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Which just means it won't conduct unless the voltage across it exceeds"Sylvia Else"
Phil Allison wrote:
"Sylvia Else"
As for whether I could substitute a 6.8, maybe I could, though I'd have
trouble getting one that has a 350 volt rating.
** Huh ??????????
No need for that whatsoever.
In operation, the resistor is not subjected to more than a few volts.
As I commented earlier, if the switching transistor fails by shorting out,
then the resistor will blow. After it's blown it'll have 340V across it.
** Irrelevant.
Once it has fused, the resistor will be OPEN circuit.
So you say, but where's the evidence?The voltage across the break can be thousands of volts.
Phil Allison wrote:
"Sylvia Else"
As for whether I could substitute a 6.8, maybe I could, though I'd
have trouble getting one that has a 350 volt rating.
** Huh ??????????
No need for that whatsoever.
In operation, the resistor is not subjected to more than a few volts.
As I commented earlier, if the switching transistor fails by shorting
out, then the resistor will blow. After it's blown it'll have 340V
across it.
** Irrelevant.
Once it has fused, the resistor will be OPEN circuit.
Which just means it won't conduct unless the voltage across it exceeds its
insulating properties.
The voltage across the break can be thousands of volts.
So you say, but where's the evidence?
That just begs the question of whether it will fuse in its designed way,"Sylvia Else"
Phil Allison wrote:
"Sylvia Else"
As for whether I could substitute a 6.8, maybe I could, though I'd
have trouble getting one that has a 350 volt rating.
** Huh ??????????
No need for that whatsoever.
In operation, the resistor is not subjected to more than a few volts.
As I commented earlier, if the switching transistor fails by shorting
out, then the resistor will blow. After it's blown it'll have 340V
across it.
** Irrelevant.
Once it has fused, the resistor will be OPEN circuit.
Which just means it won't conduct unless the voltage across it exceeds its
insulating properties.
** No, it WILL be open circuit to the voltage that caused it to fuse.
Cos the fusing behaviour will not cease until the part becomes open circuit
in the given situation.
How do you know the original wasn't rated to 350V? The issue is notThe voltage across the break can be thousands of volts.
So you say, but where's the evidence?
** You came here for the advice of folk with experience of electronics, cos
you have none.
That advice is based on many decades of ACTUAL experience, in my case.
If you simply replace the fusible resistor in the PSU with a similar part,
then it will work as well as the original one did.
As for whether I could substitute a 6.8, maybe I could, though I'd
have trouble getting one that has a 350 volt rating.
** Huh ??????????
No need for that whatsoever.
In operation, the resistor is not subjected to more than a few volts.
As I commented earlier, if the switching transistor fails by shorting
out, then the resistor will blow. After it's blown it'll have 340V
across it.
** Irrelevant.
Once it has fused, the resistor will be OPEN circuit.
Which just means it won't conduct unless the voltage across it exceeds its
insulating properties.
The voltage across the break can be thousands of volts.
So you say, but where's the evidence?
You seem to have posted the same reply twice."Stupider than Anyone Else"
As for whether I could substitute a 6.8, maybe I could, though I'd
have trouble getting one that has a 350 volt rating.
** Huh ??????????
No need for that whatsoever.
In operation, the resistor is not subjected to more than a few volts.
As I commented earlier, if the switching transistor fails by shorting
out, then the resistor will blow. After it's blown it'll have 340V
across it.
** Irrelevant.
Once it has fused, the resistor will be OPEN circuit.
Which just means it won't conduct unless the voltage across it exceeds its
insulating properties.
** No, it WILL be open circuit to the voltage that caused it to fuse.
Cos the fusing behaviour will not cease until the part becomes open circuit
in the given situation.
The voltage across the break can be thousands of volts.
So you say, but where's the evidence?
** You came here for the advice of folk with experience of electronics, cos
you have none.
That advice is based on many decades of ACTUAL experience, in my case.
If you simply replace the fusible resistor in the PSU with a similar part,
then it will work as well as the original one did.
