Building the Power Supply and Case

Case Design

The HobbyCNC power supply kit comes with most of the parts you need for the board power supply. You need to supply the case and transformer. I'm not really a fan of the equipment cases you can buy. I think they are over priced and it can be tough to find one of the right size. The HobbyCNC plans suggest a suitable case to purchase, but I decided to bend my own. Rather than just build a case off the cuff it's a good idea to do some space planning in a CAD program. I used the 2D CAD program QCAD to produce the design on the left. This is one of the best open-source CAD programs available for Linux.

Layout...

Rather than manually laying out the design, I print the CAD file as a 1-1 drawing and glue it to the 22 gauge sheet metal. I don't have a big enough printer to get the entire design on a single piece of paper, but I can piece it together from 4 separate printings so long as I'm careful with registration.

Cut...

Remove anything that doesn't look like a CNC power supply case. Handy hints: Its difficult to make large diameter holes in thin materials with regular drill bits. Using a stepped cone drill makes this easy and will also debur the edges. A sheet metal nibbling tool was used to neatly cut out the rectangular holes (line filter, DB25) and large circles (fan).

Get Bent...

Using a box and pan brake I bend up the tabs and faces of the lower case half. In this picture the metal has been bent and the paper layout has been removed. Turpentine cleans off any residual paper/glue.

Paint...

Blue glossy paint... Mmmmm...

Wire it Up

This is the bit where you are most likely to kill yourself. Since this is a metal case it adds to the danger factor. We need to carefully insulate the active and neutral wires to eliminate the possibility of them touching the earthed case. Since the case is earthed (from the line filter) any leakage current to earth would trigger the GFCI protection, but we certainly don't want to rely on that. Heatshrink is used on all exposed conductors and the crimp lugs are also of the fully covered variety.

The smoothing capacitor on the DC side is 56000 uF. In normal usage it has about 32V across it. That's about 29J of energy that can be discharged very quickly. Watch out! A resistor is installed across the cap to ensure it will drain to 0V over time, but this takes quite a while. When the power supply is switched off, it takes several seconds for the fan to stop and even longer for the LED to turn off. This capacitor must be treated with serious respect. The capacitor has been restrained in the case with some cable ties and nylon mounting blocks.

Label It

I've used a labeling machine to mark the fuse ratings and the axis designations. Everything else is self explanatory. Unplugging a stepper motor while turned on is a definite no-no: the collapsing magnetic field in the motor coil will cause a large voltage spike that can damage driver chips. To prevent inadvertant disconnections I have used DIN connectors with locking/screw-in plugs.

Complete!

Using the same techniques as before, an upper half is made to the case. Suitable vent holes are added to ensure the fan sucks in the air, blows it across the motor driver heat sink and then out. The case halves are held together with sheet metal screws. Individually the case halves flex quite a bit, but when they are screwed together the whole assembly is a rigid box.

Hobby CNC Product Support

Product support is provided via the HobbyCNC Yahoo Groups discussion forum. This forum is only open to people who have purchased HobbyCNC products. I didn't need any support to get my board working, but judging from board messages, the skill and knowledge level of purchasers varies widely.

Conclusions

The HobbyCNC 4 axis Stepper Driver is great value for money. Construction was without a hitch, and the board worked first time. The instructions were clear and straightforward. A board schematic would be useful if you run into problems, but fortunately I did not. Overall I would recommend this board to anyone who wanted to control stepper motors for homebrew CNC applications.

Resources

Here are some handy links for the HobbyCNC stepper driver board.