 both common mode and dif- quently the market for filters to block this interference is large ferential mode interference currents. and well established, with predictable performance needs. The common mode choke L consists of two identical windings on 2. There is an unfortunate tendency to add filtering as an after- a single high permeability, usually toroidal, core, configured in the thought, when it is discovered that equipment does not meet the circuit SO that differential (line to neutral) currents cancel each other. Figure 23.29 Typical mains filter circuit Filtering 23/23 This allows high inductance values, typically 1mH to 10mH, in a BS613, which specifies requirements for mains RFI filters in the small volume without fear of choke saturation caused by the mains UK, gives the maximum value for Y-configured capacitors for class frequency supply current. The full inductance of each winding is 1 appliances connected by a plug and socket as 0.005uF, and this available to attenuate common mode currents with respect to earth, value is frequently found in general purpose filters. The quality of but only the leakage inductance, which depends critically on the the components is also critical, since they are continuously exposed choke construction, will offer attenuation to differential mode inter- to mains voltage; failure of either C X or C could result in a fire ference. hazard, and failure of C, Y could also result in an electric shock Capacitors C. X1 and Cx X2 attenuate differential mode only but can hazard. Therefore only components which are rated for mains use have fairly high values, 0.1uF to 0.47uF being typical. Either may in these positions should be used. be omitted depending on the detailed performance required, remem- bering that the source and load impedances may be too low for the 23.14.3 Input/output filters capacitor to be useful. Capacitors CY1 and CY2 attenuate common mode interference and if Cx2 is large, have no significant effect on differential mode. In contrast to the mains filter, a filte