Low pass filter for 60Hz rejection

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Discussion Overview

The discussion revolves around designing a low-pass filter (LPF) using inductors and capacitors to reject a 60Hz frequency from an incoming AC mains signal. Participants explore the feasibility of achieving a DC-like output from this setup, while also considering alternative methods such as rectification and notch filtering.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant seeks to design an LC low-pass filter to eliminate the 60Hz frequency from an AC signal, aiming for an output close to DC.
  • Others argue that the mains voltage ideally has no DC component, suggesting that a low-pass filter alone would not achieve the desired outcome.
  • Some participants propose using a rectifier instead of a filter to convert AC to DC, emphasizing the importance of safety when working with mains voltage.
  • A suggestion is made to consider a notch filter to specifically target the 60Hz frequency without affecting higher frequencies.
  • One participant mentions the use of a passive twin-T notch filter as a traditional method for rejecting specific frequencies.
  • Concerns are raised about the safety of connecting homemade circuits to mains electricity, with multiple warnings about the dangers involved.
  • Questions arise regarding the purpose of removing the 60Hz frequency and what the participant intends to do with the resulting signal.
  • Some participants discuss the implications of filtering out the 60Hz component, noting that it could leave no signal at all if done ideally.
  • There are inquiries about the calculation of component values for the LC filter and the voltage output after filtering.
  • One participant emphasizes the need for a transformer to safely step down the voltage before connecting to a circuit.

Areas of Agreement / Disagreement

Participants express multiple competing views on the effectiveness of using a low-pass filter versus a rectifier for the intended purpose. There is no consensus on the best approach, and safety concerns regarding working with mains electricity are widely acknowledged.

Contextual Notes

Participants highlight the importance of understanding the hazards of mains electricity and the need for proper safety measures when designing circuits. There are unresolved questions about the specific calculations needed for the filter design and the implications of removing the 60Hz frequency.

Who May Find This Useful

This discussion may be useful for individuals interested in circuit design, particularly those exploring methods for filtering AC signals and the associated safety considerations in working with mains electricity.

pjshah72
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Hi All,
I am looking to design LPF with Inductor and Capacitor that can reject 60Hz frequency.
I tried on google about the same but did not get an idea.

Basically, I am trying to do is this, Incoming mains (AC Signal) has 60Hz frequency and I wanted to make it like a DC (close to DC) which has no frequency.

Can I do with LC-LPF?
Any help and advise will be greatly appreciated.

Thank you!
 
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The mains voltage has no DC-component (ideally), so you won't get anything from a low-pass filter. What you're looking for is a rectifier:
http://en.wikipedia.org/wiki/Rectifier

The diode-bridge is very common:
http://en.wikipedia.org/wiki/Diode_bridge

You shouldn't be messing with mains voltage if you have no experience with high-voltage applications. It's an easy way to get yourself killed.
 
milesyoung said:
The mains voltage has no DC-component (ideally), so you won't get anything from a low-pass filter. What you're looking for is a rectifier:
http://en.wikipedia.org/wiki/Rectifier

The diode-bridge is very common:
http://en.wikipedia.org/wiki/Diode_bridge

You shouldn't be messing with mains voltage if you have no experience with high-voltage applications. It's an easy way to get yourself killed.

Rectifier can convert AC into DC but I am not looking for that. In other words, I am not looking to convert AC into DC by LPF.

I want to make filter that gives me close to zero frequency when I applied AC (mains).
I want to chop AC frequency to none (close to zero).

Thank you!
 
Let's think the other way. How can I make LC-LPF filter that can reject 60Hz? (not considering this for AC signal.) Assume this is a frequency of some signal.
 
pjshah72 said:
Let's think the other way. How can I make LC-LPF filter that can reject 60Hz? (not considering this for AC signal.) Assume this is a frequency of some signal.
You could use a low-pass filter with a suitable break frequency to attenuate your 60 Hz component, but that would impact everything above 60 Hz aswell. Have you had a look at a notch filter?
http://en.wikipedia.org/wiki/Band-stop_filter

You can find tonnes and tonnes of practical guides to building them with a search on Google.
 
