Welwyn W20 Series Wirewound Resistors - Voltage Limits

[Rontone]

I have some W21 3W and W22 7W resistors and was planning to use them as HT node droppers and power tube screen resistors, I have seen the W22 larger used many times in old amps, on Marshalls as screen resistors and droppers and the W21 3W are sold as screen resistors online...

But now looking at the datasheet they have quite a low "Limiting element voltage", reading around some other topics and forums it seems this is the maximum voltage?

Or does this mean the voltage ACROSS the resistor? As those W22's pop up everywhere but are only rated for 200V

[SEL49]

Or does this mean the voltage ACROSS the resistor?

Yes, that's what I believe.

[Rontone]

Thanks, an educated guess told me they must be doing alright in those positions otherwise they would all be toasted! but wasn't quite sure,

The use of the word element made me wonder if they were referring to the actual resistor body so therefore 'across' as opposed to 'what its being hit with'

I worded my search differently online and found someone asking exactly the same on ampgarage

[acheld]

Interesting question, since I've seen quite of few of those over the years, and never gave it much thought. 

The achilles heel of this resistor is its vitreous enamel coating/insulator.  While fireproof, the insulating properties are poor when at high temps, and these can be reached whilst operating at or just above their max power rating.  See p3 of the data sheet.

"Therefore, resistors operated at near maximum temperature cannot be classed as insulated
and should not be used in contact with any conducting material." 

On the other hand, I haven't seen one of these fail, or cause problems. 

I've tended to use Vishay wirewounds because I liked the way they looked when I first started this hobby (LOL, not knowing any better!).  When I looked up the data sheet, their max "working voltage" is sqrt(PxR).  That approach makes sense to me, and fits with Steve's thought.

[HotBluePlates]

I have some W21 3W and W22 7W resistors and was planning to use them as HT node droppers and power tube screen resistors ...

But now looking at the datasheet they have quite a low "Limiting element voltage" ...

Or does this mean the voltage ACROSS the resistor? As those W22's pop up everywhere but are only rated for 200V

The voltage ratings make sense if you look at the resistance available, and the wattage rating:

   - W21 is rated for 100v across the resistor and you might use a 470Ω or 1kΩ for a screen resistor, right?
   - 100v / 470Ω = 213mA and 213mA x 100v = 21.3 watts ---> way over the 2.5 watt rating

   - W22 is rated for 200v across the resistor, and 6 watts at 70ºC.  Say you use it as a 1kΩ screen resistor:
   - 200v / 1kΩ = 200mA ---> your tube's screen melted long ago.
   - 200v x 200mA = 40 watts ---> far exceeds the power rating of the resistor, may last through temporary overload but will burn if sustained long enough.


The screen resistor is dropping volts when there is high screen current draw to keep the screen from melting.
Look at a tube data sheet for a graph of plate current with different G2 voltages, and you see very large reductions of plate current when screen voltage drops 50v or 100v (and screen current drops proportionally).

While there are likely brief blips higher, the Mullard EL34 data sheet suggests sustained screen current doesn't exceed ~50mA at full power output.  That would be 50v dropped across a 1kΩ W22, and 2.5 watts in this 6w resistor.


The voltage rating limits start making sense when you consider that wirewound resistors are rarely available above ~10-20kΩ, and tend to be wound in low resistances, passing relatively high current, and mostly low voltage-drops across the resistor itself.