Abstract: A controlled current supply circuit for semiconductor logic circuitry, such as I.sup.2 L, which has an unscaled part (10) which remains continuously active and a scaled part (12) which, when not in a data processing mode requiring a high current can be operated at a much lower curent so that the data therein is preserved and power is saved. Unless switching between the low and high currents is carefully controlled then there is a risk of data being lost. A curernt supply circuit (14) responds to a data input signal of the unscaled logic to cause the current supply to the scaled logic (12) to increase or decrease in a number of steps, there being a sufficiently long time delay between steps to allow the scaled logic to settle. The rate of increase or decrease of the injected current is such that there are smaller increments or decrements when the overall current is low causing the scaled logic to be slow and larger increments and decrements when the overall current is high causing the scaled logic to be fast.