One traditional and simple way is with a passive twin-T notch filter.
http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm
You will have to build it using high voltage capacitors and resistors.
When the power turns on or off there will be a big spike on the output.
The signal that passes will be the harmonics of 60Hz, synchronous noise from rectifiers and any signalling on the line.
 
pjshah72 said:
Basically, I am trying to do is this, Incoming mains (AC Signal) has 60Hz frequency and I wanted to make it like a DC (close to DC) which has no frequency.
What are you going to do with the signal once you have removed the 60Hz from it?
 
NascentOxygen said:
What are you going to do with the signal once you have removed the 60Hz from it?

I am very new in this field (2nd year) so I don't know much. If I will remove 60Hz successfully then I will feed to Voltage regulator and try to get some specific voltage out of it. (for ex. 12V out of voltage regulator.)

Thank you!
 
pjshah72. You are building a 12V DC linear power supply. You will need a power transformer, a bridge rectifier and a storage capacitor. You do not need a 60Hz filter, the storage capacitor will do that.
The rectified secondary voltage will have a frequency of 120Hz.
 
  • #10
Baluncore said:
pjshah72. You are building a 12V DC linear power supply. You will need a power transformer, a bridge rectifier and a storage capacitor. You do not need a 60Hz filter, the storage capacitor will do that.
The rectified secondary voltage will have a frequency of 120Hz.

Hi Baluncore,
Thank you for the input. I wanted to try the other way. With transformer and bridge rectifier with storage capacitor is common method. I want to try only.
 
  • #11
pjshah72 said:
I am very new in this field (2nd year) so I don't know much. If I will remove 60Hz successfully then I will feed to Voltage regulator and try to get some specific voltage out of it. (for ex. 12V out of voltage regulator.)

Thank you!

If you remove the 60Hz component of the signal coming from the mains you won't have any signal left at all. (Ideally, of course)

If your 60Hz signal had some sort of DC bias imposed on it (which it currently doesn't), then you could filter out the 60Hz portion and just extract only the DC bias at the output of the filter.

Like many others have suggested, in order to achieve what you've described you'd want to use is a rectifier circuit, not a filter.
 
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  • #12
Got it! Thank you all for the responses.
How to calculate voltage that comes after this filter circuit? (Input is AC mains.)
I am looking for formula or method.

Thank you!
 
  • #13
To generate DC from an AC source you must perform a multiplication such as a square. A rectifier diode is non-linear and so it generates the DC offset you need.

If you full wave rectify the 60Hz AC you will have a signal with a fundamental at 120Hz but with a DC offset.

Now imagine you used an active low pass filter to remove that 120 Hz and another for each of it's significant harmonics. The power available at the output of the active filter would be derived entirely from the power supplies of the active filter. This defeats the purpose.

By using passive resonant LC notch filters for the 120Hz fundamental and most significant harmonics, then followed that by an LC low pass filter you would have built a smooth DC power supply. Only asynchronous noise would reach your DC supply rail.
 
  • #14
Can anyone suggest value for L and C to reject 60Hz frequency?
I did google but all is about only diagram.
 
  • #16
pjshah72 said:
How to calculate voltage that comes after this filter circuit? (Input is AC mains.)
I am looking for formula or method.
You are looking for trouble!

Never connect a circuit you build to the mains, it is dangerous and what you have in mind won't do what you think it will.

You need to study a lot more before you understand the hazards of mains electricity and how circuits work.
 
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  • #17
pjshah72 said:
Got it! Thank you all for the responses.
How to calculate voltage that comes after this filter circuit? (Input is AC mains.)
I am looking for formula or method.

bold by me

I totally agree with

NascentOxygen said:
You are looking for trouble!

Never connect a circuit you build to the mains, it is dangerous and what you have in mind won't do what you think it will.

