Miniature Power Resistors

[shelly_d]

I am not sure about the voltage rating. It is possible, in my mind anyway, that the voltage rating is there as a safety measure based on the insulating properties of the outer case. I would not use the 350V rated resistor on a 500 volt line to drop the voltage to 450V. Even though the voltage drop is only 50 volts across the resistor, the outer case is being called upon to electrically isolate a voltage that exceeds the resistors voltage rating. Just my take on this as I just do NOT know what the rating means and I like to be better safe than sorry.

On power rating, I use a 2 to 1 rating to dissipation ratio in the work I do as well. I believe that is a pretty well used guideline in industry as well.

Shelly_D

 

[triode17]

So are both of these statements correct?

That the voltage rating refers to the voltage dropped across the resistor, not the voltage measured to ground. And that as long as the wattage rating of the resistor is at least twice the power being dissipated, the resistor is safe to use.

It doesn't matter how the resistor is connected, the bottom line is the voltage across the resistor can't exceed it's rating. These resistors are spiral cut and the voltage can arc across the spirals. FWIW, that voltage is a guaranteed rating but I am using a 1 meg. to Gnd. across my PAS3x main filter cap as a bleeder resistor. Its a 1/2w, rated at 350v but even with 400 v across it, it has never arc'd. So I left it there. Let's call it a beta test. Been like that for years now.

 

[GordonW]

Also to keep in mind, increasing the resistor power reduces the thermal noise, especially in high gain stage, in the tube's plate and cathode.

Yep, that's what I was talking about before. Less "shot noise"...

Regards,
Gordon.