Capacitors and switching power supplies

[moskovskiy82]

I was wondering on the best practices

[mfalkvidd]

The recommendation is to place a 4.7uF cap as close as possible to the nrf chip. I am not familiar with the adapter board but the cap should be placed after the voltage regulator.

[Oitzu]

@moskovskiy82 said:

I am using external chargers 5V for powering my sensors (some are old branded nokia ones, others are a cheap chinese replacement).
Power goes to arduino VCC (arduino pro mini 5V) and also the NRF24 adapter board (containing the voltage regulator and some capacitors - got it off aliexpress).

I wonder if adding a decoupling capacitor just before power is split to arduino and NRF24 board will help in any way. If yes what is the best value for the capacitor?

Well, the arduino should have some decoupling on the board itself, also the decoupling on the NRF24 adapter board should be sufficent.
You could add a low-pass filter after the power supply (are your supplys switching regulators?) to reduce the ripple produced, but the frequency rejection of the linear regulators should also handle this fine.

[moskovskiy82]

@Oitzu

Thanks for the great explanation!

[ronnyandre]

@Oitzu do you have a recipe for making such low-pass filter?

[AWI]

@ronnyandre Things are sooo easy (if you have a degree in electronics ;-) ) An effective design includes multiple filters and depends for a great deal on the switching frequency (and harmonics). A typical design could look like this.

An linear regulator (like LM7805) can be very effective for low frequencies (~ < 10kHz) but not for high frequencies.

In most of the cases when working with phone mains adapters the 'standard' (uno/ nano) regulator (1117) with capacitors damps sufficiently.

[ronnyandre]

Wow @AWI that looked more advanced than I thought.. :sweat_smile:

[Oitzu]

@ronnyandre The main reason for using a filter would be a crappy/cheap switching power supply.
I needed to use one of these filters on a cheap LM2596s (clone) module to filter the output to a nrf24l01+ pa/lna module.

Unlike @AWI 's solution i just added a second stage after the first stage (that is on the module).
http://blog.blackoise.de/wp-content/uploads/2016/03/lc-filter-schem1.jpg
L1: 3.3µH
C1: 220µF
Worked fine enough to get the nrf24l01+ pa/lna module working.

[ronnyandre]

@Oitzu Thanks! I'll try that one out :simple_smile:

[gohan]

@Oitzu I am about to buy some of those LM2596s, the values for the filter components are suitable for LM2596 in general or it is something more specific that you had to calculate? Did you make the impedance yourself?

[Oitzu]

@gohan these values are just the ones i used and should filter lowpass at about 5.9 kHz.
The ripple on the LM2596s is usually arround 25-50kHz. (Depends on which clone you get. ;))

I'm no electrical engineer. Maybe @AWI can comment if there may be a more suitable component selection.