# Low pass filter for 60Hz rejection

1. Aug 14, 2013

### pjshah72

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!!!

2. Aug 14, 2013

### milesyoung

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.

3. Aug 14, 2013

### pjshah72

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!!!

4. Aug 14, 2013

### pjshah72

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.

5. Aug 14, 2013

### milesyoung

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.

6. Aug 14, 2013

### Baluncore

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.

7. Aug 14, 2013

### Staff: Mentor

What are you going to do with the signal once you have removed the 60Hz from it?

8. Aug 15, 2013

### pjshah72

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!!

9. Aug 15, 2013

### Baluncore

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. Aug 15, 2013

### pjshah72

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. Aug 15, 2013

### jegues

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.

12. Aug 15, 2013

### pjshah72

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. Aug 15, 2013

### Baluncore

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. Aug 15, 2013

### pjshah72

Can anyone suggest value for L and C to reject 60Hz frequency?

15. Aug 15, 2013

### Baluncore

16. Aug 15, 2013

### Staff: Mentor

You are looking for trouble! [Broken]

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.

Last edited by a moderator: May 6, 2017
17. Aug 15, 2013

### dlgoff

bold by me

I totally agree with

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:

Last edited by a moderator: Apr 17, 2017
18. Aug 15, 2013

### pjshah72

What is the safe way? Can I put transformer between AC and circuit so it will step down voltage?

Last edited by a moderator: May 6, 2017
19. Aug 15, 2013

### pjshah72

Thanks for the image!!

Last edited by a moderator: Apr 19, 2017
20. Aug 16, 2013

### davenn

yes that's the safe way

Dave