You need to study a lot more before you understand the hazards of mains electricity and how circuits work.

Wouldn't it be better to understand how it's done in the real world instead of killing yourself? Do you know how a bridge rectifier works? If not, it's fairly simple as this image shows:

animation_bridge_rectifier_nonideal_fast.gif


Image compliments of http://educypedia.karadimov.info/electronics/javaanalogsemipassif.htm
 
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  • #18
NascentOxygen said:
You are looking for trouble!

Never connect a circuit you build to the mains, it is dangerous and what you have in mind won't do what you think it will.

You need to study a lot more before you understand the hazards of mains electricity and how circuits work.

What is the safe way? Can I put transformer between AC and circuit so it will step down voltage?
 
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  • #19
Thanks for the image!
 
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  • #20
pjshah72 said:
What is the safe way? Can I put transformer between AC and circuit so it will step down voltage?

yes that's the safe way


Dave
 
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  • #21
I'll put it a different way (this hurts my head). The AC mains have 1 frequency: 60 HZ. If you filter that out (which is not advised) you have no voltage of any kind. Zero. Nada. It does not magically appear as some other frequency or voltage. Do you understand that?
 
  • #22
You would have to dissipate the energy while dropping the line frequency from 60 Hz to 0 Hz.

Here's some energy being dissipated.


https://www.youtube.com/watch?v=4bBvmPRqfmo
 
  • #23
hey Don

That's pretty horrific, huh


Dave
 
  • #24
pjshah72 said:
What is the safe way? Can I put transformer between AC and circuit so it will step down voltage?
Even that is not safe, until you understand more about the circuit you connect it to. You are considering using an inductor in your filter. Inductors can generate a high voltage by themselves, and in conjunction with a capacitor, can produce a lethal voltage. All that is needed is a slight rearrangement of connections and you could end up with hundreds of volts where you thought there would be only 10volts. Keep studying and doing the experiments that your course provides for you, they should at least be safe!

Always, caution!
 
  • #25
Thank you everyone for alert!
i will study and learn before practical!
 
  • #26
Everyone else has said what I'm about to say in different ways. Maybe this will strike you a bit differently.

The AC mains are 60Hz AC and have an average voltage of 0. That is, there is no DC. When you filter out the AC, there is nothing left. Your filter blocks all the AC. It doesn't transform it. Filters are linear.

Rectifiers are non-linear, so they can transform it.

If you put AC through a full-wave rectifier, you have now created a non-zero average voltage equal to the Peak voltage times 0.707. Now, if you filter out 60Hz, you would have DC remaining.

Linear power supplies go through a transformer and are rectified and then filtered and possibly regulated. Switching power supplies do a bit more.
 
  • #27
can you explain for SMPS in detail too?
Thank you for the explanation!
 
  • #28
meBigGuy said:
If you put AC through a full-wave rectifier, you have now created a non-zero average voltage equal to the Peak voltage times 0.707. Now, if you filter out 60Hz, you would have DC remaining.
Not quite.
A full-wave rectifier is a frequency doubler. There would be no 60Hz fundamental to remove with a notch filter.

A full-wave rectifier would generate a DC offset, with a 120Hz fundamental and many harmonics of 120Hz. If you removed the 120Hz fundamental you would be left with DC plus many even harmonics of the missing 120Hz fundamental.

A half-wave rectifier would produce a DC offset, with a 60Hz fundamental and many harmonics.
 

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  • #29
@Baluncore

You are absolutely correct of course. I was intentially being overly simplistic and ventured into WRONG. I didn't want to get into power supply filters, And I added the full wave as a simple correction after noticing that .707 was wrong for half wave.

@pjshah72

Use wikipedia and google to explore SMPS. If you then have specific questions, come back. But first, explore transformer based linear power supplies ( full and half wave) and center-tapped transformers. Find out about the output voltage, ripple, and load characteristics of the different supply architectures, and the different filtering methods.
 
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