in the circuit of fig2 an inductor 32 is connected in parallel with the ntc resistor the capacitor resistor pair form a series damping filter for suppressing the conducted emissions using switching frequencies of the order of 600 hz 16 khz emissions are generated by the power electronic circuits as described the ntc resistor in the filter is self protecting as the current flowing to earth through the filtering capacitor increases the ntc resistor heats up and the resistance of the ntc resistor decreases thereby enabling it to stay within its power rating as the magnitude of the noise currents and emissions increases the equivalent fixed resistor in the current circuit would have to be much larger and therefore more expensive if it is to be sufficient to cope with the anticipated power the inductance is small u2014 of the order of micro henrys u2014 across the ntc resistor it acts as a shunt to divert high frequency current to reduce the power dissipation in the ntc resistor this does not directly affect the conducted emissions but it does assist by damping the resonance when using an ntc resistor with a lower current rating and or power rating this enables the ntc resistor to be smaller in size and cost than would otherwise be the case fig3 6 illustrate alternative locations for the ntc damped filter circuit as can be seen from fig3 it can be installed on either of the dc rails it can also be installed on the ac side of the rectifier shown in fig4 6 in each case the ntc resistor is preferably connected in series with at least one capacitor per rail to which the ntc is connected in the circuit of fig4 the supply.