Definitions of Technical Terms
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Time Constant

Time required for an exponential quantity to change by an amount equal to 63.2 percent of the total change that can occur. The interval required for a system or circuit to change a specified fraction from one state or condition to another.

The chart below provides a universal time constant chart for both RC and RL circuits. RC is Resistor / Capacitor circuits, and RL is Resistor / Inductor circuits. The Time Constant chart shows both the rise and fall times and can relate to either current or voltage, depending on where the measurement is taken.

Inductors resist a change in current, so the chart indicates a current change in an inductor. Capacitors on the other hand resist a change in voltage, so the chart shows the rising or falling of a voltage over a capacitor.

Both curves rise or fall at the same rate. So after 1 Time Constant [1TC] the voltage has increased to 63.2percent of the maximum, or decreased to 36.8 percent of the minimum value. Note that the larger graphs [links below] just round the numbers to 63 and 37 percent.

Rising and Falling Universal Time Constant Chart
Universal Time Constant Chart

A Time Constant [TC] could be any length of time, determined by the components in the circuit. However a time constant will always be one of two final voltage levels, depending on whether the voltage is falling or rising.

Increasing Curve

The rising curve 'A' depicts capacitor voltage on charge for an RC Circuit, or inductor current or resistor voltage on build-up for an LR Circuit. This link provides a larger view of a Rising Voltage TC Chart.

After 5 time constants the capacitor is considered charged, and most charts only show five time constants. However using the equation provided by the link any value may be calculated.

Decreasing Curve

The falling curve 'B' depicts a capacitor discharging from it's maximum stored voltage until zero volts. Or according to the graph; capacitor voltage on discharge or resistor voltage or capacitor current on charge or discharge for an RC circuit. The falling 'B' curve also shows an inductor current or resistor voltage on decay or inductor voltage on build-up or decay for an LR circuit. Another chart providing an enlarged view of a Falling Voltage TC Chart. More information may be found on the preceding linked charts; however with the Time Constant graphs you don't really need the calculations [the graph does all the work], unless you need an exact value.

Just like the increasing graph, after 5 time constants the capacitor is considered discharged, and the calculation will produce any value over time [a capacitor being used as the example].

Voltage Divider

Another way to look at the curves is as a simple voltage divider. If a voltage is applied to an RC network, see diagram to the right, and the capacitor voltage is rising, than the voltage across the resistor must be falling.

Rising TC followed by Falling Time Constant
Charging and Discharging

The point being that while these charts may show the voltage or current of one component in an RC or RL network, the voltage or current on the other component appears completely different. Using a capacitor as an example, while the voltage is increasing, the current through the circuit starts off high and than slowly drops off as the capacitor reaches the maximum voltage. On the other hand current through an LR network starts of low and slowly increases, causes the voltage across the resistor to do the same.

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