What is the use of inductor across line and how to find needed inductance value?

Thread Starter


Joined Mar 29, 2017
AC supply = 230V,50Hz at input.
L- line, N-neutral
RV1 = MOV 7d471K
C1 = 104K,275V x rated capacitor
R1 = 5.1Kohm, +/- 1%, SMD 1206 package
DL1 = 6x8 Drum core inductor 5mh ?
B1 = DB107 bridge rectifier 1000v 1A
RV2 = Mov 7d471K

The above is the specification.I want to know what is the use of inductor and how to find the value for it?
Please find the attached image.
Thanks in advance.



Joined Feb 24, 2006
You can answer your own question by calculating the reactance of the inductor at a couple of frequencies. Try 5, 50, 500, 5000, and 50,000 hz. What pattern do you see? That is your answer.


Joined Feb 24, 2006
A Low pass filter, I don't see the resistor having much effect apart from lowering the Q.
The resistor is in parallel with the IR drop in the inductor. The resistance of two resistances in parallel is lower than either one of them alone. Ask yourself: what is the resistance of R and 10R in parallel?


Also, you need to convince yourself that the inductor is in SERIES with the AC mains. It is not across them since the bridge rectifier is changing things. The filtering actions is provided by a series inductor and a shunt capacitor. While you are at it you might want to compute the impedance of that capacitor for the same values of frequency that you computed for the inductor. The pattern you should see is that the capacitor becomes more of a "short" circuit at higher frequencies.
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