PCB Design + Project box help - Page 1
Forum section: electronics / beginners

author: Kamkut17
author_rank: Contributor
timestamp: March 27, 2016, 04:05:56 pm
content: I've been working on designing a PCB for my Linear Power Supply. The design overall looks ok (I would like some feedback on it though), but it is tiny, (66.04x30.48mm), and it seems too small for any Project box I find on Maplin, Ebay etc. I cant find any project box manufacturer (From Europe) that sells enclosures around those dimensions, If you know any, reply with their name, thanks.This design uses 2 LM317's as voltage and current regulators, I'll have an ammeter and voltmeter (digital) to display the output voltage/current. I'll be using 2 10-turn pots to change the voltage and current precisely, and I'll have 2 binding posts for the output. If you would do anything differently, let me know too! (also, this is my first time designing a PCB, so let me know if there are problems and what-not)

author: michaeliv
author_rank: Frequent Contributor
timestamp: March 27, 2016, 04:47:33 pm
content: For project box there's this from China, do you need specifically from Europe ?:http://www.ebay.com/itm/Hot-DIY-Electronics-Plastic-Project-Box-Enclosure-Case-Waterproof-80x50x21mm-Fin-/291666652059For the design - depends on what you want it to do. On first look, since you're passing the output through a potentiometer your supply will have a very low current limit, ~100ma or less. Potentiometers are generally not intended to pass high currents through. Also, it wouldn't hurt to have thicker tracks on the PCB.Also your voltmeter won't display the output voltage, but the maximum output voltage. If the current limit kicks in, the output voltage will drop but the voltmeter will not show this.

author: uncle_bob
author_rank: Unknown
timestamp: March 27, 2016, 04:55:49 pm
content: HiIf a project is headed for an enclosure, the first thing to do is to pick an enclosure. You then measure out all the mounting points and transfer that information to your PC layout software. Effectively, the board mounting process defines the size of the board. It may define edges that slide in slots. It may define holes that line up with mounting bosses in the enclosure.If the project is something that will handle power, heat sinking is part of the process as well. You often will heatsink parts to a metal case rather than buying fans and heatsinks. The metal walls of the case will dictate the locations of your "hot" parts on the board. Again, the enclosure dictates the layout.If the project has connections, they also will interact with both the enclosure and the layout. For most "stuff" bringing in bulk power at the back makes sense. Connecting instruments up on the front or back may make sense. Anything you need to adjust or any fault indicators need to go on the front. In most (but not all) enclosures, the connections and indicators are mounted at right angles on the ends of the pc board.Some simple enclosure types:1) A folded metal box: They have 6 sides, generally of aluminum and a flat bottom plate. The opening in the bottom is a bit smaller than the inside X,Y dimensions of the box. Connections all are out the "top" of the box if they are PC mounted or you hook them up with wires. They are pretty easy to heatsink things to.http://www.budind.com/view/Aluminum+Enclosure/Aluminum+Chassis2) A die cast box: A cousin of the folded metal box. They are a bit better looking. They come in a few different configurations. The common ones have a full sized bottom opening. Otherwise they have the same issues as the folded metal box.http://www.hammondmfg.com/dwg.htm3) An extrusion (tube) box: The body of the box is an aluminum extrusion. It makes up the top, bottom, and left and right sides of the box. You get plates to make up the front and back of the box. The PC board slides into slots in the extrusion. This makes it easy to have PC board mounted parts out the front and back. Heatsinking to the case is not so easy.http://www.hammondmfg.com/sinkbox.htmFor most of what I do, I find the extruded boxes on eBay. I may spend a while shopping. I may get 10X more than I need. They rarely cost very much money at all.--------You might want to think a bit about what your "current limit" 317 does to the voltage regulation ...Bob

author: Kamkut17
author_rank: Contributor
timestamp: March 27, 2016, 05:31:33 pm
content: Quote from: uncle_bob on March 27, 2016, 04:55:49 pmThanks for the tips, I will probably re-design everything and do as you said, but I'm not quite sure what my current limiter will do to the voltage regulation. The only thing I can think of is it using up some of the voltage, and since my voltmeter is wired in parallel to the current limiter, it might give off an incorrect reading, but I'm not quite sure. Sorry if I'm asking some dumb questions.

author: AndrewDojo
author_rank: Contributor
timestamp: March 27, 2016, 07:15:37 pm
content: 3D printing an enclosure to fit the board? Done that a few times. Not as pretty as an injection molded box but functional.

author: uncle_bob
author_rank: Unknown
timestamp: March 27, 2016, 11:55:39 pm
content: Quote from: Kamkut17 on March 27, 2016, 05:31:33 pmHiAt least based on your schematic, the current limiter is in series with the load and after the voltage regulator. The voltage to the load will be the regulated voltage minus the drop across the current regulator. The drop on the current regulator will vary depending on the current through the device. That will degrade the voltage regulation at the load.Bob