Sylvia Else wrote:
On 23/09/2011 2:21 PM, RMD wrote:
And just by the way I don't really understand why this resistor needs
to be 2W either.

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.

**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.

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.

 

[Phil Allison]

"Stupider Than Anyone Else "

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.

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

J = 5% tolerance.



.... Phil

 

[Sylvia Else]

On 23/09/2011 10:25 PM, Phil Allison wrote:

"Stupider Than Anyone Else"


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.


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

J = 5% tolerance.

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

Sylvia.

 

[Dimmer]

On 23/09/2011 10:25 PM, Phil Allison wrote:
"Stupider Than Anyone Else"

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

 

[Trevor Wilson]

Sylvia Else wrote:

On 23/09/2011 6:25 PM, Trevor Wilson wrote:
Sylvia Else wrote:
On 23/09/2011 2:21 PM, RMD wrote:
And just by the way I don't really understand why this resistor
needs to be 2W either.

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.

**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.

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.

**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

 

[Trevor Wilson]

Trevor Wilson wrote:

Sylvia Else wrote:
On 23/09/2011 6:25 PM, Trevor Wilson wrote:
Sylvia Else wrote:
On 23/09/2011 2:21 PM, RMD wrote:
And just by the way I don't really understand why this resistor
needs to be 2W either.

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.

**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.

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.

**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.

**That would be 0.02 of a second of course.


--
Trevor Wilson
www.rageaudio.com.au

 

[atec77]

On 24/09/2011 6:34 AM, Dimmer wrote:

On 23/09/2011 10:25 PM, Phil Allison wrote:
"Stupider Than Anyone Else"

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


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

 

[Phil Allison]

"Stupider Than Anyone Else"

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.


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

J = 5% tolerance.


Yes. I kind'a already knew that,

** Bullshit you did.

Sylvia.

 

[Phil Allison]

"Trevor Wilson"

**That would be 0.02 of a second of course.

** 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

 

[RMD]

**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.

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

 

[RMD]

On Sat, 24 Sep 2011 03:16:15 GMT, rmd@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

 

[Phil Allison]

"RMD"

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.

** 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.

My thoughts are that if I put a 2.2 ohm 5W wirewound resistor across
the mains ....

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

If (say) the electrolytic capacitor fails catastrphically short
circuit

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

and the 5w wirewound resistor fails too slowly ...

** It will be quick.

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.

** 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

 

[Dennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9e52ksFq8qU1@mid.individual.net...

"RMD"


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.

** 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.


My thoughts are that if I put a 2.2 ohm 5W wirewound resistor across
the mains ....

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


If (say) the electrolytic capacitor fails catastrphically short
circuit

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


and the 5w wirewound resistor fails too slowly ...

** It will be quick.


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.

** 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

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

 

[RMD]

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