Simple AC circuit calculations
Over the course of the next few chapters,
you will learn that AC circuit measurements and calculations can get
very complicated due to the complex nature of alternating current in
circuits with inductance and capacitance. However, with simple circuits
involving nothing more than an AC power source and resistance, the same
laws and rules of DC apply simply and directly.
Series resistances still add, parallel
resistances still diminish, and the Laws of Kirchhoff and Ohm still hold
true. Actually, as we will discover later on, these rules and laws
always hold true, it's just that we have to express the quantities
of voltage, current, and opposition to current in more advanced
mathematical forms. With purely resistive circuits, however, these
complexities of AC are of no practical consequence, and so we can treat
the numbers as though we were dealing with simple DC quantities.
Because all these mathematical
relationships still hold true, we can make use of our familiar "table"
method of organizing circuit values just as with DC:
One major caveat needs to be given here:
all measurements of AC voltage and current must be expressed in the same
terms (peak, peak-to-peak, average, or RMS). If the source voltage is
given in peak AC volts, then all currents and voltages subsequently
calculated are cast in terms of peak units. If the source voltage is
given in AC RMS volts, then all calculated currents and voltages are
cast in AC RMS units as well. This holds true for any calculation
based on Ohm's Laws, Kirchhoff's Laws, etc. Unless otherwise stated, all
values of voltage and current in AC circuits are generally assumed to be
RMS rather than peak, average, or peak-to-peak. In some areas of
electronics, peak measurements are assumed, but in most applications
(especially industrial electronics) the assumption is RMS.
- REVIEW:
- All the old rules and laws of DC (Kirchhoff's
Voltage and Current Laws, Ohm's Law) still hold true for AC. However,
with more complex circuits, we may need to represent the AC quantities
in more complex form. More on this later, I promise!
- The "table" method of organizing
circuit values is still a valid analysis tool for AC circuits.
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