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A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors. It may be driven by a voltage or current source and these will produce different responses. A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit.
The RC time constant, denoted τ (lowercase tau), the time constant (in seconds) of a resistor–capacitor circuit (RC circuit), is equal to the product of the circuit resistance (in ohms) and the circuit capacitance (in farads):
Similarly, in an RC circuit composed of a single resistor and capacitor, the time constant (in seconds) is: = where R is the resistance (in ohms ) and C is the capacitance (in farads ). Electrical circuits are often more complex than these examples, and may exhibit multiple time constants (See Step response and Pole splitting for some examples.)
Figure 1: Simple RC circuit and auxiliary circuits to find time constants. Figure 1 shows a simple RC low-pass filter. Its transfer function is found using Kirchhoff's current law as follows. At the output, = , where V 1 is the voltage at the top of capacitor C 1
Since every circuit has not only resistance, but also capacitance and inductance, a delay in voltage and/or current at the load is apparent until the steady state is reached. In a pure RC circuit, the output risetime (10% to 90%) is approximately equal to 2.2 RC. [10]
The product τ (tau) = RC is called the time constant of the circuit. The ratio then depends on frequency, in this case decreasing as frequency increases. This circuit is, in fact, a basic (first-order) low-pass filter. The ratio contains an imaginary number, and actually contains both the amplitude and phase shift information of the filter.
First-order RC filter low-pass filter circuit. Roll-off of a first-order low-pass filter is 20 dB/decade (≈6 dB/octave) A simple first-order network such as a RC circuit will have a roll-off of 20 dB/decade. This is a little over 6 dB/octave and is the more usual description given for this roll-off.
Figure 1: Schematic of an electrical circuit illustrating current division. Notation R T refers to the total resistance of the circuit to the right of resistor R X.. In electronics, a current divider is a simple linear circuit that produces an output current (I X) that is a fraction of its input current (I T).