Understanding a split-mode power supply. - Page 3

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Re: Understanding a split-mode power supply.

 "Stupider than Anyone Else"

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** 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.


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** 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








Re: Understanding a split-mode power supply.
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You seem to have posted the same reply twice.

Sylvia.

Re: Understanding a split-mode power supply.

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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



Re: Understanding a split-mode power supply.

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Phil was only on about the voltage rating issue.

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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.

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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
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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.

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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.

Re: Understanding a split-mode power supply.

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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



Re: Understanding a split-mode power supply.
On Tue, 6 Jan 2009 10:25:48 -0000, "Arfa Daily"

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I would also agree. I've been doing commercial repair long before smps
arrived.

Re: Understanding a split-mode power supply.
On Tue, 06 Jan 2009 13:11:10 +1100, Sylvia Else

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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

Re: Understanding a split-mode power supply.
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.

Re: Understanding a split-mode power supply.

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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



Re: Understanding a split-mode power supply.
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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.

Re: Understanding a split-mode power supply.

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That would be most likely for that purpose.

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put

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.



Re: Understanding a split-mode power supply.

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Oops, that should be "wouldn't".

MrT.



Re: Understanding a split-mode power supply.

"Arfa Daily"


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** Such amplifier supplies are actually push pull, square wave inverters.

It is not possible to have regulated output without the using an active
PFC/regulator stage prior to the main inverter.

Also, because the inverter's output IS a square wave, full wave
rectification produces a pure DC voltage.


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** Major sales gimmick.


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** The big joke is that at least 95% of all such amplifiers are permanently
installed in venues.

Plus musos love to buy the heaviest tube amps they can find and use them
with  heaviest speakers ever made.


......  Phil




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