Understanding a split-mode power supply.

The small transistor is a C1815 - an NPN, with an annoyingly difficult to match pin-out.

Sylvia.

Reply to
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

Reply to
legg

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

Reply to
Arfa Daily

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

Reply to
rebel

Well, to avoid P&P costs, I need to use what I already have, or what I can buy from DSE or Jaycar.

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.

Reply to
Sylvia Else

"Sylvia Else"

** Looks well cleaned up.

Perfectly OK to use in any device that has only FUNCTIONAL insulation requirements.

( As opposed to "double insulation " or Class 2 safety insulation requirements )

..... Phil

Reply to
Phil Allison

2SC1815

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Reply to
Jasen Betts

If you find the spider there you should'a looked before you sat.

Reply to
Jasen Betts

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.

Reply to
Sylvia Else

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

Reply to
Arfa Daily

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.

Reply to
Sylvia Else

"Sylvia Else"

** Huh ??????????

No need for that whatsoever.

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

.... Phil

Reply to
Phil Allison

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.

Reply to
Sylvia Else

** 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

......

Reply to
Phil Allison

Which just means it won't conduct unless the voltage across it exceeds its insulating properties.

So you say, but where's the evidence?

Sylvia.

Reply to
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.

** 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

Reply to
Phil Allison

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.

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.

Reply to
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.

..... Phil

Reply to
Phil Allison

You seem to have posted the same reply twice.

Sylvia.

Reply to
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. 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

Reply to
Arfa Daily

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