🚀 go-pugleaf

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.