How to Size Resistors


Resistors oppose the flow of free electrons in a circuit. Resistors may have fixed values, or they may be variable. Resistors are rated in ohm, kilohms (1 kilohm equals 1,000 ohms) or megohms (1 megohm equals 1 million ohms), according to the amount of opposition to current flow they offer. Resistors also have a wattage rating. Resistance to the flow of electric current, like mechanical friction, generates heat and the resistor's wattage rating indicates how much heat the resistor can safely handle.

To size a resistor, you must know how much voltage the resistor is required to drop across it. You also must know the current flowing through it in amperes, milliamperes (1 milliampere equals 0.001 amperes) or micro-amperes (1 micro-ampere equals 0.000001 amperes.)

Things You'll Need

  • Pocket calculator
  • Pen
  • Paper
  • Resistor standard values charts
  • 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.

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