Resistors - not quite ten a penny!

[Nick]

With components of unknown or potentially dubious provenance, I'd never use them in a situation where an unexpected failure mode could cause problems.

Whilst these may indeed be 1% resistors, its pure speculation what their continuous voltage rating is - its nothing to do with their power rating - its all to do with breakdown.

Putting 400V across a resistor proves nothing unless its a long-term soak test - i.e. months or years - and other environmental conditions are also controlled, e.g. humidity - it also tests just that resistor, not the batch or even the next batch that someone orders - who knows what long-term quality control the manufacturer uses?

On another forum related to valve audio, a member has been dissecting cheap eBay Al power resistors. Not pretty - all over-rated by at least 100% and electrically not well insulated...

Caveat Emptor ! 

[Yorkie]

Clearly anyone who wishes to use resistors or any other components for that matter in safety critical situations wouldn't be buying bargain bags off e-bay, but as I've said before, I'm a hobbyist - I'm not putting rockets into space. That said, I can't really think of  many, if any, situations in which 0.25 Watt resistors such as these are likely to have very high voltage across them. and presumably it's the potential difference across the resistor that determines is ability to survive. Hence, if a resistor is required say to drop voltage at a point in a circuit from 250V to 150V, it needs to tolerate 100V - not 250V, or am I missing something?

The resistors at the link I gave were bought by a chum to experiment with simple 555 IC projects, for which I'm sure they'd be ideal.

Just a question of which factory in China they came from I guess. Clearly one that's well geared up to turning out bandoliered resistors by the million!

To ramble off into irrelevancies, who knows where anything comes from nowadays? Mostly China it seems - certainly much of the stuff bought from Maplin in the last 20 years or more.

Maplin Far East Operations was first established in 1992 due to the increase in the purchasing of items from Taiwan followed by Hong Kong and China. To strengthen buying power, the Maplin Electronics set up its first Far Eastern branch office in Taipei, Taiwan, in charge of product sourcing, purchasing, inspecting, shipping and payment arrangement from Taiwan and is now the Far East Operations Centre. The Hong Kong Office was established in 1995 to handle the sourcing, purchasing and shipping from Hong Kong. Most production in Taiwan and Hong Kong has moved to Shenzhen, Mainland China. The 3 offices in the Far East handle more than 500 suppliers and 5,000 buying items for the parent company.

Incidentally, Since 2014, Maplin has been owned by Rutland Partners, which  also owns Pizza Hut UK and Bernard Matthews.

Bootiful!

[Mark Hennessy]

That said, I can't really think of  many, if any, situations in which 0.25 Watt resistors such as these are likely to have very high voltage across them. and presumably it's the potential difference across the resistor that determines is ability to survive. Hence, if a resistor is required say to drop voltage at a point in a circuit from 250V to 150V, it needs to tolerate 100V - not 250V, or am I missing something?

The voltage rating question is an interesting one.

We all know that Power = Voltage squared over R. Hence, Voltage = square-root(Power times R)

From this, we can say that a 100k 0.25W resistor shouldn't have more than 160V across it - if it does, the power rating will be exceeded. Simple stuff

Of course, the voltage rating of a resistor is distinctly separate to this. Most 0.25W resistors are rated for 250V. So, although the above suggests that a 1M resistor could have 500 volts across it before the power rating is exceeded, the voltage rating will cap this...

OK, so actually, that's also pretty simple. But there's a further issue...

High value film resistors with lots of volts across them will eventually fail high or O/C. By "high", my usual rule of thumb is 100k and 100V, but I've seen failures with half those values. I've written about this before, so I won't repeat myself here.

So, I wonder if Nick was alluding to that particular failure mode? It might not be a safety thing as such - although arcing is possible - but it might be that projects working from more than a few 10s of volts might not have the long-term reliability you'd ordinarily expect.

Regarding the question about 100V or 250V, there is no simple answer - every situation is different. General thoughts: can you be sure that there will never be more than the usual voltage under all conditions (normal and fault)? If not, what might it rise to? And for how long? And how much current could be supplied? What is the effect of failure? Ultimately, it's a judgement call. And it will be guided by knowledge and experience. So, for example, my "rule of 100s" would guide me away from using 0.25W film resistors from certain situations - choosing a metal oxide type instead.

For more insight into these sorts of issues, have a look at Bob Pease's book: Troubleshooting Analog Circuits. It's an old one, but it's still very useful. I bought a paper copy many years ago, and by coincidence was reading through it just last night. Chapter 3 talks about resistors.

Cheers,

Mark

[Nick]

That said, I can't really think of  many, if any, situations in which 0.25 Watt resistors such as these are likely to have very high voltage across them. and presumably it's the potential difference across the resistor that determines is ability to survive. Hence, if a resistor is required say to drop voltage at a point in a circuit from 250V to 150V, it needs to tolerate 100V - not 250V, or am I missing something?

So, I wonder if Nick was alluding to that particular failure mode? It might not be a safety thing as such - although arcing is possible - but it might be that projects working from more than a few 10s of volts might not have the long-term reliability you'd ordinarily expect.

I was referring to that - oddly, I also raised this point in the previous thread that you referenced: http://golbornevintageradio.co.uk/forum/...9#pid46479

Its the "rated continuous working voltage" (RCWV) that needs to be looked at...

[Mark Hennessy]

I was referring to that - oddly, I also raised this point in the previous thread that you referenced: http://golbornevintageradio.co.uk/forum/...9#pid46479

Its the "rated continuous working voltage" (RCWV) that needs to be looked at...

Yes, I'd spotted that. It's an important point that bears repeating - voltage ratings generally deserve more attention than they get; that's because today, high voltages are the exception, not the norm. It was different back in the day

[Yorkie]

Many thanks guys for your erudition - I'm now better informed!

[Refugee]

I had to buy some special high voltage resistors from Farnell when I was repairing a scope as the normal ones would have just gone open circuit after a few weeks.

[Radio Fixer]

In a previous post Mark said ... " but I suspect that changes in contact area between the carbon granules is a factor as well. I could well imagine that happens after the initial compression during manufacturer - one theory suggests that the granules "relax" slightly, and I could understand that heat could contribute to this in service."

Still working on a completely stripped Philips Monoknob and the interesting thing, and related to the above, that the cable driven potentiometers all go miles high being typically three times that specified (for example a 600K that measures 1M6. With use of WD40 this increases to 1M8).

I have 'collected' two other chassis (different models) and they are the same, of course all pretty well worn out.

Gary

[Radio Fixer]

Sorted this one: the high R is at the connection tags.

Gary

[Mark Hennessy]

I've noticed a similar thing with the popular "Allan Bradley" potentiometers that were fitted to many sets during the 1970s. 50k was used a lot by Hacker, and they usually measure about 65k these days. Not a problem for the operation of the volume controls, but interesting nonetheless...

When these go bad, the problem is usually corrosion of the "wiper", but not the part that contacts the carbon track, but the part that contacts the metal track that is connected to the wiper tag. I did take some photos of the last one I did. I repair them by swapping parts from similar potentiometers - I look out for all examples of these, whatever the track value, because they are so easy to take apart and swap parts.