inductances cancels out large dc bias currents if both conductors carrying a given load current pass through the choke in the same direction noise currents enter the welding machine as on all conductors in an equal amount this is referred to as common mode noise a common mode choke blocks the common mode noise currents while allowing the differential signal to pass unimpeded a typical common mode choke will present impedance of hundreds of ohms to the common mode noise while inserting only a few ohms to the differential signal output choke or inductances 112 is shown in detail in fig2 and includes three j type ferrite toroids 201 u2013 203 with two turns of each weld cable 205 and 206 through them cables 205 and 206 are connected at one end to output circuit 103 and to the output studs of power supply 100 at the other end the turns are placed in such a way that the flux produced by the weld current cancels each other the output common mode choke has a 66 uh common mode inductance and a 0 42 uh differential inductance the differential inductance has no effect on the welding current waveform the common mode inductance has minimum impedance of 820 ohms at 2 mhz other embodiments provide for a common mode inductance of at least 60 microhenrys or at least 30 microhenrys and a differential mode inductance of no more than 0 5 microhenrys or no more than 1 microhenry or for a ration of.