First layout, any pointers? - Page 1
Forum section: electronics / projects

author: ratio
author_rank: Regular Contributor
timestamp: December 01, 2017, 12:32:14 am
content: Here's me trying to get a little experience with KiCad & the whole EDA thing. This is a little R/2R network to allow my power logger to log three binary channels on it's 10V analog channel. Feel free to poke it apart-you won't hurt my feelings.The power source is nominal 24VAC, actual could be as high as a strong 27 volts RMS (60Hz), a bog-standard (in the US) 24V xfrmr. The regulator is an amazingly-high-input-voltage 3 term linear regulator I found after I realized that a 7810 wouldn't cut it with the peak input voltage. Only 3mA output current, but the impedance of the analog channel is appx 225.6k per the manual, that'll be plenty? The cap values were taken straight from the data sheet.A handful of things I already know about: I just grabbed footprints for the caps, resistors, & diodes. I'm pretty sure they're the wrong size. I couldn't find a polyfuse small enough for the output, they don't make one that'll hold 3mA, trip around 30mA (Ipeak from the data sheet)? The board layout is atrocious-I just threw the parts down. The first version I laid out pretty much like the schematic, but then I wanted to put some tracks on the bottom side. The combined board PDF was made by printing an XPS & converted to PDF via a tool, I believe it left a few artifacts, missing pads & barely connected tracks.

author: ataradov
author_rank: Super Contributor
timestamp: December 01, 2017, 02:33:40 am
content: Screenshot from a CAD would be much better. How do you expect us to review this?

author: ratio
author_rank: Regular Contributor
timestamp: December 01, 2017, 11:33:29 am
content: Sorry, I thought PDFs were wanted. Is this better?And thanks for your time.

author: daqq
author_rank: Super Contributor
timestamp: December 01, 2017, 11:56:55 am
content: Hi,So, my objections:Functional:@60Hz and a current of 3mA you will not get a good rectified output with a 1.0uF cap. Try a bigger one. Try simulating it.In the schematic:You do not have connection dots in between the diodes, namely between D2 and D4 or D1 and D3.You do not have the ground named as such (or connected to a ground symbol) - cosmetic for such a small device, but useful.Naming nets adds to the schematic readability.In the PCB:QuoteThis pretty much negates anything useful we can add :-) When you route it with the correct packages, then feedback from us will be more useful. That said:Try doing it with one layer.Think on the layout of individual blocks rather than the whole. In this case, try laying out the rectifier block, then the LDO block, then the divider block. The layout will work, it's just not nice.Don't forget to add mounting holes or other means of mounting.

author: ataradov
author_rank: Super Contributor
timestamp: December 01, 2017, 04:53:25 pm
content: Yes, the whole board can be clearly laid out in one layer. And also, put some board outline.

author: mariush
author_rank: Super Contributor
timestamp: December 01, 2017, 06:21:34 pm
content: Well, my suggestions would be1. Consider using surface mounted components, it's easier to design square pads for 0805 resistors and diodes of similar shape.2. Consider using a DIP or SIP bridge rectifier instead of 4 separate diodes (but 4 diodes is also ok for your need)3. 20 K resistors would not be as common as other values, maybe it would be worth just reusing 10kohm resistors so put 2 10k resistors in series there (you also simplify your BOM this way)4. No need to go to some weird regulators, there are common regulators which can work with 40v DC or higher on the input. Seehttps://www.digikey.com/short/q30vc3(and yeah, you need a bigger capacitor on the input, go for 100uF 50v rated or more/higher rating)5. don't put the output connector so close to input, especially if it's same connector. You may be drunk one day and shove 24v AC in the wrong connector

author: ratio
author_rank: Regular Contributor
timestamp: December 01, 2017, 06:53:44 pm
content: Thanks for the input. I'll go over things this weekend & we'll what it looks like the next time through. Bear in mind that I'm learning this as I go, so if you think "why even do that?!?", it's because I don't know any better yet.

author: ratio
author_rank: Regular Contributor
timestamp: December 02, 2017, 05:20:34 pm
content: Here's the latest revision, back to single side. I haven't yet simulated it daqq, I'm assuming that you mean LTSpice? Is there something else I should be looking at? I'm looking at the other regulators, but one step at a time. I'm trying not to get overwhelmed with learning everything at once.

author: ataradov
author_rank: Super Contributor
timestamp: December 02, 2017, 05:53:40 pm
content: Why it is so sparse?

author: stmdude
author_rank: Frequent Contributor
timestamp: December 02, 2017, 06:12:22 pm
content: What's going on with those caps? There's two extra pads underneath each

author: ataradov
author_rank: Super Contributor
timestamp: December 02, 2017, 06:13:27 pm
content: Quote from: stmdude on December 02, 2017, 06:12:22 pmSo you can mount whatever size you have?

author: ratio
author_rank: Regular Contributor
timestamp: December 02, 2017, 08:33:01 pm
content: It's sparse because I don't really know what I'm doing. I can see some things I can do to tighten things up, and I'll be working towards that as well.The cap footprints are indeed multi-sized. It might not be apparent from the screenshot, but the two cathodes on the cap aren't connected in the footprint. I'm pretty pleased that I caught that-it sounds like missing that would be a rookie mistake!

author: ratio
author_rank: Regular Contributor
timestamp: December 04, 2017, 02:01:46 am
content: I think this one would actually work. I changed the regulator to a LV8-still an oddball, but this one has internal limiting, so I should be ok eliminating the polyfuse on the output, right? I put a ground plane on the back side, the pads that are highlighted are connected to it. I tried to add some copper fill on the top side too, around the regulator to heat sink it, but while they all show in outline, only one shows as filled, either when I click "show filled areas" or in the 3d view.