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

#### sprabhujh

Joined Mar 29, 2017
13
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?

#### Papabravo

Joined Feb 24, 2006
13,702
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.

• sprabhujh

#### Dodgydave

Joined Jun 22, 2012
9,156
5mH inductor @ 50Hz is 1.57 ohms...

• sprabhujh

Joined Jul 18, 2013
20,652
Also see:
Max.

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• atferrari and sprabhujh

#### BR-549

Joined Sep 22, 2013
4,938
Where is the isolation transformer?

#### sprabhujh

Joined Mar 29, 2017
13
Also see:
Max.

Thank you very much. You exactly gave me what i'm looking for.From your attached file,i understand it as filter.But may i know why inductor is in paralled with a resistor & what is the type of filter?

Joined Jul 18, 2013
20,652
A Low pass filter, I don't see the resistor having much effect apart from lowering the Q.
Max.

• sprabhujh

#### Papabravo

Joined Feb 24, 2006
13,702
A Low pass filter, I don't see the resistor having much effect apart from lowering the Q.
Max.
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?

$$\frac{1}{R_T}=\frac{1}{R}+\frac{1}{10R}$$
$$R_T=\frac{10R^2}{R+10R}=\frac{10}{11}R$$

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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• sprabhujh

Joined Jul 18, 2013
20,652
Across the line circuit and 5.1k? No real significance from what I can see?
Max.

#### DickCappels

Joined Aug 21, 2008
6,379
It is probably there to damp ringing from transients from the power line or current spikes from the power supply itself. Probably needed to pass conducted EMI tests.

• sprabhujh and John88

#### John88

Joined Aug 15, 2017
1