How to get the capacitor filter value in a dc power supply?

Hello everybody, I'll use a 12V/5A power supply. Actually I got it (already bought it).

I'll use it for a university project, but one of my requirements (it has to be followed in order to achieve a good grade) is to use a filter so the ripple voltage decreases.

I've seen some of the different filters that are used in ac/dc power supplies and I thought that It'd be easy to add a "capacitor filter" as shown in the image below.

I have some questions and would really aprecciate your help.

1.- Is the capacitor filter the best choice if I want to decrease the ripple voltage and isn't as difficult as the other filters? or you recommend another one?

2.- In case the capacitor filter is ok, how can I get the value of the capacitor if I only have the frequency, the power supply current and voltage values and maybe the current my project will consume (I think the maximum current used at the same time will be like 3A)

3.- I don't know if the power supply I bought has already another filter inside. Would something happen if I add the new filter? I dropped an image of my power supply. Btw, do you know what is the orange screw on the right below side?

Captura de Pantalla 2021-10-25 a la(s) 17.06.09

All power supplies have internal filters. Some do better than others.

The first thing I would do is to find the specifications of the supply. Read the ripple specification and see if you believe it needs to be reduced.

The orange screw is a potentiometer (similar to a volume control). It controls the output voltage. Usually it has a very small range like 4.8V to 5.2 volts but each unit is different.

1 Like

This is what I found on the web, is the S-60-12 model (2nd column). So... what does it mean?
Sorry, I'm a rookie

The spec states that the maximum ripple and noise is 120 mV peak to peak, subject to the conditions of Note 2. What does that note state?

I see no point in attempting to reduce that ripple further, and wonder about the reason for that part of your project requirement.

1 Like

Because in the beginning of the project (before buyin' the power supply) I put "to design and implement a system to energy the project".

So let's say it's an specific objective that I need to achieve, but now I don't know what to add if the filter isn't neccesary.

That seems like a pretty broad goal statement, and the concept of "system design" certainly includes selecting from commercially available parts.

1 Like

The power supply you bought is nothing like the schematic you showed. You have a SWITCHING power supply. Note the huge inrush current parameter. It's ripple frequency will be many hundreds of Hz, making the filtering much easier to do.

1 Like

For a simple Arduino board the 120 mv ripple is not an issue.
However if you were to use a sensor or A/D converter this amount of ripple could be an issue.
Also you said your professor is expecting you to add a filter, and I assume you will need to explain your choices.

The specification doesn't say but the vast majority of this type of supply uses a switching frequency between 18K and 25kHz.

The simplest form of filter is a capacitor.

The value can be estimated but a lot of assumptions must be made (and I don't think I can explain it is a post.

However there are two things to think of:

  1. The spec indicates the startup time is 50ms. And the voltage is 5v. So using the equation I = C dv/dt
  • I = startup current into capacitor in amps
  • C = capacitance in farads
  • dv = change in voltage in volts
  • dt = change in time for the above dv

The I must not exceed the specification for the chosen capacitor

  1. You should chose a low inductance or low impedance capacitor, likely aluminum.
1 Like

Thanks for your answers.
Actually I'm using a cnc shield board on my arduino UNO to control 2 0.4A steppers and a 2A solenoid, but they won't work at the same time, possibly the solenoid and a stepper (that's why I round the consumption to 3A), so it leaves ~2A free.
So, I leave my startup current as 3A?

2.-Why the capacitor must be low inductance?

Thank you, again.

I see, actually I was thinking in making a pcb with an array of capacitors and connecting in to the dc output of the power supply. Is it wrong?

and how can I make the filter? Is there any specific equation I need to use?

Thanks for your help

for instance, if I use a solenoid and a stepper, the ripple won't drop and will keep itself in 120mV no matter what I connect?

You're driving motors. Why are you worried about ripple that is only 1% of the supply voltage?

1 Like

To answer that, you have to dig deeper into the data sheet. For example, what does "Note 2" say? It's referenced next to the 120mV spec.

Pretty much.

1 Like

Note 2 says : "Ripple & noise are measured at 20MHz of bandwidth by using a 12" twisted pair-wire terminated with a 0.1uf & 47uf parallel capacitor."

dunno what it means.

And yes, I am driving 0.4A steppers and a 2A solenoid. But since I'm a newbie, I don't know certain things. I learn from you, too. Thanks for that.

Inductance can reduce the effectiveness of the capacitor. The higher the ripple frequency the more important having low inductance is.

Unless I misread your first post, the additional filtering was suggested as required by your professor.
So you even probe him/her wrong or add the additional filtering.

1 Like

You are misinterpreting the professor, I'm sure. Your diagram shows a bridge rectifier not a power supply. For that, a capacitor filter is definitely required and that is 100% likely what the professor was talking about. If the assignment calls for the design of a power supply, you've failed by utilizing a commercial unit. Adding a capacitor to that, would simply highlight your misunderstandings in the view of the professor. So you need to rethink the whole approach.

Technically, though, the switching supply is far superior to any rectified AC/DC converter like diode bridge and smoothing capacitor.

You can approximate the appropriate filter capacitor value by deriving the desired slew rate from design ripple voltage targets, then use the fundamental capacitor formula I=C(dV/dt) (after rearranging it) to match the load current to the desired max slew rate.

It's a cheesy approximation, but it gets you close very quickly.

1 Like

Yep. To this point I dunno if it's neccesary or not to add the additional filtering because I gonna drive a solenoid and a stepper and don't know if the ripple will increase or keep in 120mV when they're used.

What do you recommend?

If I can remove the requirement, is it ok? and if there's no chance to remove it, will the additional filtering affect my switching supply?

What is the purpose of this assignment?

1 Like

Possibly I misinterpreted my teacher, but now I have my switching power supply. Actually my professor and me chose the assignment and I think it can be viewed from different points of view (which helps me now that I bought the power supply). The advice of using a capacitor filter was from my professor, he already knows I bought a power supply but we need to achieve the assignment, that's why he recommend to use a filter and calculate the value so we can say "it was a way of designing the project energy supply".