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RMD
Guest
Fri Sep 23, 2011 4:21 am
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
Guest
Fri Sep 23, 2011 4:34 am
RMD wrote:
Quote:
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
Guest
Fri Sep 23, 2011 5:01 am
"RMD"
Quote:
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
Guest
Fri Sep 23, 2011 5:59 am
On Fri, 23 Sep 2011 15:01:28 +1000, "Phil Allison" <phil_a_at_tpg.com.au>
wrote:
Quote:
"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
Guest
Fri Sep 23, 2011 8:10 am
On 23/09/2011 2:21 PM, RMD wrote:
Quote:
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
Guest
Fri Sep 23, 2011 8:25 am
Sylvia Else wrote:
Quote:
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
Guest
Fri Sep 23, 2011 11:52 am
On 23/09/2011 6:25 PM, Trevor Wilson wrote:
Quote:
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
Guest
Fri Sep 23, 2011 12:25 pm
"Stupider Than Anyone Else "
Quote:
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
Guest
Fri Sep 23, 2011 1:48 pm
On 23/09/2011 10:25 PM, Phil Allison wrote:
Quote:
"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
Guest
Fri Sep 23, 2011 8:34 pm
Quote:
On 23/09/2011 10:25 PM, Phil Allison wrote:
"Stupider Than Anyone Else"
He just can't resist it, can he...
Trevor Wilson
Guest
Fri Sep 23, 2011 8:54 pm
Sylvia Else wrote:
Quote:
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
Guest
Fri Sep 23, 2011 9:01 pm
Trevor Wilson wrote:
Quote:
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
Guest
Fri Sep 23, 2011 9:38 pm
On 24/09/2011 6:34 AM, Dimmer wrote:
Quote:
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
Guest
Fri Sep 23, 2011 11:51 pm
"Stupider Than Anyone Else"
Quote:
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.
Quote:
Sylvia.
Phil Allison
Guest
Fri Sep 23, 2011 11:56 pm
"Trevor Wilson"
Quote:
**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
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