Power SuppliesThe drives require low voltage direct current power. Since power from the wall receptacle is a higher voltage and alternating current, a specially made power supply is required. There are commercial units available but they tend to be expensive, and the cheap ones can be underpowered. Therefore many DIY builders make their own.The wall voltage has to be lowered, fortunately this is easy with the use of a transformer. The current also has to be converted from AC to DC. This too is straightforward with the use of a rectifier. A catch is the rectified current will pulse since it was converted from AC. This too is easily remedied with the use of a capacitor, which acts something like a buffer or storage tank; pulses in, smooth flow out.  The
transformer has to be able to handle the large
current that the
steppers require, so a little $5 doorbell transformer, shown below
with the toroid, though perhaps of the correct voltage, will be unable
to deliver enough power to some systems.Larger transformers, for the popular drives and steppers, cost in the $35 to $100 range. The less expensive larger transformers, shown on the right, are less efficient than the more expensive toroids (donut shaped) transformers, shown below.  (The ruler with the toroid is 6 inches long; the
rectangular
transformer on the above right is around 4 inches wide and high.)Toroids usually run cooler and can deliver a higher current than similarly sized laminated body transformers. ............. The
secondary or low voltage wires from the transformer are connected to
the
rectifier, which is a set of diodes, or one way valves, that allow the
current to pass in only one direction, thus turning AC into DC.  The rectifier has to be sized for the power that will pass through it or it will cook. A rectifier can be made from four diodes, or a bridge rectifier can be used which combines the four internally. The image on the right is a bridge rectifier; it is about 1-1/4 inches square. The DC positive and negative sides of the rectifier are attached to the capacitor which smoothes out the pulsing DC. ............. 
The capacitor also has to be sized for the power, and large capacitors
can
cost a few tens of dollars. However, it is possible to use two half
sized capacitors attached in parallel to do the same job.The smaller capacitors are significantly less expensive, so a few dollars can be saved, even though two are used in place of one. .............
The math for deriving the sizes
of components is in a document from Geckodrive (pdf document page 11)
that is linked on this site’s Links page. The document also includes wiring diagrams. The variables include the maximum voltage or current the steppers and drives can tolerate before they are destroyed by overheating. The math is simple, but for those unfamiliar with this sort of work the entire process can be daunting and dangerous. Fortunately there are other options. ............. Xylotex sells a power supply
with the package system; simply attach two wires to the drive board and
plug the other end in the wall and the power supply set-up is done.
This site's Xylotex page shows more.HobbyCNC sells all of the properly sized components for a power supply with the exception of the transformer itself, but a supplier’s link and model number are given, so there is no risk of using mismatched components. However, the parts do have to be wired together by the builder. .............
Other options include computer power supplies, and rewinding microwave
oven transformers.Computer power supply outputs are usually too low for many CNC systems, but they have been used. Rewound microwave oven transformers work but the process can be tedious.  The image on the right is of a homemade power supply made from some of the HobbyCNC components as well as a rewound microwave oven transformer. The transformer's original secondary coil was removed with a hacksaw and rewound with new wire. It took around 15 feet of wire; one turn produces around 1 volt. These microwave oven transformers can kill. If in doubt about the process do not attempt it. This homemade microwave oven transformer works okay, but not as well as the toroids that were used on other systems. It is also a lot of work and potential danger just for saving a few dollars. The microwave oven transformer becomes much hotter than the toroids; it also interferes with the computer monitor when close to it, but the transformer was free from the scrap bin. The fan is powered by a stray wall wart; were there room, another smaller coil could have been wrapped around the microwave oven transformer, and once rectified, used to power the fan. Or a 115 volt fan could have been powered by the main. .............
 Antek sells toroids at a good price. One of their $55
toroids was used in this shop to
power some Gecko stepper drives, and it has proved to be a good product
at a good price. It is the smaller unit in the right photo. The
larger Antek power supply shown on the far right is used with a bench
mill; it was around $200.As shown on the Controls page, a Plitron toroid was used for the servos on this shop's 4 x 8 ft. CNC table; it also is a good unit and can be custom built to specific voltages, but the toroid's cost is around $100 delivered. |