Understanding a split-mode power supply. - Page 2

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Re: Understanding a split-mode power supply.


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Which Library?
I've seen mint books in some TAFE libraries that are of the same vintage.

Re: Understanding a split-mode power supply.
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Erie Community College, Williamsville, New York state, USA.

The postage cost more than the book did.

I'll probably get an email from them now saying it was stolen and that
they want it back :(

Sylvia.


Re: Understanding a split-mode power supply.

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How about Q1 being a thyristor as a crow bar on the bias of that second
transistor?

    In which case, your symbol is incorrect. And would show the reason
why you're getting low ohm reading is my guess on what you call the
   base-emitter and Collector being opened which is actually the M1
  terminal etc..

    That's just a guess of course.


http://webpages.charter.net/jamie_5 "


Re: Understanding a split-mode power supply.
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The small transistor is a C1815 - an NPN, with an annoyingly difficult
to match pin-out.

Sylvia.

Re: Understanding a split-mode power supply.
On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else

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Looking at the package's flat surface, with leads pointing down, the
pin-out of 2SC1815 is ECB.

RL

Re: Understanding a split-mode power supply.

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i.e. absolutely standard pinning for just about any and all Japanese TO92
transistors starting 2SA, B, C, or D ...

FWIW, the '1815 is about the commonest general purpose small signal NPN
Jappo transistor to be found anywhere, and anything similarly general
purpose will sub for it. It's generally not hard to rearrange the leads of a
differently pinned device, with a bit of sleeving on one or two legs to stop
them touching.

Arfa



Re: Understanding a split-mode power supply.
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Well, to avoid P&P costs, I need to use what I already have, or what I
can buy from DSE or Jaycar.
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Yes, I'd already concluded that given its position in the circuit, any
vaguely similar type will suffice, and that rearranging the leads is
practical, if not very elegant.

I've obtained a suitable resistor (not fuisible, but OK to test the
solution) and will give that a go later. Shame it's getting so hot outside.

Sylvia.

Re: Understanding a split-mode power supply.

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Why stress?  Replace with the same part# - WES have them in Sydney.

Re: Understanding a split-mode power supply.

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2SC1815  

http://www.toshiba.com/taec/components2/Datasheet_Sync/50/6455.pdf



Re: Understanding a split-mode power supply.



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**What are the type numbers of the two transistors?


--
Trevor Wilson
www.rageaudio.com.au



Re: Understanding a split-mode power supply.




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**Scratch that. You already told me.


--
Trevor Wilson
www.rageaudio.com.au



Re: Understanding a split-mode power supply.
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.



Re: Understanding a split-mode power supply.

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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.

Otherwise, if it definitely is 6.2 ohms, and you really want to replace it
with exactly that value, your reasoning with making such a value in the way
that you suggest, would be quite valid.

Arfa



Re: Understanding a split-mode power supply.
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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.

Re: Understanding a split-mode power supply.

"Sylvia Else"


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 **   Huh   ??????????

No need for that whatsoever.

In operation, the resistor is not subjected to more than a few volts.




....   Phil






Re: Understanding a split-mode power supply.
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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.

Re: Understanding a split-mode power supply.

"Sylvia Else"
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** 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





......




Re: Understanding a split-mode power supply.
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Which just means it won't conduct unless the voltage across it exceeds
its insulating properties.

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So you say, but where's the evidence?

Sylvia.

Re: Understanding a split-mode power supply.

 "Sylvia 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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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.
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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.

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