Now suppose that the voltage bias abruptly changes, switching from its stationary positive value to a higher magnitude constant negative value: then, since a certain amount of charge has been stored during forward conduction, diode resistance is still low (i.e. the anode-to-cathode voltage VAK has nearly the same forward conduction value). Anode current does not cease but reverses its polarity (i.e. the direction of its flow) and stored charge Qs starts to flow out of the device at an almost constant rate IR. All the stored charge is thus removed in a certain amount of time: this time is the storage time tS and its approximate expression is