CNC Motor Power Supply

A table of this size required building fairly large power supply to drive the table's servos. This documentation describing power supply formulary says that two servos requiring 10A should never exceed 2 * 10 * 67%, or 13.4A.

The manufacture Plitron sells toroidal transformers. They have some useful technical notes. To calulate the desired transformer voltage I used the formula: (68VDC/ 1.4) = 48.6 VAC. However, when I constructed my power supply I used a formula supplied by Plitron which uses a slightly different method of:

(68VDC + 2) * 0.8 = 56 VAC.

Using this required that I make the modification described below. But since I went with a 55VAC toroid I went with a 13.4A * 55VAC = 743 VA rated toroid transformer. I purchased plitron transformer 117042201, with two 55v secondaries @ 9A each, $139.73. See plitron's toroidal ratings.

At this amperage and using the formula ((80000 * I) / V) I estimate I would need around ((80000 * 18) / 68) = 21167 uF filter capacitor. I purchased five Model#: 3VTLM153M80V, 15000uF, 80V electrolytic caps on ebay and I'll wire two in parallel. I also purchased 4, 25 Amp 200 Volt bridge rectifiers,$4 each.

I made a spread sheet that helps perform these calculations.

CAUTION: the capacitors in the supply store a lethal charge after powering up. The resulting discharge has the potential to be very unsafe. This is typically experienced when you're just hooking the thing up to the rest of circuit and comes in the form of a firecracker-sized explosion. This is primarily a problem with the power supply is not hooked up to anything and its best to keep the output terminals shunted when not in use. Fortunarely the manufacturer of the Gecko drives provisioned for removing the energy stored in the caps and the overall is system is much safer when the Geckos are connected to the supply. Knowledgable designers should consider adding circuitry that will safely discharge across a power resistor; although I have not been able to get advice on exactly what that circuitry would look like.

The suply employs a full wave bridge circuit and put into a canibalized Sun harddrive enclosure. Its a nice box that comes with fans and a 5VDC power supply. Most wiring was done with 14 guage wire, terminals, and screw-down terminal blocks. After completion I looked at the voltage on my osciloscope and there was absolutely no ripple.

Over-voltage repair

VDC = 1.4 * VAC

The voltage is 79vdc. Lowering the voltage of the transformer required that I remove some wraps from the Plitron torroid. I drilled out the epoxy core on the drill press in about ten minutes. The heat shrinkable wrapping around the windings completely prevented the epoxy from entering any windings. After chipping out the remaining epoxy block I removed the heat shrink wrap. The secondaries wires were very accessable and I didnt have any problem unwinding them.

I went with about 5 windings first prior to testing. I carefully checked for shorts between any of the exposed wires. (Its a bit of an act of faith that the enamel around the wires will prevent any shorting but it does the job.) After checking as much as I could, I hooked up the transformer and measured the voltage. The first time the voltage the wasnt even close. I unwrapped some more, eventually got to the right voltage, trimmed the secondary wires, soldered new connectors and put heat shrink tubing around solder joints.

Heatshrinkable sheet was wrapped around the donut. I didnt like that result but left the sheet on, and followed up with lots of wraps of electrical tape. I popped the transformer back in the power supply enclosure and I'm operating at the right voltage.