[Amps] Need some 220K 2 watt carbon resistors

[Jim Kearman]

From: R; Measures <r at somis.org>

> >
> > I think his question was how to test the voltage rating.
> 
> Simple:  Apply 10% of the V-rating, measure I, calculate Ohms.  Apply 
> 100% of the V-rating, measure I, calculate Ohms again.  If the 
> resistances are not within 1% of each other, the V-rating is 
> questionable.

Is this test method valid for wirewound resistors? Also, my intuition is that a resistor that will withstand X volts at room temperature may not hold up at higher temps. It's clear that the only way to be sure is to test one to destruction and hope the rest of the batch has similar characteristics. 

It's worth going to Mouser and downloading datasheets. Vishay uses a rule-of-thumb method for specifying working voltage of their CW series of silicone-coated wirewound resistors: The square root of Power x Resistance. Ohmite, OTOH, gives a specific working voltage based on the power rating, for their 20- and 270-Series wirewounds. 

Panasonic MOF metal-oxides vary from 250 to 750 V depending on wattage. The minimum wattage to withstand 400 V is 5W (500 V). Using two or more, 2-W MOFs in parallel, for example, would increase total dissipation, but the working voltage of the 2-W parts is only 350 V. 

73,

Jim, KR1S
http://kr1s.kearman.com/