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50 Hz line filter

Hello, dear authors, journalists and readers!

I'll tell you about my homemade.


This is a 50 Hz mains filter.

How can it be useful.

Anyone who is involved in amplifiers, sound and measuring equipment, knows what the background of alternating current. Getting rid of it can be difficult.

As a basis, I took the double T bridge.

The principle of its operation and calculation of elements is quite complicated.
This is described in various sources, you can read if you wish.

With fine tuning, it can greatly weaken the noise, and also not introduce noise, since there are no semiconductors (transistors) in its circuit.

And does not require a power source. It has small dimensions.

This is a schematic diagram.


I pointed to homemade entry and exit. Although I suppose that they are reversible, I did not check, there was no need.

The following details were used:

Three identical capacitors KMBP and six resistors MLT 0.125 watts. In the diagram, I showed only three resistors, and six in the photo. This is due to the fact that I did not have exact values ​​and had to be selected using a digital multimeter by parallel connection.


Materials:

Solder, a small piece of unfolded textolite, some glue and a mounting wire and solder.

Instruments:

Soldering iron, wire cutters, scissors, tweezers and a scalpel.

When repeating the design, the following should be considered.

Capacitors can be taken KMPB, MBM and others, except ceramic and oxide (electrolytic). These types have a strong temperature dependence of capacity.

If possible, select the ratings of the parts using a digital instrument. From the selection accuracy depends on the suppression quality of the network pick-up of 50 Hz.

Or look at the percentage accuracy of the part rating. +/- 5% already gives good noise reduction quality.

On the graph, I showed the frequency response of the device. Of course, in practice it is somewhat smoother.


In conclusion, I will add. By changing the ratings of the parts, the filter can be made to suppress interference of a different frequency.

Sincerely, author.

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74 commentary
I repeat what the author asked:
Why make things worse if you can do better with almost the same amount of time and materials?
With the filter from the thermal relay, everything is clear: for some reason (either the dimensions did not allow shoving, or simply there were no capacitors of a larger capacity), the author resorted to the non-classical construction of the filter. Therefore, he, apparently, to compensate for the decrease in capacitance, he also reduced the value of the resistor. The fact that the filter will work is undoubtedly shown Nruter on their charts. But he will not work optimally, which is also evident from his schedules.
Why make things worse if you can do better with almost the same amount of time and materials? Or is the final version not important to you?
Gentlemen, I did not quite understand your dispute, what is at stake is about suppression tips, which should already be less than the useful signal, or the frequency of 50 Hz with equal to the useful signal level? In the first case, the suppression of 20 dB (10 times the level) can be considered "steep", in the second - very weak.
I agree with Nruter
And tell me where they put it, in what chains.
Author
New Standard, read what I wrote Ivan_Pokhmelev. And the scans attached. But the main scan from the Soviet book of 1964 with the secret denominations of the edements of the double T bridge, where it turns into an amplifier, I did not attach and will not attach. For now. The gain is small, of course, but this fact does not fit into the head. How can a circuit of passive components, without a power source, amplify?
Author
Ivan_Pokhmelev. I have a surprise for you. I found a Soviet book in my house, and there is a thermal relay circuit and at the entrance there is a T bridge filter at a frequency of 50 Hz. Do not try to convict me of plagiarism. Filter elements are in a different order and labeled differently. And the denominations are something like mine !! And capacitors, as you see, are all of the same nominal value of 0.5 mKF. This is your C2 on the advice of internal votes should be different. HA HA HA!
See 2 scans.
But that is not all. In another Soviet book of 1964, which I found yesterday, secret values ​​of the details of the double T bridge are given, in which it turns from a fence ... it turns into a circuit with a transmission coefficient of 1.2, that is, it amplifies the signal !!!

Calculations give: C1 = 388.38 nF, C2 = 2C1, R3 = 0.5R1! smile
Yes, and R2 should be 4.1 kOhm.
The correct filter should not press at 34 dB, but at 60 dB. Presumably due to the fact that C2 is half as much as in the classic filter.
In addition, given the use of not the best components, a fine tuning element is needed to accurately set the frequency, the easiest way is in the R2 circuit.
Author
New Standard. They are now attacking me that the C2 rating is incorrect. I do not want to disassemble (break) the filter. I carefully solder and measure this capacity. I’ll inform you about the results. What is there 0.25 uF should be on the advice of their internal votes or 1 uF? He cannot do anything else. We will find out.
Ivan_Pokhmelev But is he not tuned to the frequency of the notch, perhaps it is not clearly visible, for 50 Hz there is a wider coverage of frequencies?
Author
New Sdandart, he himself did not understand what he showed. 19 dB suppression at 40 Hz !!!!! This is a cool indicator. I have not yet selected the exact details; this is written in my article.
Quote: Ivan_Pokhmelev
in the band above the notch frequency, the characteristic is blocked. .
Looked closely at the characteristics. I overlooked that the frequency sweep is linear. ((Therefore, I remove the claims for the obstruction of the frequency response.But the suppression of the network frequency is too small.
Ivan_Pokhmelev As far as I understood from the graphs, it well suppresses exactly 50Hz, what was planned, but about C2 I almost understood it needs a larger capacity or something.
And where did you see "positivity"? In the band below the notch frequency, the characteristic is close to that of the correct 2T bridge (there should be a 10 dB attenuation at F / 2), but in the band above the notch frequency, the characteristic is overwhelmed. In addition, there is little attenuation at the notch frequency.
Nruter It’s hard to think of a more powerful and positive advertisement for the “50 Hz network filter” than those of your “5 black squares))
Quote: Ivan_Pokhmelev
an inner voice tells me that the C2 rating is wrong

The inner voice is right :).
I turned on Multisim and this is what I got. I apologize for the abundance of large pictures, but ...






Looking at the frequency response of this miracle filter, we can say that it is useless. At a frequency of 20 Hz, the suppression is already 7 dB. At 40 Hz, it’s already 19 dB.
Are you going to cut them something?
And tell me where they put it, in what chains.
Quote: R555
in the double T bridge, the suppression frequency can be changed with a variable resistor in the widest range!
In a real double T-bridge, a single resistor cannot change the frequency without significantly impairing the suppression. There is a scheme somewhat resembling a double T-bridge, but even there it doesn’t smell like the “widest range”; it works effectively in the range of about two octaves.
I pointed to homemade entry and exit. Although I suppose that they are reversible, I did not check, there was no need.
Why guess and check? Everything is visible according to the scheme.
In addition, an inner voice tells me that the C2 rating is incorrect.
Author
Korolev, I just found a 1964 book. So, it shows how, it turns out, in a double T bridge it is possible to change the suppression frequency with a variable resistor in the widest range! What is not a reason for my new homemade. A ?
Author
Act, I respect Om Georg, one of my idols! You, apparently, that the input of the load will feed the constant? Yes ? So if it has an input resistor to ground. And if not, it won’t.
R555
if you ask about the constant, then no
I’m embarrassed to ask, but Ohm’s laws in your homemade product no longer apply? scratch
Author
Korolev, if you ask about the constant, then no, it goes through the series-connected resistors R1 and R3. So even infrasound and direct current will pass. If we don’t need it, before or after the filter we put a capacitor and all things.
And what is the voltage drop across the filter? scratch

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