On Mar 28, 2006, at 8:09 AM, Jim Kearman wrote:
>
> From: R; Measures <r@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?
For any R because when the resistance begins to change with V, it
indicates that something has begun to breakdown. At some point, with
carbon-comp resistors, it appears that current paths are beginning to
go around the carbon granules instead of them and the resistance
decreases with increasing voltage -- i. e., the resistor is beginning
to act like a varistor. High-value WW resistors suffer from the same
problem which is why a 100k-ohm, "100W" size unit may be max-rated at a
potential that results in only 70W of dissipation.
> Also, my intuition is that a resistor that will withstand X volts at
> room temperature may not hold up at higher temps.
Mine too
>
> 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.
Good point. A 24-hr test might shed light as well.
>
> 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.
Some of the WV ratings are case to gnd potential , and have nothing to
do with the max-J rating.
>
> 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.
Which is why I use their 3W-rated, 100k-ohm MOF resistors for
equalizing 450WV and 500WV electrolytics.
end
>
> 73,
>
> Jim, KR1S
> http://kr1s.kearman.com/
>
>
Rich Measures, 805.386.3734, AG6K, www.somis.org
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