The DC load needs an enclosure and finding one always turns out more difficult than you think. This is a common fate of many electronics project, often you start with a prototype, the PCB and wires etc are on the workbench, and in a second iteration you rebuild everything so that it fits in a standard Hammond enclosure. These boxes are expensive, some of my ham friends make them from double sided PCB. An alternative is to look for a machining tool which allows you to make almost any aluminium enclosure that you like.
For this project I made an aluminium box, something I’ve never done before. To machine 1 mm aluminium sheets you need a tool that cuts and bends the material, the used HBM tool can even roll sheets and bars. There is only one drawback, it weighs 45kg and I need it on the second floor, so I threw my back out but a week later I’m doing a lot better. Aluminium sheets are affordable, 1000 by 1000 mm aluminium at 1mm is about 36 euro, please ask them to cut it as 300 by 1000 mm because of a limitation with the HBM tool unless you got more space in your workshop to get something larger. But 300 mm is a nice size, if you need something larger then consider to reuse 19 inch rack enclosures. Also, if you go beyond 300 mm then 1 mm sheets are not enough to provide stability so that you need a frame within the box. Up to 300 mm you don’t need thicker aluminium or a frame unless something heavy like a transformer is part of the project.
The first thing to machine for this project is a conduit for the heater coil in the DC load. The maker-beam frame in the second image is only used for orientation and placement of all components before you start on the box. The conduit consists of two L shaped parts and in the end I used rivets to attach them together. Forget about 3mm nuts and bolts, instead use self drilling screw and rivets.
And after about 2 days of work you get the following:
Mind that the relays have a 8 Amp limitation up to 250 Volt. All loads that you connect should have a separate fuse. If you go over 8 amps then chances are that the relays start to fail, contacts weld together etc. A worst case failure is that relay R1 and R2 and R3 stay in a M (make) position (the used relays have a make and a break contact). In case of a failure the 0.15 Ohm resistor is your only savior, and a fuse between the battery or power supply and the DC load should take over. It actually happened once which is the reason to print the warning on the front plate of the enclosure.
Last update: 13-Jan-2020 7:20 (spelling correction, and additions)