DC load in an aluminium enclosure

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.

plaatwerk
HBM combi machine for cutting, bending and rolling, weight: 45kg

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.

IMG_0444
Left: conduit with heater coil and computer fan,  right: PCB with relays.

And after about 2 days of work you get the following:

IMG_0467
The aluminium enclosure consists of two U shapes that fit into another. Due to the limitation of the HMB combi machine you have to make each U shape from three parts. Finally use rivets, self drilling screws and washers to keep everything together.
IMG_0468
Display and rotary encoder / push button switch installed, separate system fuse and power and measurement connectors on front and backside,
IMG_0469
DC load in action discharging a battery

Schematics:

IMG_0470
The software and hardware is rather straightforward Arduino stuff, you select an resistance (2.5, 5, 10 or no Ohm) and the discharging starts, every minute it measures the unloaded and the loaded voltages and both values are shown on the display.
IMG_0471
This is the correct relay logic. The monitor port is not used by the Arduino, it is intended for a separate volt meter to obtain the current. The conversion of voltage over the 0.15 Ohm resistor to amperes requires a calibration constant.

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)

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