9.3 The Universal Time Constant Chart

This is the third week of Module 9, "Relationships of Current, Counter EMF, and Voltage in LR Circuits." These modules are part of the Navy Basic Electricity and Electronics series from the 1970s. The third section of this module is titled, "Using the Universal Time Constant Chart."

9.1 Rise and Decay of Current and Voltage
9.2 LR Time Constant

The so called universal time chart gives the percentages of maximum I (current) or E (voltage) for each of the five time constants it takes for a circuit with an inductor in it to reach the maximum when powering up or to reach zero if powering down. The chart is to the right.

The time constant, as we learned in the previous section, is calculated by dividing the inductance by the resistance. So you can see on the chart that TC1, TC2, TC3, and so forth are expressed as L/R, 2L/R, 3L/R and so forth.

The line that starts at zero and increases to 100% is the powering up line. So at one TC (time constant), the voltage or current will be at 63.2% of its maximum value. At T2, it will have reached 86.5% of its maximum value, and so forth.

The line that starts at 100% and decreases to 0 is the counter-EMF coming from the inductor as the circuit powers down (E_L). It has the percentages opposite the percentages on curve A. So at one time constant interval, the circuit will still have 36.8% of what its maximum voltage was. At T2, voltage will be down to 13.5%. At T3 it will be at 5%. At T4 you will have 2% left. Then final at T5 the circuit will have completely powered down.