Filter circuit for DC voltage

Thread Starter

pinkyponky

Joined Nov 28, 2019
118
Hi all,

I have a 5V, 12V and 24V supplies in my circuit, but as we know that all are DC voltage, so there is no frequency. In this case, how we can design a filter circuit for example RC and LC filter circuits?.
 

Irving

Joined Jan 30, 2016
2,055
All DC outputs derived from AC, whether mains or via high-frequency SMPS, will have a small superimposed AC component which ideally would be <<1% of the DC voltage (so 5mv peak-peak ripple on a 5v supply could be considered acceptable). So that's where your 'frequency' comes from. But you would not normally add power-supply filtering outside of the power supply itself unless you knew it was noisy and required additional filtering for a specific need.

The other aspect in analog circuits is where you wish to set a DC bias point, eg on one input of an opamp where an AC signal is applied to the other input. Depending on the design, filtering may be necessary to prevent the DC bias being impacted by the AC signal.

Why do you think you need to design filters?
 

ericgibbs

Joined Jan 29, 2010
13,579
I have a 5V, 12V and 24V supplies in my circuit, but as we know that all are DC voltage, so there is no frequency. In this case, how we can design a filter circuit for example RC and LC filter circuits?.
hi P,
Your query makes little sense, as already asked 'why' .?
Do you have an actual practical power supply in mind.?

E
 

Thread Starter

pinkyponky

Joined Nov 28, 2019
118
All DC outputs derived from AC, whether mains or via high-frequency SMPS, will have a small superimposed AC component which ideally would be <<1% of the DC voltage (so 5mv peak-peak ripple on a 5v supply could be considered acceptable). So that's where your 'frequency' comes from. But you would not normally add power-supply filtering outside of the power supply itself unless you knew it was noisy and required additional filtering for a specific need.

The other aspect in analog circuits is where you wish to set a DC bias point, eg on one input of an opamp where an AC signal is applied to the other input. Depending on the design, filtering may be necessary to prevent the DC bias being impacted by the AC signal.

Why do you think you need to design filters?
Thank you for good explanation Irving.

Here, I'm explaining that where I would like to use the filter stages:

1) Im powering the DC-DC converter (24V-5V) (which is in PCBA) from the outside power supplies. Is it require the filter stage in this case?. If so, which one is good to use either RC or LC?. How to calculate the component values?. I don't know how much noise it has to filter out?.

2) I have 5V analog signal which is coming from outside of the PCBA and it is a one of the input to the amplifier circuit. So, in this stage do we require the filter stage?. If so, which one is good to use either RC or LC?. How to calculate the component values?. I don't know how much noise it has to filter out?.
 

Irving

Joined Jan 30, 2016
2,055
Thank you for good explanation Irving.

Here, I'm explaining that where I would like to use the filter stages:

1) Im powering the DC-DC converter (24V-5V) (which is in PCBA) from the outside power supplies. Is it require the filter stage in this case?. If so, which one is good to use either RC or LC?. How to calculate the component values?. I don't know how much noise it has to filter out?.

2) I have 5V analog signal which is coming from outside of the PCBA and it is a one of the input to the amplifier circuit. So, in this stage do we require the filter stage?. If so, which one is good to use either RC or LC?. How to calculate the component values?. I don't know how much noise it has to filter out?.
There's no value in further filtering the external supply if you have a DC-DC converter onboard. Filtering the output of the DC-DC converter may be beneficial but you'd need to use a 'scope to look at the 5v rail and other key points to see how noisy it was. Generally if you have good decoupling capacitors (eg 100nF in parallel with 10uF) at major chips or major switching points that will suffice.

Analog inputs often need signal conditioning to exclude RF interference (including switching noise from PSUs) or generally to reduce noise by limiting bandwidth. Passive RC filters or active filtering may be necessary but only you can decide that based on the expected frequency range of the analog signal and how often you sample it. For example, if the 5v analog signal is a battery voltage sampled once a second then a simple 1-pole RC filter with a cutoff of a few Hz may be all you need (or you do it in software). Without more information about the signal, the circuit impedance and your needs I can't tell you much more.
 

DickCappels

Joined Aug 21, 2008
7,687
If you don't have noise problems, there is no point in adding filtering. When cleaning up noise, sometimes on a power supply getting into a signal chain, I would tailor the filter design to the type of noise and the amount of attenuation needed.
 

Irving

Joined Jan 30, 2016
2,055
The TS doesn't really know and, I suspect, doesn't really know/understand how to assess. Look at their other posts about this same project...
 