BTW:
The maker's max voltage rating relates to a functional resistor - not a
blown one.
There is a HUGE amount of electronics YOU have no clue about.
I must say that in this case, I agree with Phil. It's sometimes possible toPhil Allison wrote:
"Stupider than Anyone Else"
As for whether I could substitute a 6.8, maybe I could, though I'd
have trouble getting one that has a 350 volt rating.
** Huh ??????????
No need for that whatsoever.
In operation, the resistor is not subjected to more than a few volts.
As I commented earlier, if the switching transistor fails by shorting
out, then the resistor will blow. After it's blown it'll have 340V
across it.
** Irrelevant.
Once it has fused, the resistor will be OPEN circuit.
Which just means it won't conduct unless the voltage across it exceeds
its insulating properties.
** No, it WILL be open circuit to the voltage that caused it to fuse.
Cos the fusing behaviour will not cease until the part becomes open
circuit
in the given situation.
The voltage across the break can be thousands of volts.
So you say, but where's the evidence?
** You came here for the advice of folk with experience of electronics,
cos
you have none.
That advice is based on many decades of ACTUAL experience, in my case.
If you simply replace the fusible resistor in the PSU with a similar
part,
then it will work as well as the original one did.
BTW:
The maker's max voltage rating relates to a functional resistor - not a
blown one.
There is a HUGE amount of electronics YOU have no clue about.
You seem to have posted the same reply twice.
Sylvia.
Phil was only on about the voltage rating issue.I must say that in this case, I agree with Phil. It's sometimes possible to
get too pedantic about these things. I wouldn't think for one moment that
the value of 6.2 ohms is in any way critical. If it were, then they wouldn't
be using a bog standard 5% tolerance part, which could be up at over 6.5
ohms and still be in spec.
It was a 5% tolerance component. Maybe at 5% and with allowance forFar more likely that either the manufacturing
department had a shedload of that value in stock, or were being offered them
for virtually nothing, or the designer had some bee in his bonnet about
calculating some 'correct' value for some aspect of his design that he
thought important enough to warrant it. If it really really did need to be
*exactly* that value, then I'm afraid that I would rate it as a poorly
designed circuit, with insufficient tolerance of component spread and aging.
It's credible, given that there was no sign of distress, but it's oddLike Phil, based on decades of service work, with switch mode power supplies
figuring in most of that, my experience of fusible resistors is that they
often fail open for no apparent reason - which is maybe what happened in
your case
Well, you wouldn't if they've been correctly specified. Phil wascatastrophic enough to blow the resistance material off the substrate. Even
where these resistors have failed either benignly or catastrophically in the
high voltage side of switchers, I can't ever recall seeing any secondary
problem caused by continued arcing or leakage within the failed part.
It's not really the case that I was trying to match the exactIf you are really that bothered that the exact characteristics of the
original failed device should be preserved, then you should not really be
contemplating making up the value from two series connected resistors with
totally *different* characteristics.
Well, I hear what you're saying, and you could debate this in detail forArfa Daily wrote:
I must say that in this case, I agree with Phil. It's sometimes possible
to get too pedantic about these things. I wouldn't think for one moment
that the value of 6.2 ohms is in any way critical. If it were, then they
wouldn't be using a bog standard 5% tolerance part, which could be up at
over 6.5 ohms and still be in spec.
Phil was only on about the voltage rating issue.
Far more likely that either the manufacturing department had a shedload
of that value in stock, or were being offered them for virtually nothing,
or the designer had some bee in his bonnet about calculating some
'correct' value for some aspect of his design that he thought important
enough to warrant it. If it really really did need to be *exactly* that
value, then I'm afraid that I would rate it as a poorly designed circuit,
with insufficient tolerance of component spread and aging.
It was a 5% tolerance component. Maybe at 5% and with allowance for aging
it will stay within the limits required by the design. But all bets would
be off if I stuck in a 5% component of some other value.
