# Low Pass Filter design......

Discussion in 'General Electronics Chat' started by pjshah72, Aug 13, 2013.

1. ### pjshah72 Thread Starter New Member

Dec 17, 2012
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0
Hi All,
I have attached circuit for Low Pass Filter at cut-off frequency 60Hz.
My input is 240VAC, 60Hz for this filter. My goal is to block all AC signal at the output of the filter (means I am looking for DC.).
Two questions: -
(1.) If I use attached circuit, can I get DC as an output of this filter?
(2.) What will happened to 240Vac voltage? Will voltage (240Vac) stay same after output of this filter? (If yes, how can make it to ground?)

Thank you!!!!!!

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2. ### Papabravo Expert

Feb 24, 2006
11,860
2,520
1) No
2) There will be some loss due to the resistance of the windings, but you will have a hard time making the output close to ground. At 60 Hz. the reactance of the inductor would have to be >> 38 Ohms, like say 3.8 KΩ

In this case the output would be about 2.375 VAC. As you can see that is a small fraction of 240 volts but it is NOT GND.

On the other hand if the reactance is .38Ω then the output will be 237.62 VAC assuming no IR loss in the inductor.

Maximum power will be transferred to the load when the reactance is 38 Ohms and the voltage across the resistor will be 120 VAC. The power dissipation in the resistor will be 378.9 Watts.

Personally I've never seen a resistor that could handle that kind of power, but you may have other sources.

Are you trying to convert AC to DC without using a transformer? Oh foolish noob -- if so this thread may be locked.

Last edited: Aug 13, 2013
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3. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
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What do you mean by this thread may be locked?
Is it not proper place for this problem?
Yes trying to block AC and in other words wanted to grounded incoming AC.
Like a Capacitor can block DC. Can we have anything that can block AC?

4. ### #12 Expert

Nov 30, 2010
18,076
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You can reduce the amplitude of AC by quite a lot, but there is no DC in a wall outlet, so nothing useful will be left. The problem with this quest is that this website does not allow talk of plugging directly into the outlet and using that without some way to make it safer.

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5. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
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I understand that wall outlet doesn't have DC. I am just looking how to ground/zero AC. But as you said this is not allowed here so I don't know how to get help.

6. ### wayneh Expert

Sep 9, 2010
15,515
5,739
Unplug it.

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7. ### crutschow Expert

Mar 14, 2008
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If you block all AC, then what DC are you looking for? There is none.

8. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
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that was my question, how to block all AC? (If it is possible.) I thought to use LPF and trying to make filter that gives me close to DC from AC. If i will not get anything, then it would be fine.

9. ### crutschow Expert

Mar 14, 2008
20,812
5,915
The mains voltage has a DC average of zero volts so that is what a filter will give. It's a pointless exercise.

10. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
0
ok. I am not sure this is a proper question to ask or not!! But wanted to know answer so I am asking here:
How can we differentiate AC voltage and DC Voltage for power supply? In other words, in real world, can we make single Input (AC or DC) power supply? Is there a circuit that can tell us whether it is AC or DC?

Thank you!!!

11. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Are you in a part of the world where the local (mains?) supply voltage could be either AC or DC ?

12. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
0
Supply voltage is always separate. Either AC (Mains) or DC.
I am just asking (if it is possible) can we make circuit that can take both (one at a time)?

First question came in my mind is- in real world, is this kind of power supply exists?

13. ### wayneh Expert

Sep 9, 2010
15,515
5,739
You can block DC with a coupling capacitor that will pass AC. It's not really practical in a power supply but it provides an easy way to detect AC. If something comes through, there is at least some AC component. There is also such a thing a zero-crossing detector which might be relevant.

But, you could just use a full wave bridge. It'll accept either source and reliably ensure that only (pulsing) DC is at the output.

14. ### Papabravo Expert

Feb 24, 2006
11,860
2,520
If you look at the derivative of the voltage output you can tell.

for DC the derivative is 0
for AC(sinewave) the derivative is a cosine wave.

15. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
As mentioned, running the supply through a bridge rectifier will produce either pulsing DC or straight DC minus the diode drops. Then run the voltage into both low-pass and high pass filters. Indicators could be attached on these outputs.
Is this on the right track ?

16. ### #12 Expert

Nov 30, 2010
18,076
9,676
It works for me. I have used the FWB approach to make DC inputs "idiot proof". One diode will produce, "won't work" for a backwards polarity of DC input. Four diodes will simply correct the polarity. They will also rectify an AC input and deliver the polarity correctly. Whether the magnitude of the input voltage is correct is different aspect to consider.

17. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
0
That was I thought last night. How can we detect frequency? Because AC has 50 or 60 Hz frequency but DC has always zero frequency. once we can detect "zero frequency" vs "non-zero frequency", we can route two signals accordingly.

I hope there is something that can detect zero and non-zero frequency.
Thank you,
pjshah

18. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
0
Is there a bridge rectifier that can take AC mains as an input and also DC (30V max for my application)?

Nov 30, 2010
18,076
9,676
Yes.
......

20. ### pjshah72 Thread Starter New Member

Dec 17, 2012
24
0
Hi,
How did you calculate 2.375 VAC at 38ohm resistor?