BobTPH

Joined Jun 5, 2013
3,648
Let’s start over. Is there an actual problem that you are trying to solve? Or did you just decide you need filters on your DC power supplies for some reason?

Bob
 

Papabravo

Joined Feb 24, 2006
16,772
Im trying to filter out the voltage of the MOSFET and that voltage is measuring to the ADC. This is discussed in my thread
High voltage and current handling circuit
https://forum.allaboutcircuits.com/threads/high-voltage-and-current-handling-circuit.181467/
Filters won't do squat for high DC voltages and currents. They only work on AC voltages and currents, so I am tempted to ask why you have AC voltage and currents on your DC supplies. For all I know you might suspect that gremlins are responsible.
 

MrChips

Joined Oct 2, 2009
24,134
All signals, regardless of being DC or AC, have frequency components.
Theoretically, DC has only 0Hz component. In real life it is impossible to create a signal that has zero bandwidth.

Thus, when attempting to filter a DC signal you are trying to prevent two things:

1) self induced fluctuations
2) externally induced noise

You can characterize both of these in two ways which are the same, one being the reciprocal of the other, in other words, by bandwidth (or frequency response) or by time constant.

Thus, if you know the bandwidth of the induced noise as in (2) you can design a filter tailored to that situation.
Similarly, if you are attempting to reduce changes in a DC source as in (1) you can use the time constant approach.
In both cases, you are applying a low-pass filter that can be characterize by frequency response or time constant.

For digitizing any type of signal you must follow the Nyquist Theorem. You apply an anti-aliasing filter (low-pass filter) that attenuates all frequencies higher than one-half the sampling frequency.
 

andrewmm

Joined Feb 25, 2011
1,752
You seem to be suffering from being "smart", and having posted parts of questions in different forums ,

This leads to partial answers,

Suggestion,
start a new thread , with the complete story and question,
and stop the other threads
 

Papabravo

Joined Feb 24, 2006
16,772
All signals, regardless of being DC or AC, have frequency components.
Theoretically, DC has only 0Hz component. In real life it is impossible to create a signal that has zero bandwidth.

Thus, when attempting to filter a DC signal you are trying to prevent two things:

1) self induced fluctuations
2) externally induced noise

You can characterize both of these in two ways which are the same, one being the reciprocal of the other, in other words, by bandwidth (or frequency response) or by time constant.

Thus, if you know the bandwidth of the induced noise as in (2) you can design a filter tailored to that situation.
Similarly, if you are attempting to reduce changes in a DC source as in (1) you can use the time constant approach.
In both cases, you are applying a low-pass filter that can be characterize by frequency response or time constant.

For digitizing any type of signal you must follow the Nyquist Theorem. You apply an anti-aliasing filter (low-pass filter) that attenuates all frequencies higher than one-half the sampling frequency.
So let the TS clarify his purpose and intent, and consider what his intentions are in light of post #1 in this thread.
 

MrChips

Joined Oct 2, 2009
24,134
I have a 5V, 12V and 24V supplies in my circuit, but as we know that all are DC voltage, so there is no frequency. In this case, how we can design a filter circuit for example RC and LC filter circuits?.
Your assumption is that since it is DC there is no frequency content. That is a false assumption.
Your aim is to create a DC signal that has no frequency greater than 0Hz which is not possible to do.
Hence you have to specify how close to 0Hz you would like to achieve. With that criteria you can design the filter components
 

Papabravo

Joined Feb 24, 2006
16,772
Your assumption is that since it is DC there is no frequency content. That is a false assumption.
Your aim is to create a DC signal that has no frequency greater than 0Hz which is not possible to do.
Hence you have to specify how close to 0Hz you would like to achieve. With that criteria you can design the filter components
Doing a passive filter for high current and high voltage is going to run into a number of problems involving component values and properties. I don't know if an active filter will do anything for the TS because he has offered nothing like a specification for such a filter.
 

BobTPH

Joined Jun 5, 2013
3,648
The problem here is that the TS is unable to communicate what he wants, possibly because he has no clear idea himself. In the other thread he wants us to tell him how to measure a current and a voltage without knowing what the circuit is!

Bob
 

LowQCab

Joined Nov 6, 2012
1,078
Did anyone notice that his first Post that he linked to was in "Homework Help" ?
It would appear that the Thread-Starter doesn't have a good grasp of the basics,
and is hoping that there is a one paragraph answer to an entire text-book of theory.
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