🚀 go-pugleaf

"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:496343d1$0$26491$afc38c87@news.optusnet.com.au...
> Arfa Daily wrote:
>
>> 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.

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