Power Supplies |
|
The 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 many DIY builders make their own. The wall voltage is lowered with a transformer. The AC current is converted to DC with a rectifier. The rectified current's pulses are smoothed with a capacitor. |
|
Transformers
The transformer has to be able to handle the current that the motors require, so a little doorbell transformer, shown below with the toroid, though perhaps of the correct voltage, may not be able to deliver enough power to some systems. Larger transformers for the popular drives, steppers and servos, cost in the $35 to $100 range. The lower cost larger transformers, shown on the upper right, are less efficient than the 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. 
Laminated body transformer.
Doorbell and toroid transformers.
|
|
Rectifier
The secondary or low voltage wires from the transformer are connected to the rectifier, which is a set of diodes that allow the current to pass in only one direction, thus turning AC into DC. A rectifier can be made from four diodes, or a bridge rectifier can be used. The rectifier has to be sized for the power that will pass through it or it will cook. 
Bridge rectifier, ~1.25 inches wide.
|
Capacitors
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 cost much less, so a few dollars can be saved, even though two are used in place of one. 
One large = two small capacitors.
|
|
Documentation
The math for deriving the sizes of components, with wiring diagrams, is in a document from Geckodrive (PDF document page 11) that is linked on this site’s Links page. A number of suppliers such as Xylotex and HobbyCNC include power supplies, or the components and directions, with their packaged systems. |
|
Power Supplies
The image on the right is a power supply for the HobbyCNC system. It uses the components that are supplied in the HobbyCNC 3 axis package (switches, rectifier, fuses, capacitor, etc.) as well as the transformer that is recommended by HobbyCNC. This power supply unit was wired together according to the HobbyCNC directions. The directions are thorough and straightforward. Minor alterations were made to power the additional fan since separate boxes for the drive and power supply were used. Other options include computer power supplies, and rewinding microwave oven transformers. Computer power supply outputs can be too low for many CNC systems, but they have been used. Rewound microwave oven transformers work, but the process can be tedious. The transformer's original secondary coil is removed with a hacksaw and then rewound with new wire. Each turn produces around 1 volt. The homemade microwave oven transformer shown here works okay, but it was lot of work and potential danger for its quality. It becomes much hotter than the toroids, and it interferes with the computer monitor when close. 
Power supply using HobbyCNC components.
Rewound microwave oven transformer.
These microwave oven transformers can kill. If in doubt about the process do not attempt it. |
|
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. It is the smaller unit in the right photo.
The larger Antek complete power supply shown on the far right is used with a bench mill. This pre-made power supply was ~$200. The second image is of a temporary setup of a toroid from Plitron that was used for the servos on this shop's 4x8 CNC table. It also is a good unit and can be custom built to specific voltages, but the price was higher at ~$100 for a custom size (toroid only, similar to the non-custom $55 Antek). At the top of the capacitor in the second image are four resistors that are wired in parallel. These bleed the charge from the capacitor when the power is off. A pilot light, as described below, has proven to be a better alternative. 
Small homemade and larger commercially made power supplies.
Toroid and capacitor with bleeder resistors.
|
|
Pilot Light
In the helpful hint department. To prevent electrical shocks, the capacitors should have a resistor across their terminals so they will discharge when the power is turned off. In this shop a 110v 4 watt night light bulb is used for this load. The lamp serves as a pilot to show that the power supply is on, and it dims out as the capacitors discharge after the power is turned off. These small bulbs have worked well with a variety of power supplies. They will glow in the 30v to 75v range of voltages used in this shop. 
Pilot light with power supply.
|