Understanding a split-mode power supply.

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

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.

"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

2SC1815

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.

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

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.

"Sylvia Else"

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

No need for that whatsoever.

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

.... Phil

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.

** 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

......

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.

** 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

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.

"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

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

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

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

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.