Hi,
As others have mentioned you need a resistor that can handle the power. I meant to say something about this but i had to do something yesterday and could not get back soon enough. Story about this nutty adventure at a later time
Anyway, a resistor has a Ohmic value like 100 Ohms, but it also has another 'rating' or rather specification, and that is the power handling capability. The resistor must be able to handle the power developed in it due to the voltage across it or the current through it.
Knowing the voltage across it we get the power from:
P=V^2/R
where V is the voltage in volts and R is the resistance in Ohms.
Knowing the current through the resistor we get the power from:
P=I^2*R
where I is the current through it in Amperes and R is the resistance in Ohms.
So you can use either of these, and it is quite easy to do.
For example, say you have 10 LEDs in series each 3.2 volts. That's ten times 3.2 equals 32 volts. With a resistor of 100 Ohms, the power would be:
P=32^2/100=1024/100=10.24 watts.
To get a more safe value for the power we would double this, and that would mean we'd need a 20 watt resistor at 100 Ohms.
Doubling the power rating means we get a resistor that runs cooler, but going even higher like 4 times the rating means an even cooler running resistor. Of course this assumes that the resistor gets fresh air through vent holes in the case of the unit it is installed inside of. If there is no fresh air supply then other means have to be employed such as heatsinking to the side panel or something like that.