Filtered 3V3 pin?

I have made a couple of sensor boards and I used the 3V3* pin to supply power. I realized that the Photon wont have that pin, so I switched to using the 3V3 in my latest board.

It looks like my sensor readings are a bit spiky now though.

How can I reproduce the filter from the 3V3* on the 3V3 pin ?

Hi @Rockvole

On the Spark Core schematic on github it is shown as taking the 3.3V supply through a ferrite (it says bead but it could be some other type of inductor) and then adding two capacitors from the other side of the ferrite to ground, one 100nF and one 10uF electrolytic to form the filtered 3.3V* (called VDDA in the schematic).

Anything close to that kind of filter will work. You want some kind of choke followed by high and low frequency filter caps to ground.


Given the Photon will have a (1.5MHz) switching regulator on board, does that change the selection of filtering componentsÉ

Thanks @bko
I saw the diagram on another post :

I’m still confused about what to do with only the 3V3 pin though.
I guess the 3V3 will still have the ferrite on it ?

Do I need to add a 100nF Cap to ground and a 10uF Cap to ground ?

Hi @Rockvole

That is indeed the schematic of the Spark Core. In this case the 3.3V pin on the upper right of the diagram is the power input to LC filter. You connect that the 3.3V pin of the Spark Core/Photon.

The pin labelled VDDA and has the red arrow pointing to it is the power output, equivalent to the 3.3V* pin on a Spark Core.

As @peekay123 says above, you might want to change the value of the 100nF capacitor but I am sure it will work fine.

Does this look right ?

I don’t see the inductor or the other capacitor in your fritzing diagram. I am not where I can draw something up for you unfortunately, but you have to start with one inductor and two caps at least.

I added the inductor, Im not sure where to put the other Cap - should it just go between the 3V3 and GND the same as the first Cap ?

Hi @Rockvole

I can’t make the Fritzing breadboard look good so I am enclosing a schematic.

You need two capacitors and a different inductor–ferrite core, not air core. Fritzing does not seem to have a ferrite core inductor in the the library that is not huge.

C1 is 100nF (also known as 0.1uF)
C2 is 10uF electrolytic which means it has a positive and a negative terminal–pay attention here.


Thank-you @bko

I drew it up how I would solder it.
I couldn’t find a ferrite core inductor in Fritzing so I just added a note.

Hi @Rockvole

You are really close but you have the capacitors on the wrong side of the inductor–just move their positive leg to supply rail at the bottom of the breadboard (the other end of the inductor) and you are there!

Like this :

1 Like

Hi @Rockvole

Yes! You have it now! Perfect.


Hi @bko
I finally got down to the local electronics store. I didn’t realize inductors came in so many sizes. The assistant didn’t know what size I would need, but he seemed to think that ferrite cores are used when you need 1 henry or more, but he said those are quite big.

Any idea what size inductor I should use ?

Hi @Rockvole

The one used in the Spark core is very small and is a ferrite bead in a surface mount package. They don’t give an inductance on the data sheet, but give a total impedance instead:

@bko, just to add to what you said, the impedance of 220 ohms seems to be measured at 100MHz (stated as Test Frequency). Based on that, the inductance can be calculated to be 0.35uH :smiley:

@bko would this be an equivalent in through hole mount ?

I was looking for something like those inductor beads, but this is the closest I could get :

@Rockvole, those are just beads, not inductors.

I thought I am looking for ferrite beads ? I think that is in the Spark Core ?