resistorsthermalto220
I purchased this 0.5 Ohm power resistor, which comes in a TO-220 Package, with the aim of pushing 3A through it (i.e. 4.5W)
The datasheet has the following information:
It says it will rise 6.5 K/W, so it should rise 29.25C + 23C ambient = 52.25C. Using my thermometer probe, the temperature on the metal body is well over 150C, so I turned it off.
Have I messed up something in my calculations, or is this datasheet incorrectly specified?
The libraries in the official distribution are not very extensive. The gEDA way is to find/create your own, you'll likely have your own personal preferences about pad lengths, symbol sizes, silkscreen formatting etc. There are no resistor arrays in the default libraries to my knowledge.
1206 is a common large SMT resistor package, as you guessed. It refers to a 120 mil by 60 mil rectangular package. Some newcomers to SMT prefer to use individual 1206 components, but most use 0805 (80 by 50 mils...you get the idea) for general resistors that have to be hand soldered. I like 0603, personally, but I like to solder under a binocular microscope.
However, in automatic soldering, it's also a popular outline for arrays: the 4 resistors fit in a space 120 mils by 60 mils. You'll also find them in 0805 and 0603, but you'll want to stick with 1206 if you're new at this. After peering into my crystal ball, I'm guessing that you might be looking at a page on RS like this and datasheets with packages like this:
...and footprints like this:
You need to draw the second image as a footprint in PCB, using the dimensions in the table.
Some parting thoughts:
Lead resistance is not a problem. Look at the IRFB7545PBF data sheet. Lead cross-section is ~0.38 mm x 1.14 mm (minimum). Cross-section area is ~ 0.5 mm\$^2\$. This is equivalent to 20 ga wire, and has a resistance of ~10 mΩ/ft. A lead length of 0.3 inch will give a total resistance of 0.25 mΩ. Two leads, of course, will have a total resistance of 0.5 mΩ (less than 10% of the stated 5.9 mΩ). Total power dissipation for the package is 26 W (67 x 67 x .0059), which is well within limits for a TO220.
Under this load, each lead will dissipate 1 W. For a long run in free air, this is enough to melt the lead, but that is not how a TO220 is used. The leads will be soldered into a pc board with very heavy traces (to avoid delamination of the board), and these traces will serve to conduct heat away from the leads.
And for what it's worth, the data-sheet specifies the 67 A figure for a case temperature of 100°C.
ETA - According to a FAQ note at irf.com, the TO220 package has a current limit of 75 A, so the original suspicion that a TO220 can't handle high currents is almost correct.
Best Answer
The level of heatsinking you need depends on the maximum ambient temperature you can guarantee (let's assume 50C). At an operating temperature of 150C you have a temperature gradient between the resistor's internals and the air of 100C. You want to push 5W (rounded up) through this gradient, which means that the total thermal resistance can not be more than 100C per 5W or 20C/W (C/W is how a heatsink is rated).
If the 6.5C/W is the correct internals-to-case heat resistance that leaves 13.5C/W for the heatsink. To put that in perspective: the heatsink below is rated at 11C/W. Note that for heatsinks a lower value is better (and larger, and heavier, and more expensive), so stay below the 13.5C/W figure.
You can do such calculation the same way you would calculate with voltage (temperature in C), current (power in W), and resistance (thermal resitance in C/W).
Note1: With a 13.5C/W heatsink that heatsink will be at (13.5/20)*100 = 67.5C above the ambient temperature. If that is too hot for your purpose you will need a (much) bigger heatsink.
Note2: The heat must be transferred to the ambient. Even th largest heatsink is useless when it is put in a thermally isolated box!
Note3: The C/W of eany heatsink can be improved (lowered) dramatically by using forced circulation (= a fan). But think about what happens when the fan fails, and the air must still go somewhere.
Note4: A a quick rule of thumb anything in a TO220 casing can dissipate up to 1W, but the case will get too hot to touch. Above 1W a heatsink is probably needed.