On 2/6/2013 3:41 AM, Bob Axtell wrote:
> On 2/6/2013 2:35 AM, Ruben J=F6nsson wrote:
>>> On second thought, considering the price of 5W 0.1ohm 5% wirewound
>>> resistors (http://www.futurlec.com/Res5W.shtml), winding your own may
>>> not be worth the time. I can easily 4-parallel them to get a 0.025ohm
>>> +/-1.25% 20W resistor block that can sink 30A - more than enough for my
>>> purposes. And for those prices, can make a 10-parallel block for $2,
>>> getting increasingly more accurate as more resistors are added.
>>>
>>> Can always stick them in the vodka freezer I keep next to my
>>> high-voltage lab bench if it ever comes down to keeping them cold.
>>>
>> Note that some resistor manufacturer even state the rated power at 25 de=
gree C
>> (most at 75 degree C). Then the power derates linearely until it gets to=
 0 W at
>> the maximum operating temperature.
>>
>> The temperature is the ambient temperature so if the resistor is in an
>> unventilated compartement it will contribute to the heating of the ambie=
nt
>> temperature itself. If your equipment is rated for a maximum operating
>> temperature of, say 50 degree C, and the resistor is in an enclosure it =
may not
>> take very much before the temperature inside this enclosure has rised 25=
 degree
>> C, taking into account other heat producers such as transformer, linear
>> regulators and other power electronics at worst case.
>>
>> /Ruben
>>
>>
>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>> Ruben J=F6nsson
>> AB Liros Electronic
>> Box 9124
>> 200 39 Malm=F6 Sweden
>> www.liros.se
>> Tel +46 40142078
>> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>>
> Ruben is absolutely right, proper removal of resistor heat is the most
> critical factor in the proper useage of power resistors, ESPECIALLY
> those used in measurement applications.
>
> --Bob A
>
I think someone else, maybe Russel had an anecdote about power resistors=20
heating up.  I have one too.
Working on a 24  ~350 watt motor controller, I wanted a load to test it=20
out.  I bought a bunch of Radio Shack 50 ohm 10 watt resistors and=20
soldered 10 of them in parallel to some 18 gauge wire.  No real=20
mechanical connection, just solder blobs holding them on.  After a few=20
minutes at max power they were hot enough to melt the solder and start=20
dropping off the bus wires (about 11.5 W per resistor).

Joe W

--=20
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