Selecting a Case

I need to fit most of this lot into a case

 

Aluminium, not too big and I want it to have some sort of decent aesthetic given it will sit on my desk for some time.   There are some very nice cases out there, mostly at an exorbitant price!  It’s not that easy to search them out either: forget eBay or Amazon as there is no way of filtering by size and I don’t have the patience to go through page after page of irrelevant entries.  I started out with a ‘rough’ size requirement but the main suppliers don’t provide filtering on internal dimensions; once you start you rapidly find out that there can be a fair bit of difference between the two, especially if the case is extruded aluminium.  Anyway, after quite a few hours of searching, I settled on one from Takachi Industrial but after some thinking about the ease of cutting holes and consideration of supporting weight - it was, essentially, two 1mm U shaped pieces that wrapped together - I went with something a little sturdier and a bit better looking, still from Takachi: MS desktop series 210mm x 280 x 88mm - PDF catalog - with the optional mounting plate. 

 

This has flat front- and rear-panels which will make cutting out mounting holes easier, something that turned out to be an extremely wise decision.  It came really well packed and is very good quality.  2mm aluminium and working surfaces, by which I mean those that will take components, all have a protective film.  There are spare screws for both the case and mounting plate. 

Layout and Drilling Templates

Before committing, I wanted to make sure that the components would actually fit in without being too cramped and ensure that I could get a reasonable panel layout.  Doing this also ensures that the components that project into the case from the panel have room to do so without interfering with each other or with components inside. 

 

I made a to-scale layout plan using Omnigraffle (for the few who don’t know, a MacOS equivalent to Microsoft Visio) of an internal, top down view and front/rear panel layout.  It’s then possible to play around with layouts, wire routing and so on.  Here’s what I ended up with (1:1 scale, although the images are reduced in size of course):

 

Case internal layout

This is the internal dimension from rear of back panel to rear of front panel.  The internal projects are where panel mounted components will project.  The dashed line is the usable extent of the mounting plate.

Case panels, front and back

The dashed line is the extent of the 4Duino PCB and the gap in that on the left side is where the USB cable will enter.  It has a rigid connector which extends approximately 4cm so I had to be sure it wouldn't interfere with the load switch or banana sockets.

 

Drill pattern for front and back panel

From these layouts, a set of drill and cutout templates can be created, again 1:1 scale.  I can print these on transparencies and stick to the relevant parts of the case:

Drill pattern for the mounting plate

The cut-outs are precision-sized in the templates although in spite of component designers thinking otherwise, I don’t actually own any 10.3mm or 18.2mm drill bits.  Crack the rifflers out!

 

Affixing the 4Duino

This thing might be functionally good but 4D Systems have done nothing to help with mounting.  They don’t provide any form of fixing kit and you can see from the pics below that clearance for mounting bolts is tight - M3 holes, but one is so close to the Headers that it is unusable; others will have the nut (or bolt head depending on how it is mounted) extremely close to components on the PCB and LCD connector ribbon.

 

The visible screen area is an awkward subset of the overall LCD size so I don’t want the whole screen ‘poking’ through the panel.  That will require some precision cutting of a 49mm x 36.7mm panel hole that is very neatly finished - as seen in the template!  I also don’t want screw heads showing on the panel so I need to give some thought to how this will actually securely mount behind the panel.

 

This thing really needs access to a 3D printer to design and make your own bezel and it’s proving a bit of a headache to decide how I will mount it.  I worked out something which can be seen below, but I think it’s a bit of a shame that 4D Systems didn’t invest enough in this aspect of it.  It feels like a great device let down by its execution.

 

Cutting Out

I do have access to a pillar drill which will make most of the holes easy to create - a step bit for the bigger ones.  I also have a deburrer and some good finishing files.  For the rectangular holes I have a piercing saw and fret saw, with fine teeth blades (32tpi) and plenty of spare blades. 

 

Panels and mounting board prepped up, ready for drilling

 

And after cutting and removal of protective film.  Shiny goodness.

I’m pleased with the rectangular holes.  They aren’t perfectly square but for something done by eye and tongue stuck out the corner of my mouth they ain’t bad.  A CNC router would have made easy work of this I’m sure.

