When a more constant output voltage is desired, the Joule Thief can be given a closed-loop control. In the example circuit, theSchottky diode D1 blocks the charge built up on capacitor C1 from flowing back to the switching transistor Q1 when it is turned on. A 5.6 Volt Zener diode D2 and transistor Q2 forms the feedback control: when the voltage across the capacitor C1 is higher than the threshold voltage formed by Zener voltage of D1 plus the base-emitter turn-on voltage of transistor Q2, transistor Q2 is turned on diverting the base current of the switching transistor Q1, impeding the oscillation and prevents the voltage across capacitor C1 from raising even further. When the voltage across C1 drops below the threshold voltage Q2 turns off, allowing the oscillation to happen again. If the load requires even lower ripple, in this example some delicate digital circuitry like a microcontroller, a linear regulator can be used after this to smooth the ripple out.