A rainbow night light - Choosing the capacitors. - Page 1
Forum section: electronics / projects

author: Jaunedeau
author_rank: Regular Contributor
timestamp: August 04, 2018, 07:08:45 am
content: Hi,I'm making a rainbow-shaped night light. The V1 is about ten years old and used height 5mm leds. The V2 is on its way and will use 24 SK9822 (APA102).The PCB is rather simple : On front side there is a serie of SK9822 with caps. On the backside, an Atmega328pb with its two caps - I will add capacitiv bouttons later.There is an a 2x10 connector for ISP (will later be replaces by four pads / pogo pins I think).Also on the back, I have two large ground and +5V planesThis is the current status (see attachement 1)Now from what I read somewhere, it seems that I could duplicate my back-side polygons on the front-side and invert them so that I have a +5V plane on the other side of the GND and GND on other side of the +5V. Apparently this should create a giant capacitor and maybe I can remove all the caps from the SK9822, keeping only those for the MCU just to be sure. The PCB would be really simpler (see attachement 2 for a quick made test). Adding capacitiv buttons and maybe a few other things (an RTC ?) become much easyer.Do you think I can remove the caps as in second attachement ? (The four planes have an area of about 20cm² each).Thank you,Jaune.

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2018, 08:32:02 am
content: Quote from: Jaunedeau on August 04, 2018, 07:08:45 amIt will indeed create a physically large capacitor.You can calculate roughly how "giant" the capacitance would be from the geometry and the standard equation, and compare that with the capacitance you need.

author: Jaunedeau
author_rank: Regular Contributor
timestamp: August 04, 2018, 09:11:41 am
content: QuoteI tryed to do the math and found 0.129pF ( 50cm² area * 4.35 permittivity for FR4 * 8,84.10-12 / 1.5mm thickness). SO it apparently won't help

author: Jaunedeau
author_rank: Regular Contributor
timestamp: August 04, 2018, 11:56:37 am
content: I tryed a new version :On the back side, instead of haing two equaly large ground and power plane, I put one ground plane, and one 10 +5V rail on the inner borderOn the front side, I put a ground planeThis way, the connection of the capacitors to ground are cleaner, and it is easy to have one wire going to the capacitor then to +5V.Am I doing this the right way ?Thank you,Jaune.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2018, 12:20:30 pm
content: By cutting the top side pour fully into pieces with traces, and not stitching them with vias, you've not changed much! Fortunately this is easily addressed.The capacitors will be very small, so if the LEDs need, say, 10nF or more, you aren't going to get it here (at least, not without 6+ layers and a lot of interleaving). Planes are more about limiting the extent of electric and magnetic fields. They aren't relevant to bypassing until you get into the ~100MHz range.Tim

author: Jaunedeau
author_rank: Regular Contributor
timestamp: August 04, 2018, 12:45:49 pm
content: Quote from: T3sl4co1l on August 04, 2018, 12:20:30 pmoops, I attached the wrong picture. I edited the message to add the right versionIt's not a really big change, mainly instead of trying to connect led -> cap -> psu on each side (+5V and GND), I do it only on +5V side and use a ground plane on both sides.I made a few PCB before, but they drew nearly no current, and they worked even if I didn't solder the caps. This one will have 24 leds, 1.44A at 4.7kHz (sk9822) or 19.2kHz + 440Hz for global illumination (APA102) and capacitiv touch sensing... I am a software developer and don't know much about analog electronic. Any rview / hint / link to things I should read will be appreciate !Thank you,Jaune.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2018, 12:50:11 pm
content: Ah yeah, that looks better...Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 04, 2018, 01:24:35 pm
content: The manufacturers of the APA102 appear to be patent trolls so consider using some other part.https://hackaday.com/2018/07/31/are-patent-claims-coming-for-your-ws2812/

author: Jaunedeau
author_rank: Regular Contributor
timestamp: August 10, 2018, 01:13:09 pm
content: Hi,I *think* I'm done. Since it will be the first board I make that doesn't only have low frequency and low current digital signals, I'd be happy to have some review.Two things I am not sure about :-Can the capacitive touch button (the discs ont the back side) be just behind the LEDs ?-Do I need a big capacitor between ground and +5V in addition to all the decoupling capacitors on each SK9822 and those on the MCU ?But I probably missed a few things I am not even aware ofThank you,Jaune.(eagle files in the zip file)

author: Jaunedeau
author_rank: Regular Contributor
timestamp: August 12, 2018, 08:00:30 pm
content: Hi,I need to order the parts. The sk9822 will be bought form aliexpress ($14 for 100 pieces). I'm not sure about the caps thought. I remember reading somewhere that the 100nF ceramics caps can be bought anywhere ( I need 56 of them), but the 10µF is more subject to bad quality of fake value.Can I just order a pack of 100 "10µF 0805 SMD ceramic capacitors" from aliexpress, or is it a bad idea ?Thank you,Jaune.