LED circuit with capacitor and variable resistor · CAP 2013 #26
Physics · electromagnetism
Problem
A light emitting diode (LED) is connected in series with a super capacitor of capacity 3000 F and internal resistance ∼ 1 mΩ, and a microprocessor-controlled resistor. The capacitor is charged to 5 V. The LED requires constant current of 800 mA. At this current, the voltage drop over the LED is 2.7 V.
a) What should be the dependence of the variable resistor on time for the current to remain constant?
b) What should be the initial value of the resistor? Does the value of the resistor stabilize after some time, and if so, to which resistance?
c) How long can the LED operate at full brightness before the capacitor needs to be recharged?