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Ohm's Law and Power

Schematic for use with Ohm's Law calculator.

    Given two of the four values for resistance (R), voltage (V), current (I), and power (P), this solver calculates the remaining two values.  When dealing with AC, use the RMS voltage or RMS current. 

    For reference, the equation shown in the following figure can be used to determine how the values are calculated.  To use it, select the quantity you want to find from the inner circle to find the three equations located within its quadrant that yield the selected quantity.

Ohm's Law wheel to quickly find the needed equation.

    It is worth noting that Ohm’s Law is useful for finding values in more than just resistors. For instance, suppose you have a three terminal voltage regulator as shown in the following figure. 

    Knowing that the adjustment pin (the one attached to ground) has very little current flowing out of it and can therefore be ignored, you can determine the amount of power dissipated by the regulator.  The voltage across the regulator (the difference between the unregulated and regulated voltage) and the current flowing through it (the current used by the load) can be used to determine the power dissipated by the regulator.  This information is useful in determining if a heat sink is needed and if so, how large it should be.

Schematic showing how to extend Ohm's Law to less obvious situations.

    In general, Ohm’s law can be used in any DC circuit with little hesitation.  This even applies to devices such as motors.  In the case of a motor, part of the power comes out as heat due to motor efficiency and part as mechanical power.

    In AC circuits some caution is in order.  When dealing with reactive components (inductor and capacitors), or other devices that cause a phase shift between the  voltage and current, Ohm’s Law cannot be applied directly.  The phase shift between the voltage and the current must be accounted for and treating it like a DC circuit will most probably lead to wildly inaccurate results.

copyright © 2021 John Miskimins


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