Is brass a good choice for projects case?
Basher
Posts: 25
I've finally finished tweaking my projects circuit boards and its time to install them in some kind of enclosure.
I was originally planning on making something out of acrylic sheet or possibly casting something out of epoxy resin.
However, I've found a couple of old saxophone bells in my spare parts box. ( I repair saxophones and other woodwind instruments for a living.)
There's enough good sheet brass in one for me to hammer and braze into a sturdy enclosure.
I just wanted some advice before I start.
1. How safe will a brass shell be? my project runs on a Li-ion battery which is charged in circuit from a 5v wall adapter.
I suppose if something went badly wrong with the wall adapter the case could get connected directly to 240v ac.
How likely/unlikely is this and what could I do to mitigate the danger?
2. Is a brass enclosure likely to introduce noise into my circuit? I've read that when designing PCB's you should avoid creating areas of unconnected copper because they act as antennas picking
up noise which then gets coupled into your circuits. My project consists of a z80 , a 128k sram, a propeller, 2 SDcards, a LCD screen and CPLD's connecting them all together. It all
works fine as bare pcb's on my workbench. I don't want to waste time making a case only to find it interferes with the operation of the circuit.
I assume that it would be best to connect the case to the projects ground at one point as close to the wall adapter connection as possible.
3. Would I be better off making a case out of plastic? if so why?
Any thoughts and/or advice would be welcome.
I was originally planning on making something out of acrylic sheet or possibly casting something out of epoxy resin.
However, I've found a couple of old saxophone bells in my spare parts box. ( I repair saxophones and other woodwind instruments for a living.)
There's enough good sheet brass in one for me to hammer and braze into a sturdy enclosure.
I just wanted some advice before I start.
1. How safe will a brass shell be? my project runs on a Li-ion battery which is charged in circuit from a 5v wall adapter.
I suppose if something went badly wrong with the wall adapter the case could get connected directly to 240v ac.
How likely/unlikely is this and what could I do to mitigate the danger?
2. Is a brass enclosure likely to introduce noise into my circuit? I've read that when designing PCB's you should avoid creating areas of unconnected copper because they act as antennas picking
up noise which then gets coupled into your circuits. My project consists of a z80 , a 128k sram, a propeller, 2 SDcards, a LCD screen and CPLD's connecting them all together. It all
works fine as bare pcb's on my workbench. I don't want to waste time making a case only to find it interferes with the operation of the circuit.
I assume that it would be best to connect the case to the projects ground at one point as close to the wall adapter connection as possible.
3. Would I be better off making a case out of plastic? if so why?
Any thoughts and/or advice would be welcome.
Comments
Well those were the days of building HAM radio gear with tubes. 500 volts flying around, no problem, never mind the puny mains supply voltage.
What you must have as a minimum:
a) A transformer between your mains and your low voltage side. Which you have in the wall wart.
b) VERY IMPORTANT: A very well secured connection between the metal case and mains ground. I'm talking big fat green wire with an eyelet, or whatever it is called (help someone), crimped on the end which is bolted to the box. Use lock washers as well.
c) VERY IMPORTANT: Fuses. Have a fuse in the low voltage power input as soon as it enters your box. Size the fuse as small as possible.
What worries me with these things is that a regular wall wart does not have a ground connection.
A well grounded metal box should be very good at keeping noise out of your circuit and stopping noise getting in.
A nice polished shiny brass box could be very nice I think.
They have been extensively tested for safety. Very unlikely as long as the metal box is connected to the ground plane or VSS of the Prop. You can't get a better shield from external noise in the environment than with a nice metal enclosure. More is better.
The standard rule of thumb is ground to the metal chassis at every enclosure entrance and every opportunity possible.
The exception is for some low level analog signals but everything else should be grounded. I can't see any technical reason why plastic would be better than metal.
Esthetic reasons maybe. I bet you can make the metal look better than plastic, especially in a steam punkish way.
My favorite metal enclosure are tin candy and cookie boxes and soldered together raw copper clad PC board material.
Duane J
This is extensively tested, (I used to do this testing in my former life), so I don't have much worry about them, unless their damaged in some way.