If this had been the 200K component, (also a non-standard value), I'd
happily have stuck in a 220K of 180K, and thought nothing of it. But it's
a low value component connected to the feedback system. Who's to say what
a 10% change to 5.6 ohms, let alone a 30% change to 4.7 ohms would do.
Maybe nothing, but it's not worth the possible trouble to find out.
Like Phil, based on decades of service work, with switch mode power
supplies figuring in most of that, my experience of fusible resistors is
that they often fail open for no apparent reason - which is maybe what
happened in your case
It's credible, given that there was no sign of distress, but it's odd that
Q1 was taken out as well. I still can't see a mechanism for that, which is
why I was somewhat surprised that the repair worked.
- but if they do fail for a 'real' reason, that failure is usually
catastrophic enough to blow the resistance material off the substrate.
Even where these resistors have failed either benignly or
catastrophically in the high voltage side of switchers, I can't ever
recall seeing any secondary problem caused by continued arcing or leakage
within the failed part.
Well, you wouldn't if they've been correctly specified. Phil was
essentially arguing that I could substitute a component with a lower
voltage rating.
As part of my research for this reply, I found a data sheet for a 0.22 ohm
0.25 W fusible, which can be reached from this page
http://tinyurl.com/8jy2zx
It quotes a number of different voltage levels. In particular, it quotes a
specific "Maximum withstand voltage after fusing." To my mind, in the
particular circuit in question, that value would have to be 340V or
greater.
If you are really that bothered that the exact characteristics of the
original failed device should be preserved, then you should not really be
contemplating making up the value from two series connected resistors
with totally *different* characteristics.
It's not really the case that I was trying to match the exact
characteristics. Indeed, I haven't, because a 4.7 ohm fusible requires
more current to fail than a 6.2 ohm fusible of the same power rating.
Instead, my aim was to reproduce the original resistance, within the
tolerance chosen by the designer, to minimise the chance of destabilising
the circuit, while preserving a safe failure more should the transistor
fail shorted. In this context, by safe I mean a failure mode that would
have a fair chance of protecting the upstream components, thus leaving the
board in a repairable state.
Sylvia.
** Massively wrong presumption.Phil was only on about the voltage rating issue.
It's credible, given that there was no sign of distress, but it's odd that
Q1 was taken out as well. I still can't see a mechanism for that, which is
why I was somewhat surprised that the repair worked.
Well, you wouldn't if they've been correctly specified. Phil was
essentially arguing that I could substitute a component with a lower
voltage rating.
Instead, my aim was to reproduce the original resistance, within the
tolerance chosen by the designer, to minimise the chance of destabilising
the circuit, while preserving a safe failure more should the transistor
fail shorted. In this context, by safe I mean a failure mode that would
have a fair chance of protecting the upstream components, thus leaving the
board in a repairable state.
Phil, it's a matter of record. Just go back and look."Stupider than Anyone Else"
Phil was only on about the voltage rating issue.
** Massively wrong presumption.
You could try construing the point as being an invitation to explain howIt's credible, given that there was no sign of distress, but it's odd that
Q1 was taken out as well. I still can't see a mechanism for that, which is
why I was somewhat surprised that the repair worked.
** Ignorance is neither a virtue nor ever a basis to argue from.
Another matter of record, Phil.Well, you wouldn't if they've been correctly specified. Phil was
essentially arguing that I could substitute a component with a lower
voltage rating.
** Massively wrong presumption.
I come here for comment, to help drive my thinking about a problem. LikeBTW:
You came here for the advice of folk with experience of electronics, cos
you have absolutely none.
So you say, but how am I to know?The advice I gave is based on many decades of ACTUAL experience.
Phil was only on about the voltage rating issue.
** Massively wrong presumption.
Phil, it's a matter of record.
It's credible, given that there was no sign of distress, but it's odd
that Q1 was taken out as well. I still can't see a mechanism for that,
which is why I was somewhat surprised that the repair worked.
