Welwyn W22 resistors

[Murphyv310]

So they should be derated for low values. Seems strange they have just slapped on a 200v figure across the range. Granted I agree 're their size but the rating could be confusing as in the op's question.

[Terry]

Back in the late 60s I prided myself on the breadth and quality of the component stock I had built up and sold large numbers of them.

However, there were limits. I could see no point in stocking resistor values beyond the E12 series as these were perfectly adequate for our service purposes and the constructional articles in magazines that our component buyers wanted them for.

However, many of these project designs came from engineers working on professional and military electronics at places like Marconi. I'm quite sure that a lot of the designs were based mainly on rule of thumb calculations or mental arithmetic as the proliferation of 1k, 2k2 and 10k values for collector load resistors  appeared to indicate. There was, of course, another commonly used value which supported this theory: 5k1 ...

A succession of boys, mainly from the local grammar school, got a crash course in tolerances when they asked for 5k1 resistors - fortunately most turned up with the relevant magazine in their hands so I was able to point out to the statement  at the bottom of the component list which said "All resistors ½W, tolerance ±20% unless otherwise stated" and prove to them that my 4k7 ±5% resistors were guaranteed to be much closer to 5k1 than a resistor of the 'correct' value with a tolerance of ±20% - even assuming that such a component even existed anywhere on the planet!

The fun began when a bewildered mum turned up in the workshop holding a piece of paper on which her son had copied out the list of values he wanted her to buy ...

[Bushbaby]

Thanks all.
On the face of it they appear perfect (and indeed are) for the application that I want to put this to.

Regards

Rob

[Alistair D]

I always thought that the rated voltage was partly related to what the resistor could withstand after failure. In the case of a WW resistor the gap between the damaged ends of the winding could be very small and could even be carbonised. Perhaps not so much in the case of vitreous coated resistors but the gap between the windings either side of the burn out will also limit the max voltage that can be applied.

Al

[Mark Hennessy]

Moved Terry's post (#12) into this one, and hidden the others...

[Terry]

These limits apply to various resistor types - I produced a table once for lower power resistors to illustrate this, but I haven't got it on this PC. If I can locate it, I'll post it here.

Found it on my laptop - I'd omitted to put it in my Dropbox!

So, here it is:



It shows the voltage drop across each resistor value which results in a dissipation equal to the resistor's maximum rating.

However, each series has a maximum specified voltage rating, just like the Welwyn wirewounds, and the maxima shown here are typical values although resistors with higher maximum limits are available - Vishay seems to spring to mind.

Obviously, if I hadn't stopped the calculation when these these limiting figures were reached, the voltages across the resistors would have continued to rise with increasing value.

Jeffrey will no doubt be delighted to see that the figures in the table have been rounded to whole numbers ...

[ppppenguin]

The Vishay (previously Philips) VR37 resistors are rated at 2500V. Their VR68 series at 10000V. Available in high values so very suitable for all sorts of EHT applications.