Since the first revision of the ATX standard, PSUs were required to have a 3.3 V output voltage rail. Rarely, a linear regulator generated these 3.3 V, supplied from the 5 V and converting the product of voltage drop and current to heat. In the most common design this voltage is generated by shifting and transforming the pulses of the 5 V rail on an additional choke, causing the voltage to rise delayed and rectified separately into a dedicated 3.3 V rail and getting the rising idle voltage cut by a device type TL431, which behaves similar to a zener diode. Later regulators managed all the 3.3, 5 and 12 V rails. Cutting the pulse by the voltage regulator the ratio of the 3.3 and 5 V is controlled. Some of these PSUs use two different chokes, feeding to the 3.3 V rail from the transformer to manage changing loads by pulse with ratio between the 3.3 and the 5 V outputs. In designs using identical chokes, the pulse width manages the ratio.