** Ignorance is neither a virtue nor ever a basis to argue from.
You could try construing the point as being an invitation to explain how a
failure of the resistor could take out Q1, or vice versa.
Well, you wouldn't if they've been correctly specified. Phil was
essentially arguing that I could substitute a component with a lower
voltage rating.
** Massively wrong presumption.
Another matter of record, Phil.
** There is no such thing in the data sheet.I note you've snipped out the stuff about the data sheet that directly
contradicts your claim that the potential across the fused resistor could
be several thousand volts.
BTW:
You came here for the advice of folk with experience of electronics, cos
you have absolutely none.
I come here for comment, to help drive my thinking about a problem.
The advice I gave is based on many decades of ACTUAL experience.
So you say, but how am I to know?
It's not at all uncommon to see switchers that are secondary-sideThis PS circuit seems rathed old fashioned. I opened a larged A/C unit
from the same manufacturer, and which was installed at the same time. It's
circuit board is only about half the size.
Although this is a switch mode PS, it doesn't appear to use switch mode
regulation. As far as I can see, there is no feedback from the secondary
side back to the primary feedback circuit. Instead, there is a linear
regulator in the secondary side to set the voltage for the electronics.
For what it does, the PS primary circuit seems rather complicated.
Sylvia.
Though the PS and basic electronics are running 24/7, not just when theIt's beyond me why it would be worth
the added complication of a switcher over a linear, given the energy-gulping
nature of the appliance in the first place, and the potentially hostile
environment that it has got to work in ...
It's possible that the original is just a flame-proof part - notArfa Daily wrote:
"Sylvia Else" <sylvia@not.at.this.address> wrote in message
news:4961f0fa$0$25649$afc38c87@news.optusnet.com.au...
Ok, as I mentioned elsewhere in this thread, I managed to get the A/C
working again by replacing Q1 and the 8.2 ohm resistor.
Actually, I'd misread it, and when I looked more carefully, it realised it
was 6.2 ohm, as is the other one on the board with the same markings
(i.e., a not blown one). Nice standard value that.
So now I have to replace the resistor with a fusible, but sourcing a 6.2
ohm fusible is problematic.
The situation is not made any easier because if the switching transistor
fails by shorting out, then after it blows the fusible will have 340V
across it.
From the suppliers I now know about (thanks, Trevor), I can get a 0.5 watt
340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 1 watt
non-fusible. My reasoning is that the 1.5 ohm resistor will have only 1/3
the power dissipation, and will handle twice as much. Therefore the 4.7
ohm will go open circuit before the 1.5 ohm could get hot enough to be a
problem.
Does this stand up?
Sylvia.
Are you absolutely sure that it is definitely 6.2 ohms ? That is a *very*
odd value, particularly for a fusible type ... FWIW, I really don't think
that the circuit would give a damn if you replaced it with a 6.8 ohm, which
is a standard value. Did you actually measure the one that's ok with an
accurate low ohms meter, and get a reading of 6.2 ? Seems to me that a grey
band and a blue band might easily be misread one for the other, with some of
the banding paints I've seen used over the years. I find that it is often
very easy to misread red for orange or brown, especially if the resistor
runs warm in normal use.
Blue Red Gold Gold = 6.2 ohms, 5%. It is in the E24 series, but I've
certainly never seen one before.
When I made up an equivalent, my meter gave the same reading (allowing
for tolerance) for the equivalent as it does for the identical resistor
on the board.
So, yes, I'm pretty sure.
As for whether I could substitute a 6.8, maybe I could, though I'd have
trouble getting one that has a 350 volt rating. The only supplier I know
of that purports to have them doesn't give the rating. The suppliers
that give ratings don't carry that value.
If I understood the circuit better, I'd be more comfortable about
changing the value. The mere fact that an unusual value has been used
gives me pause - maybe it's the value that's required there. Even if
another value worked, I'd not know the ramifications.
Sylvia.
That would be most likely for that purpose.So what was the
advantage of complicating things by using a switcher ? I can only assume
that it's primarily because it's a portable item of band equipment, so it
makes it much lighter to carry around.
