Resistor wattage?

[Illum]

some time ago someone recommended me this: http://led.linear1.org/1led.wiz
for calculating resistance needed for some of my earlier projects.

I was wondering...um, How do you increase the overall wattage if resistor combinations?
say 4 1/2 watt resistors in parallel. does that sum to to 2 watt?

heres the requirements for my project [USBing the desklamp by mounting two stars in parallel at 250ma each and a heatsink to where the incan bulb was]
http://led.linear1.org/1led.wiz?VS=5;VF=3.5;ID=500
the solution was 3.3 ohms at 2 watt
I have a combo soldered previously for another project consisting of 3x10 ohm [1/2 watts] tied together in parallel.
I'm not sure whether i can be used without starting a fire





I remember I made a thread on this somewhere, but I couldnt find it

[B@rt]

Yes, with resistors in a parallell setup you can add up their wattage rating.

[evan9162]

What Bart says is true, but with one caveat.

If you cluster your resistors closely together, I would derate them somewhat - the resistors are able to disspate less heat when they are tightly clustered together (i.e. twisting the leads together and making a "resistor clump") since air is less able to move around the resistor body and leads.

So if you combine 4 1/2W resistors together in a tight clump, I would derate to 1.5W just to be safe.

[Illum]

thanks B@rt

Evans...When I actually mount the resistors they will be packed in with thermal epoxy on to the heatsink where I mounted my stars on, so I dont think ventilation would matter, but yeah...its safe to be conservative

[HCaul]

One (obvious) caution: adding up the power ratings only works for resistors of the same value. For mixed values, you have to calculate the power dissipation of each resistor separately. A 1/4 Watt 1 Ohm resistor in parallel with a 1/4 Watt 1M resistor does not make a 1/2 Watt .99999 Ohm resistor....

And the advice about derating the wattage slightly is very good, especially when you consider that two 5% resistors of the same value may be different from one another by as much as 10% and still be in spec.

H. Caul

[Illum]

from what I have on hand, I'm assuming 2 x [10 ohm 1 watts] + 1 x [10 ohm 1/2 watts] wouldn't burn anything

[HCaul]

from what I have on hand, I'm assuming 2 x [10 ohm 1 watts] + 1 x [10 ohm 1/2 watts] wouldn't burn anything

If you're burning 2 Watts through those three parallel resistors, each is
dissipating 2/3 Watts. Your smallest resistor is only rated for 1/2 Watt,
so this is pushing things.

You probably won't start a fire, but you will overheat the resistor, which,
in turn, can cause its value to change beyond its tolerance, which may
cause your circuit to fail if it depends on the resistor value. I forget whether carbon resistors tend get more or less resistive as they heat up, but when they fail completely, they tend to open rather than short..