What is the difference between a 1206, 0805 and a 0603 SMD resistor, as far I know they all have a resistance of 10.7K?
I going to need one for my little 5v DC buck converter.
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The codes you are referring to are size codes, e.g. 1206 means 0.125x0.060 inches.
There are some other differences, except for size.
Smaller resistors probably feature a lower power and voltage rating, but can have smaller parasitic components, ESL and parallel capacitance. I would not expect the parasitics to be an issue, just be sure to get one with the required power rating.
If you need to hand solder them, I suggest not going too small,e specially if you do not have any experience. 0603 should be fine for almost everyone, 0805 even more so, 1206 is a huge beast you can solder with your hands tied and your eyes closed.
As usual, if you have doubts always refer to the manufacturer datasheet.
They are different sizes.
1206 means 0.12 inches by 0.06 inches (3.2 x 1.6 mm). 0603 means 0.06 by 0.03 inches (1.6 by 0.8 mm). And so on. (Wikipedia)
The smaller ones will most likely have lower power and voltage ratings, because they are smaller.