 

Encased Components

Mounting the 4Duino was a bit of a head scratcher but in the end I went with some nylon PCB mounts, wood and super strong fixing tape (55KG!)  The PCB mounts at 10mm plus the thickness of LCD and PCB came to just over 15mm and as luck would have it I had some 15mm scrap wood so with the tape the tops of the wood and mounts were level.  I cut out a couple of ties from waste aluminium and screwed the 4Duino to the wood mounts:

Not pretty, but that tape is super strong and it isn’t going anywhere.  I left it subject to gravity overnight and it hasn’t moved at all.

 

Building the case up was a bit of a jigsaw, but not too hard, although the instructions were interesting

And here it is populated:

Wiring is a bit of a mess but everything is secure.  The transformer has Kapton tape on the bolt head under the mounting plate and will have some of the bolt end before the top goes on.  The USB cable will allow me to re-program the 4Duino without opening up the case as well as use the USB as a serial port for writing/reading data.  The device can be powered by both its 5V regulated supply as well as the USB switching between the two as necessary.  Usefully, if it switches during a data write it doesn’t corrupt data - that scenario should never arise in practice but, you know....  I had a thread going not so long ago about mounting the thermistor to the top of the MOSFET: you can see my solution - two small pieces of mounting tape (red) and it seems to work, keeping the end of the thermistor securely on the MOSFET.  The lead leading out of the top of the image is for the Case thermistor which will be taped to the case top when I close it up.

 

Finished Supply

Here in all its glory, top, front and back panels:

I’m hoping that I can tune in the Pot dials to specific voltages and amperages - certainly at the 0 points it is supplying 0V 0A!  But if not, they’re pretty cool dials.  If you’ve followed along with the blog you will have seen it in operation and that hasn’t changed so I won’t bore you with a repeat of that.

 

Files

I’ve updated Post One with all the files I created as I went: design files, Gerbers, BOM, Spice model, calculations etc.  These are the most up to date. 

 

In Summary

As Vinnie Jones once said, “It’s been emotional”.  I’m very happy with the result although the journey has had its highs-and-lows.  Having said that, I have been able to take a ‘not quite working’ design and modify it to do so and add some improvements; designed and had manufactured PCBs; tested the circuit out with LTSpice; scratched my head on data sheets and figured out problems as they arose.  Given that this time last year I’d only just found out that a LM7805 was not a helicopter registration number I’m definitely chalking this one up as a successful first major project.

 

Of course, it absolutely wouldn’t have been possible at all without the amazing help and encouragement from Element 14 members - you guys are awesome.  I can’t thank you enough for helping me out when I’ve needed it, keeping me steered in the right direction.  This is really an amazing community.

 

And next?  Believe it or not, I’m going to have a go at another one, but this time I’ll design it from scratch.  Maybe not immediately   I’ve learned a heck of a lot about SMPS and there are a lot better switcher ICs out there to choose from now. 

 

Should you build one?  Well, it’s expensive and you could buy one of the cheaper supplies from Farnell or even a reasonable second hand supply for the price.  There is flexibility in the design to reduce costs though.  The Power switch, transformer and AC to DC power board can be removed and power supplied from a 24V 4A wall wart - I have a 24V 6A one which cost about £10 from Amazon.  That would mean a smaller case and one less PCB so there’s about £75 saved on all that alone!  You could use an Arduino Uno and a simple 96x2 Lcd by junking most of the code and that would save a further £30 or so - or do away with it altogether and just hook up a DMM on the output when you want to use it (take out the sensing INA260 to save yet more and bridge the gap)!  You don’t need the thermistors either and you could choose cheaper potentiometers and dials.  Altogether, it could probably be built for about £40 in components + £40 for a display/controller + £30 for a shipped 4-layer PCB + a case.  I do have spare PCBs, free for those who helped me out

 

Really, sad to say, I don’t recommend it given the cost.  I did it for the learning experience and interest without any expectation of ending up with a cost-effective bench tool.

 

That, as they say, then is that!  Ironically, two days before I take receipt of the Tenma power supply that baldengineer ran the competition for.

 

Back: Part 12 - Design revisited: reworking the layout and PCB

Back: Part One - Introduction