Something else ive just thought of. Since im using an AC PSU and a rectifier i need a capacitor. Most of the solenoids are fine but the flipper solenoids buzz a bit because of DC ripple. In this case (or for any other circuit) how do i know what capacitor to use? Ive been told that 24uf is enough but i dont know where that figure came from. Is there a name for this type of equation?