Or the reason a lot of gear uses SMP these days, easier to supply variousA lot of other kit uses switchers
these days because of the energy efficiency, particularly when they are
put
into a 'sleep' mode, but an A/C unit ?
Oops, that should be "wouldn't".(Another major factor of less shipping weight would apply to
air-conditioners)
It's not at all uncommon to see switchers that are secondary-side
linear-regulated only. I worked on a mixer / PA just a couple of weeks ago
which had a full blown switcher in it. The main rails for the output
stages had no regulation at all, and were completely dependant on the
input line voltage.
So what was the advantage of complicating things by using a switcher ?
I can only assume that it's primarily because it's a portable item of band
equipment, so it makes it much lighter to carry around.
I would also agree. I've been doing commercial repair long before smps"Sylvia Else" <sylvia@not.at.this.address> wrote in message
news:49630537$0$4046$afc38c87@news.optusnet.com.au...
Phil Allison wrote:
"Stupider than Anyone Else"
As for whether I could substitute a 6.8, maybe I could, though I'd
have trouble getting one that has a 350 volt rating.
** Huh ??????????
No need for that whatsoever.
In operation, the resistor is not subjected to more than a few volts.
As I commented earlier, if the switching transistor fails by shorting
out, then the resistor will blow. After it's blown it'll have 340V
across it.
** Irrelevant.
Once it has fused, the resistor will be OPEN circuit.
Which just means it won't conduct unless the voltage across it exceeds
its insulating properties.
** No, it WILL be open circuit to the voltage that caused it to fuse.
Cos the fusing behaviour will not cease until the part becomes open
circuit
in the given situation.
The voltage across the break can be thousands of volts.
So you say, but where's the evidence?
** You came here for the advice of folk with experience of electronics,
cos
you have none.
That advice is based on many decades of ACTUAL experience, in my case.
If you simply replace the fusible resistor in the PSU with a similar
part,
then it will work as well as the original one did.
BTW:
The maker's max voltage rating relates to a functional resistor - not a
blown one.
There is a HUGE amount of electronics YOU have no clue about.
You seem to have posted the same reply twice.
Sylvia.
I must say that in this case, I agree with Phil. It's sometimes possible to
get too pedantic about these things. I wouldn't think for one moment that
the value of 6.2 ohms is in any way critical. If it were, then they wouldn't
be using a bog standard 5% tolerance part, which could be up at over 6.5
ohms and still be in spec. Far more likely that either the manufacturing
department had a shedload of that value in stock, or were being offered them
for virtually nothing, or the designer had some bee in his bonnet about
calculating some 'correct' value for some aspect of his design that he
thought important enough to warrant it. If it really really did need to be
*exactly* that value, then I'm afraid that I would rate it as a poorly
designed circuit, with insufficient tolerance of component spread and aging.
Like Phil, based on decades of service work, with switch mode power supplies
figuring in most of that, my experience of fusible resistors is that they
often fail open for no apparent reason - which is maybe what happened in
your case - but if they do fail for a 'real' reason, that failure is usually
catastrophic enough to blow the resistance material off the substrate. Even
where these resistors have failed either benignly or catastrophically in the
high voltage side of switchers, I can't ever recall seeing any secondary
problem caused by continued arcing or leakage within the failed part.
If you are really that bothered that the exact characteristics of the
original failed device should be preserved, then you should not really be
contemplating making up the value from two series connected resistors with
totally *different* characteristics.
FWIW, I just looked at a couple of types in one of my catalogues, and one
was rated at 250v ac, and the other at 350v ac, so either would be fine at
340v dc. If the ones that you find listed don't have a voltage rating in the
catalogue, check them out on their manufacturers' websites, where they
surely will.
Arfa
I obtained a second hand ex-library copy for a few dollars plus postage.N_Cook wrote:
Simplified design of Switching Power Supplies
by John D Lenk, 1995
Thanks for the reference.
Sylvia.