I’m trying to build a fan controller. My power source is a +24 V battery which is commonly used in cars.

I want use a regulator module (buck step-down regulator based on the CN3903).

This is my schematic:

Regulator Part

[EDIT : wrong cap number] C4: for +24 V battery input ripple
C9: for +5 V output ripple

Fan Motor Part

How can I calculate the capacitor values when drawing a circuit using these regulator modules?

If I have an oscilloscope, do I apply the required maximum load and set it to the maximum noise and ripple-free value?

  • \$\begingroup\$ Where did this schematic come from? As drawn this circuit will run the fan constantly. If FAN1 is indeed drawing 3A average, D1 is also going to get very hot, burning several watts and probably going to melt. \$\endgroup\$
    – Matt S
    Dec 29, 2022 at 8:33
  • \$\begingroup\$ The SS34 has a maximum forward current of 3A so it's not really suitable anyway. \$\endgroup\$
    – Finbarr
    Dec 29, 2022 at 9:44
  • 2
    \$\begingroup\$ There are no such caps in the image that you refer to in the text. Also, what caps you need to use depends on what your regulator module needs, if any, and what your other module needs, if any. Was there a manual or schematic of products available? \$\endgroup\$
    – Justme
    Dec 29, 2022 at 10:16
  • 1
    \$\begingroup\$ Everything given in the datasheet: dcx-ic.com/uploads/soft/210625/1-210625131439.pdf so what exactly is your question? \$\endgroup\$
    – datenheim
    Dec 29, 2022 at 10:19
  • 1
    \$\begingroup\$ @datenheim That's why you would need the data sheet of the module. Or schematics. So you don't need to remove the capacitors to be able to measure them. You could also just not add any extra capacitors and see if it works. If it powers a fan controller then what difference it makes how much ripple there is if the fan controller can work with it. Ending up with no ripple means infinite capacitance. Small ripple is allowed. \$\endgroup\$
    – Justme
    Dec 29, 2022 at 14:52

1 Answer 1


Since you only require 5V / 100 mA nearly any module using this chip, whatever capacitors it has from the manufacturer, would be up to the task without any additional capacitors at all (it is very far from the maximum power possible, so no stress).

If you would draw the max. advertized current all the time you may notice differences between modules from different manufacturers due to varying component values and qualities. In some cases adding carefully chosen capacitors may lead to lower ripples or so.

If you need to optimize something in such a case, or want to integrate the ic yourself, just inspect the datasheet and follow the formulas and text carefully regarding capacitor values and their ESR.

Thank you @Justme for clarifying the OP's question.

  • \$\begingroup\$ I've read datasheets of different switching regulators. most these datasheets have a fixed value of the capacitor and the capacitor that needs to be changed according to the input/output voltage thank you i'll read datasheet more carefully. \$\endgroup\$
    – KimJiHoon
    Dec 29, 2022 at 23:00

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