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AC / DC distinction and filtering

istvanb

Dec 10, 2012
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Dec 10, 2012
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Hi,

The topic title is may not be very descriptive, let me describe my dilemma here. As we learnt in school:
DC is when the voltage is solid, lets say constant 5V
AC is when the voltage reverses direction periodically, lets say 110V AC in the US

But there is a certain gray area between them, lets say the load on the DC supply changes by time and the voltage drops below the DC voltage.

I am wondering how I should filter such a signal on my computer. If I have a true DC voltage then I just use a digital filter which cancels all the AC signals (ex.: lowpass 10Hz filter). If it is an AC signal then I can use a bandpass filter (ex. bandpass 60Hz filter), to get rid all the superponated voltages.

Now what an earth should I do if I use a DC source, but because of the load the the voltage level drops. It doesnt became an AC signal, since its not periodical, but not a DC signal anymore either.

What would be the right approach?

Thanks.
 

Relayer

Dec 15, 2012
39
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Dec 15, 2012
Messages
39
Hi,
DC is when the voltage is solid, lets say constant 5V
Correct.

AC is when the voltage reverses direction periodically, lets say 110V AC in the US
Yes every 60 times a second.

But there is a certain gray area between them, lets say the load on the DC supply changes by time and the voltage drops below the DC voltage.
I am wondering how I should filter such a signal on my computer.
Voltage regulator IC's and error circuitry compensate for various fluctuations in Input and Output voltage rails. PC power supplies are excellent for this, that's one of the reasons for their bulky size.
AC from the mains is rarely a steady 110 volts. It tends to fluctuate with respect to consumer demand.
Voltage regulation is quite universal in most consumer and industrial products.

If I have a true DC voltage then I just use a digital filter which cancels all the AC signals (ex.: lowpass 10Hz filter). If it is an AC signal then I can use a bandpass filter (ex. bandpass 60Hz filter), to get rid all the superponated voltages.
The superponated (I've never heard of this term) voltages (ripple) is so small in a well regulated power supply its often dismissed.
I'm not sure why you would want to further filter out any hash that comes from the mains supply. Installing a filter would make extremely little difference.

Now what an earth should I do if I use a DC source, but because of the load the the voltage level drops. It doesnt became an AC signal, since its not periodical, but not a DC signal anymore either.
No, it won't become an AC signal, but its still a DC signal regardless, even if it dropped down to micro volts.
Think about an audio signal: It varies its amplitude constantly, so would that be considered as AC or DC. In fact its still AC, but on a DC level, since its amplitude never goes into a negative polarity (though this is a bit of a misnomer, its should be called something else other than AC). This is why transistors can amplify an audio signal without rectifying out any negative polarity signal.

What would be the right approach?
Do nothing :)

Regards,
Relayer :D
 

istvanb

Dec 10, 2012
18
Joined
Dec 10, 2012
Messages
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I'm not sure why you would want to further filter out any hash that comes from the mains supply. Installing a filter would make extremely little difference.

Do nothing :)

Thanks for your reply. I would agree although you approache the problem with assuming that I am talking about very slight variation of the DC level. In this case this is not correct, I constantly overload the power supply therfore it drops its voltage significantly. Think about car batteries. When you crank the voltage drops a lot.

Looking fwd to hear your thoughts.
 

davenn

Moderator
Sep 5, 2009
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Thanks for your reply. I would agree although you approache the problem with assuming that I am talking about very slight variation of the DC level. In this case this is not correct, I constantly overload the power supply therfore it drops its voltage significantly. Think about car batteries. When you crank the voltage drops a lot.

Looking fwd to hear your thoughts.

Well you need a better power supply then, one that can handle the load you are putting across it. The voltage is only dropping because you are trying to draw more current than what the PSU can supply.
NO amount of filtering in the world is going to solve that problem

Dave
 

istvanb

Dec 10, 2012
18
Joined
Dec 10, 2012
Messages
18
Well you need a better power supply then, one that can handle the load you are putting across it. The voltage is only dropping because you are trying to draw more current than what the PSU can supply.
Dave

Actually thats the intended operation so getting another PS is not an option. I understand that none of us meet this type of an issue in our daily life and normally a bigger PS would be the solution.

I recommend to consider this as a theoretical problem.
 

davenn

Moderator
Sep 5, 2009
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there's not really any theory to discuss ;)

OK so if you dont wanna go that way,
use really huge capacitors to help with the peak demands, wont solve it totally but will help.
Its a trick that every young car owner does with his excessively hi powered audio amplifier

Dave
 

Relayer

Dec 15, 2012
39
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Dec 15, 2012
Messages
39
The only way you may possibly prevent load drop outs is to use a pass transistor or several in your PSU, but this depends on the way the power supply is designed.
If its a switch-mode type, then you have little chance of making any major improvements without having to drastically change its design.
Is there any chance that you have a schematic diagram of your unit?
If not, could you possibly take some snapshots of it?
Perhaps we may be able to work something out.
Regards,
Relayer :D
 
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