Calculate the resistance for a series voltage-dropping resistor when the required voltage drop is 6 volts and the current flowing through it is 0.25 amperes. The Ohm's Law formula is R = E/I where \"R\" equals the resistance in ohms, \"E\" equals volts and \"I\" equals amperes. This gives R = 6/0.25 = 24 ohms.
Compare this computed value against the standard values given in the Electronics Industry Association (EIA) Standard Resistor Value Table. You will see that 24 ohms is a standard value. Choose that value.
Compute the required wattage rating for this resistor. The Ohm's Law power formulas give two formulas to choose from for this calculation. You may choose P = I * E or P = I^2 * R where \"P\" equals the power in watts, \"E\" equal the voltage drop (VD) and \"R\" equals the resistance in ohms.
The first formula will be the easiest to use, because it is simple multiplication. P = E * I = 6 X 0.25 = 1.5 watts.
The second formula produces the same results. P = I^2 * R = 0.25 X 0.25 X 24 = 0.0625 X 24 = 1.5 watts.
Use a 24-ohm (?) 2-watt (W) resistor for this circuit.