Welwyn W22 resistors

[Bushbaby]

I bought some Welwyn w22 300R resistors (7 watt) to use one of this in my amplifier project. According to the info that I read about these on the Farnell website these are only rated at 200v??? Is this correct as it appears that they may unusable in mains voltage projects?

Many thanks

Rob

[ppppenguin]

I can't see a problem: http://www.farnell.com/datasheets/1449996.pdf

I can see 200V on some Farnell pages but really don't know why. 200V across a 300R resistor would result in a burnout very quickly indeed as it would be dissipating over 130W. The data sheet says the proof voltage is 500V. In other words the enamel coating could touch the chassis even if the resistor is 500V above chassis voltage.

[Bushbaby]

Thank you Jeffrey.
I couldn't understand why they would indicate 200v but I must admit I didn't see 500v although I didn't look that closely. I did think it strange as the are used in many items of test equipment which I would image operates in some instances much higher than 200v.

Best regards

Rob

[ppppenguin]

These Welwyn resistors are top quality parts. The sort you use with the confidence that they won't let you down.

[Bushbaby]

I've checked with the data sheets....W23 are 500v and W22 are 200v. I cannot believe this is correct??

[Terry]

Rob, I think you are missing the point that Jeffrey has made.

Forget the data sheet and use simple formulae such as Ohms Law.

The current through the resistor will determine the voltage drop across it. Multiply the two together to find the power dissipated in the resistor. If it exceeds 7 watts, it won't be suitable for your purpose!

You should find that if the voltage drop across a 300R resistor is 45.825V, the current through it is 152.75mA and the power dissipated is 6.99976875W.

This is rather academic as it assumes a ±0% resistor tolerance and you should derate these figures in practice but the important thing here is that no way does the voltage drop approach 200V!

The 200V figure is the maximum for any value in the W22 series - the voltage drop will increase for resistors with higher values but must not exceed that limiting figure. 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.

[ppppenguin]

To give you an idea of actual numbers, 200V across a 13.3k resistor would disspate 3W. 200V across a 5.7k resistor woudl dissipate 7W. You're just not going to get 200V across most wirewound resistors without overdisspating them

"6.99976875W" Terry, that's spurious precision. If I'd done calculations like that at university or even at school I'd have been marked down. 46V, 153mA, 7W.

[Bushbaby]

Apologies as I didn't understand the term Limiting Element Voltage which is the 200v max for the W22 series.

Regards

Rob

[Murphyv310]

I'm sure it's nothing to do with the voltage across the resistor itself but it's insulation properties. The W23 is larger in diameter and consequently has a greater flash over voltage of 500v whereas the W22 being smaller has a lower flash over voltage.

[ppppenguin]

There's no chance of 200V or 500V flashing across something that big. You'd need kilovolts for that to happen. Except for the highest values the voltage across the resistor is limited by power disspation as discussed above. Short term pulse overload is fine unless it blows the wire like a fuse.

Limiting Element Voltage is defined here. It's simply the largest steady or RMS voltage allowed across a resistor. Subject to not exceeding power disspation.
http://www.ttelectronicsresistors.com/te...-terms.asp