🚀 go-pugleaf

On Tue, 06 Jan 2009 13:11:10 +1100, Sylvia Else
<sylvia@not.at.this.address> wrote:

>Arfa Daily wrote:
>> "Sylvia Else" <sylvia@not.at.this.address> wrote in message 
>> news:4961f0fa$0$25649$afc38c87@news.optusnet.com.au...
>>> 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.
>
>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.

It's possible that the original is just a flame-proof part - not
advertised as 'fusible' by the part mfr. You'll easily find these as
6R2 in 5% and 2% ranges.

The fact is that metal film resistor values below ~ 18R in 1/4 watt
and 10R in 1/2 watt are demonstrably fusible, under a certain range of
conditions, providing the coating is flame resistant. This
characteristic is often used to establish predictable single fault
abnormal behavior, where the overstress is predictably high under the
expected fault. It doesn't work as well for slower, lower voltage
fault surges.

If you want to know if the part you've selected is suitable, run tests
with a few samples by attaching them to the end of a line cord and
plugging them into the applicable line voltage, making sure that they
are located in a suitable container of some sort. The result may
suprise you - often the fusing event may be hardly audible for some
constructions.

This is only normally practical if you have access to parts in volume,
and control over the purchasing specification.

RL