what should be the value of resistor for a current rating of 300mA?

Well E from I..The wattage is a range because you didn't give a voltage (which you didn't know) but here is some examples.

converting mA to Amps its .0003 amp..so if the voltage was 12v...036watt now in general we work with divisions for larger amperage resistors in 1/8watt increments so 1/8 watt =.125 sac o you can see that the heat dissipated by even a 1/8 watt would more than cover your voltage ..Next extreme would be higher voltage Vm=300vac ,,,P= 300 x .0003=.09 watt so a selection of a 1 or even a 2 watt would do well. here is a list of Pc values

Rectangular passive components values I got from Wikipedia....

01005 (0402 metric) : 0.016" × 0.008" (0.4 mm × 0.2 mm) Typical power rating for resistors 1/32 watt

0201 (0603 metric) : 0.024" × 0.012" (0.6 mm × 0.3 mm) Typical power rating for resistors 1/20 watt

0402 (1005 metric) : 0.04" × 0.02" (1.0 mm × 0.5 mm) Typical power rating for resistors 1/16 watt

0603 (1608 metric) : 0.063" × 0.031" (1.6 mm × 0.8 mm) Typical power rating for resistors 1/16 watt

0805 (2012 metric) : 0.08" × 0.05" (2.0 mm × 1.25 mm) Typical power rating for resistors 1/10 or 1/8 watt

1206 (3216 metric) : 0.126" × 0.063" (3.2 mm × 1.6 mm) Typical power rating for resistors 1/4 watt

1210 (3225 metric) : 0.126" × 0.1" (3.2 mm × 2.5 mm) Typical power rating for resistors 1/2 watt

1806 (4516 metric) : 0.177" × 0.063" (4.5 mm × 1.6 mm)

1812 (4532 metric) : 0.18" × 0.12" (4.5 mm × 3.2 mm) Typical power rating for resistors 1/2 watt

2010 (5025 metric) : 0.2" × 0.1" (5.0 mm × 2.5 mm) Typical power rating for resistors 1/2 watt

2512 (6332 metric) : 0.25" × 0.12" (6.35 mm × 3.0 mm) Typical power rating for resistors 1 watt

theses are passive resistors which are on a Pc board and can be observed to have the value on the top in a numerical value..

the formula for finding the ranges is P=I x E there are other manipulations of this simple layout but for the question you asked this will do...set up a table and solve for the minimum you expect and the max...If .3 is the largest by ALL 1/2watt etc...you can't go wrong getting something larger...

Have a good one from the E.. ..

Depends on what voltage you need, just apply ohms law.

If you want 3 volts corresponding to 300 mA, that is a resistor of 10 ohms.

100 ohms would give you 30 volts

1 ohm would give you 300 mV

This assumes that your current of 0 to 300 mA is not dependent on voltage. In practice, unless you have a regulated current source, it will be, so you should use as low a value of resistance that you can use and still get a voltage range you need.

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the easily forward voltage of each and every l.e.d. won't inevitably be precisely 2.2 on the pronounced modern-day (in spite of that would desire to be). The sum of the forward voltages would be approximately 11, so the voltage around the sequence resistor would be 2.8 to 3 while the engine is working (it rather is a 12V gadget?) Your parent of 300mA seems a techniques too intense. 30 is extra life like or perhaps this is greater than i'd assume. in spite of the indisputable fact that, calculating for 30mA ability a resistor of a hundred ohms is needed. the potential in a hundred ohms at 30mA is decrease than 100mW, so a 1W resistor would be better than sufficient.

The equipment you are using may be having different value. If you know voltage just divide current by volatge to get resistance value.

And if you don't have that, the last option is this, Use multimeter to know all values.

Sir: What would be the voltage that you are using to ''drive'' a current of 300mA.

I would advise you to use Ohm's Law to make these calculations.