Difference between 1/4W and 1/2W resistors?

[david]

Timothy Daniels wrote:

[snip]

Assuming that P=VI (power = voltage x current), or V**2/R,
that works out to .033watts, or about 3.79 times the 1/8watt rating.
The 1/4watt resistors mentioned previously in this thread should
suffice quite nicely.

*TimDaniels*

There's a problem there .033W (33mW) is a lot less than 1/8W (125mW).

Tim hit the multiply button instead of the divide button on his
calculator.

 

[Nobody > (Revisited)]

To prevent overheating. If you take a resistor and operate it at its
rated power, it will easily reach 100 Celcius, and if it's rated for
at least about 1/2W it can also burn your fingers, melt wire
insulation, make circuit boards discolor or char, and even soften
solder. I've been told to use resistors rated for at least twice the
maximum power they'll have to dissipate (or 4x the max for hot
environments, like in a car's engine compartment), and usually when
I've found a cracked or burned resistor, it had been running at very
close to its power rating.

You've just shown the problems that have always existed when a resistor
is used in a circuit which dissipates a lot of power.

The biggest reason why a lot of the situations you describe happened
wasn't due to the wattage rating, it was the mechanical designs used.

A lot had to do with airflow. Stuffing the resistor flat to the board
may have contributed to a little bit of heat-sink action, but unless
there were "big around planes" underneath... all you had was an eventual
board-burn. Raising the resistor off the board a ¼" cured a lot of that.


Pad-burn, melted solder, gray joints were also common with just 90° lead
bends. Simple thing like adding a full loop about ¼" often cured that.
Just another basic heat-sink idea...

As for "underhood stuff", blame the bean-counter mentality on that.

 

[david]

"david" tried again:

Which says the same thing. Learn some arithmetic.

No, it's not the same thing at all. Learn some arithmetic.

X does not equal 1/X.

Which is the same thing. Learn some arithmetic.

*TimDaniels*

No, it's not the same thing at all. Learn some English.

 

[david]

Conclusion: A 1/8W resistor is quite enough, a 1/4W resistor is safer
and conservative.

*TimDaniels*

Of course, this isn't what you said earlier...

Maybe you'll finally realize that 0.033 does not equal 0.125 * 3.79, but
it is equal to 0.125 / 3.79.

Or, more likely, instead of admitting your mistake, which was pointed out
by several, you'll continue to insist you were right all along.

In either case, I could care less about anything that you say, since you
have lost all real credibilty.

 

[david]

[.....]
In either case, I could care less about anything that you say, since
you have lost all real credibilty.

Promises, promises...

*TimDaniels*

As I suspected...

"Or, more likely, instead of admitting your mistake, which was pointed
out by several, you'll continue to insist you were right all along."

Idiot.

....plonk...