, ▁as ▁well ▁as ▁other ▁communic ations ▁equipment ▁and ▁industrial ▁equipment . ▁Early ▁var ic ap ▁di odes ▁usually ▁required ▁a ▁reverse ▁voltage ▁range ▁of ▁ 0 – 3 3 ▁V ▁to ▁obtain ▁their ▁full ▁capac it ance ▁ranges , ▁which ▁were ▁still ▁quite ▁small , ▁approximately ▁ 1 – 1 0 ▁p F . ▁These ▁types ▁were ▁– ▁and ▁still ▁are ▁– ▁extens ively ▁used ▁in ▁television ▁tun ers , ▁whose ▁high ▁car rier ▁frequencies ▁require ▁only ▁small ▁changes ▁in ▁capac it ance . ▁ ▁In ▁time , ▁var ic ap ▁di odes ▁were ▁developed ▁which ▁exhib ited ▁large ▁capac it ance ▁ranges , ▁ 1 0 0 – 5 0 0   p F , ▁with ▁relatively ▁small ▁changes ▁in ▁reverse ▁bias : ▁ 0 – 5   V ▁or ▁ 0 – 1 2   V . ▁These ▁newer ▁devices ▁allow ▁electron ically ▁tun ed ▁AM ▁broadcast ▁rece ivers ▁to ▁be ▁realized ▁as ▁well ▁as ▁a ▁mult itude ▁of ▁other ▁functions ▁requiring ▁large ▁capac it ance ▁changes ▁at ▁lower ▁frequencies , ▁generally ▁below ▁ 1 0   M Hz . ▁Some ▁designs ▁of ▁electronic ▁security ▁tag ▁readers ▁used ▁in ▁ret ail ▁out lets ▁require ▁these ▁high ▁capac it ance ▁var ic aps ▁in ▁their ▁voltage - control led ▁oscill ators . ▁ ▁The ▁three ▁lead ed ▁devices ▁dep icted ▁at ▁the ▁top ▁of ▁the ▁page ▁are ▁generally ▁two ▁common ▁c ath ode ▁connected ▁var ic aps ▁in ▁a ▁single ▁package . ▁In ▁the ▁consumer ▁AM / FM ▁tun er ▁dep