Duane J
Fuses. I take the max current rating from the data sheet of each component in my circuit add them all together and fit a fuse as near that as possible to that figure?
Sorry if thats a dumb question. I'm self taught with electronics and I usually just try things and see what happens, but that's not really appropriate here is it?
But then there is the recent case I was called in to look at.
Equipment A was connected to Equipment B via an RS422/485 link. Only trouble is everytime they made that connection one of those two stopped working, permanently.
Why they called me I don't know, I'm supposed to be the software guy. Anyway a quick probe around with a scope showed that one of those two units was not grounded and was floating 50 odd volts up and down at mains frequency. Boom, there goes the RS422 inteface chip.
Oh, yeah, I know one young guy whos entire lifes posessions were recently destroyed when his appartment burned out. Cause - a faulty wall wart.
be U.K. only though. I note one of my power blobs(*) has 14 different logos on the label - only one
of which I recognise (UL)
(*) "Wall-wart" never applies to me, I never plug them into the wall, its the 4- or 6-way adapter lead that plugs into the wall
and they sprout from that.
Strictly speaking this thread belongs in the general discussions, not the Propeller forum. If you had mentioned a Prop rather than a Z80 then it would have been eligible
However I'm always intrigued by the number of projects that are designed and *then* they start looking for a project box. I can't help but look at enclosures first to get the right look and feel and *then* build the project to fit. The RPi is a classic what not to do in terms of picking an enclosure last. Their aim was to fit it all onto a credit card sized board but totally neglected all sense and sensibilities related to actually incorporating it into an enclosure or with other equipment. The connectors are not flush (to put it mildly) with each other and they are placed on every side of the board as well. Try fitting that into any kind of enclosure neatly. There's another project which is an oscilloscope for Android and you would thing that obviously it would have to be placed in an enclosure yet they amateurishly sandwich it between two sheets of perspex because they can't find an enclosure to fit!
Now back to the brass. Is there no enclosures you can find to suit anywhere? Sometimes I buy a cheap doodad just for the case or use the case off some old bit of equipment. True, I have various metal sheets here that I could theoretically bend up into a box if I am really desperate but I haven't had the occasion yet to do so. Plastic is so easy to work with and I have a small high speed Dremel type drill with a small bit on the end of a flexible shaft that I use for cutting as well as a scroll saw and a cordless jig-saw too (amongst other things). Then there are the polyester laser label sheets I use for prototypes which are really good looking, just like the real thing. In fact I make various labels and when I settle on one I can easily pass the artwork to the label manufacturer if the unit makes it to production knowing exactly what I will get.
So it may be that you have an OTS PCB, perhaps even from Parallax themselves and I understand the problem, just like I mentioned about the RPi. It really is sad and frustrating when these boards are designed without any consideration for enclosures.
I think you are right.
However I am willing to forgive the Pi designers on this as they seem to have spawned a whole industry of Raspi case makers. Some of those cases are realy cool. http://shop.pimoroni.com/products/pibow
Thats more or less what I did to my Nascom1's UART, when i stuck a Creed 7B on it. No earth on the Creed and hello half mains voltages from the suppressor caps etc.
Alan
I was inspired to start this project because I found a cheap digital photo frame in my local supermarket and I thought, "If I drill some extra holes in either side of the screen and fit a small joystick and a couple of buttons that would make a cool handheld retro game." However I had to abandon the original case idea eventually because my main press and peel circuit board had outgrown it and secondly I realised that most of the sinclair zx spectrum games my project runs need more than 2 buttons and a joystick so I've added the 40 spectrum keys around the screen.
I don't think I really designed it at all, it just sort of grew
I agree with the points you've made, for my next project I'm going to set out some proper design objectives at the beginning.
As for the brass I was thinking of using it because it's recycled therefore free which I like! I want to be sure it's safe and not likely to cause problems for the circuit before I use it though. It's a one off project so I don't really have to consider cost too much and I don't mind spending a bit of time on it, obviously a brass case would be too expensive both in labour and materials to put into production.