🚀 go-pugleaf

"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:49630537$0$4046$afc38c87@news.optusnet.com.au...
> Phil Allison wrote:
>>  "Stupider than Anyone Else"
>>
>>>>>>> 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.
>>>>>>  **   Huh   ??????????
>>>>>>
>>>>>> No need for that whatsoever.
>>>>>>
>>>>>> In operation, the resistor is not subjected to more than a few volts.
>>>>> 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.
>>>> ** Irrelevant.
>>>>
>>>> Once it has fused, the resistor  will be OPEN circuit.
>>> Which just means it won't conduct unless the voltage across it exceeds 
>>> its insulating properties.
>>
>>
>> ** 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.
>>
>>
>>>> The voltage across the break can be thousands of volts.
>>> So you say, but where's the evidence?
>>
>>
>> ** 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.
>>
>
> 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