Understanding a split-mode power supply.

[Phil Allison]

"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.




..... Phil

 

[Sylvia Else]

"Sylvia Else"




** 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. There's not much point in having a fuisible there if it
remains conductive through insulation break down after it has fused.

Sylvia.

 

[Phil Allison]

"Sylvia Else"

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.

The voltage across the break can be thousands of volts.

The issue is a total furphy.


....... Phil





.......

 

[Sylvia Else]

"Sylvia Else"

** 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?

Sylvia.

 

[Phil Allison]

"Sylvia Else"

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.

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.





...... Phil

 

[Sylvia Else]

"Sylvia Else"


** 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.

That just begs the question of whether it will fuse in its designed way,
or whether the excess voltage will cause it to fuse in an unintended,
and possibly damaging, way.

** 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.

How do you know the original wasn't rated to 350V? The issue is not
whether it will work, but whether it will fail safe.

Sylvia.

 

[Phil Allison]

"Stupider than Anyone Else"

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.

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.



...... Phil

 

[Sylvia Else]

"Stupider than Anyone Else"



** 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.




** 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.

 

[Arfa Daily]

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

 

[Sylvia Else]

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.

 

[Arfa Daily]

Phil was only on about the voltage rating issue.


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.


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

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.


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.

Well, I hear what you're saying, and you could debate this in detail for
ever, but again, I would say to you, based on many years experience of
domestic and commercial repair work right down there at component level, if
the circuit is that critical of an emitter resistor value and type, then
it's poorly designed. If you believe this to be the case, and you see it as
an ongoing safety issue, then you should only replace it with an exact same
device. If you were able to look at a schematic for the board, and it showed
this resistor as a 'designated safety component', then I would not hesitate
to say that it should not be substituted. But in the event that you can't
confirm one way or the other, then it has to be your judgement call alone,
and no amount of advice from any of us, can change that.

Arfa

 

[Phil Allison]

"Stupider than Anyone Else"

Phil was only on about the voltage rating issue.

** Massively wrong presumption.

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.

Learn that and you will be an entirely better person.

Shame how no-body can teach old bitches new tricks.

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.

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.

** The fusible resistor in question is there ONLY to prevent the PCB
material catching fire - as might occur if a non fusible type were used -
cos they can act like an incendiary.

A more accurate name for fusibles is " flame proof " types - as described
in your link.

Protecting delicate parts like semis is not part of their job description -
at all.

BTW:

You came here for the advice of folk with experience of electronics, cos
you have absolutely none.

The advice I gave is based on many decades of ACTUAL experience.

If you simply replace the fusible resistor in the PSU with a similar part,
then it will perform just as well as the original one did.

There is a HUGE amount of electronics YOU have no clue about.

Nor any other fucking thing either.





...... Phil

 

[Sylvia Else]

"Stupider than Anyone Else"


** Massively wrong presumption.

Phil, it's a matter of record. Just go back and look.

** 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.

** Massively wrong presumption.

Another matter of record, Phil.

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. Like
any other reader of NGs, I'd be foolish to accept anything said here as
gospel, because I have no way of judging the motives of other posters.

The advice I gave is based on many decades of ACTUAL experience.

So you say, but how am I to know?

Sylvia.

 

[Phil Allison]

"Stupider than Anyone Else"

Phil, it's a matter of record.

** Then go read what was written properly.

I know you will never do that as you are completely autistic.

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.

** Been explained already - the likely scenario is that the high base
current that drove the switching transistor hard on passed to over to Q1
after the fusible resistor opened.

Q1 is a relatively fragile, low voltage device - so it failed.

Another matter of record, Phil.

** Then go post under those words.

I know you never will do that since they do not exist.

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.

** There is no such thing in the data sheet.

Correctly interpreting component maker's data is a whole science in
self - that also takes long experience to become expert at.

Counts YOU out completely.

I come here for comment, to help drive my thinking about a problem.

** Your totally bizarre thinking is not my problem.

So you say, but how am I to know?

** I was explaining that the basis for the advice given was experience -
shame I cannot supply a URL for that.

Now piss off -

you trolling PITA idiot.




...... Phil

 

[Sylvia Else]

This 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.

 

[Arfa Daily]

This 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.

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. The low rails for the preamps had only simple linear regulators. At
first, I thought this was a little odd, but of course, it is actually quite
common for the main 'big' rails not to be regulated in this sort of kit,
when it uses a conventional transformer-based supply. 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. A 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 ? It'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 ...

Arfa

 

[Sylvia Else]

It'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 ...

Though the PS and basic electronics are running 24/7, not just when the
A/C is running.

Maybe a hypothesis of yours applies here - they got a good deal on
transformers suitable for use in a switch mode PS.

Of if I were being cynical, which of course, I never am, I might suspect
that it was to ensure that the boards were uneconomic to repair (except
DIY), thus increasing the market for replacement boards.

Although I don't know how much a replacement costs, because 3 days after
I asked, Daikin still haven't replied to my email.

Sylvia.

 

[legg]

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.

It's possible that the original is just a flame-proof part - not
advertised as 'fusible' by the part mfr. You'll easily find these as
6R2 in 5% and 2% ranges.

The fact is that metal film resistor values below ~ 18R in 1/4 watt
and 10R in 1/2 watt are demonstrably fusible, under a certain range of
conditions, providing the coating is flame resistant. This
characteristic is often used to establish predictable single fault
abnormal behavior, where the overstress is predictably high under the
expected fault. It doesn't work as well for slower, lower voltage
fault surges.

If you want to know if the part you've selected is suitable, run tests
with a few samples by attaching them to the end of a line cord and
plugging them into the applicable line voltage, making sure that they
are located in a suitable container of some sort. The result may
suprise you - often the fusing event may be hardly audible for some
constructions.

This is only normally practical if you have access to parts in volume,
and control over the purchasing specification.

RL

 

[Mr.T]

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.

That would be most likely for that purpose.

A 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 ?

Or the reason a lot of gear uses SMP these days, easier to supply various
countries differing mains voltages.
(Another major factor of less shipping weight would apply to
air-conditioners)

MrT.

 

[Mr.T]

(Another major factor of less shipping weight would apply to
air-conditioners)

Oops, that should be "wouldn't".

MrT.