[Amps] Need some 220K 2 watt carbon resistors

[R at contesting.com]

On Mar 28, 2006, at 8:09 AM, Jim Kearman wrote:

>
> 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?

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