 D.I.Y. Power Supplies. Here are a couple of basic suggestions. Linear (transformer) power supplies are fairly simple, and very easy implemented. The first hurtle you run into is the cost of the transformer. Where to get a inexpensive (cheap, man) transformer? A common everyday 12 volt battery charger! 30 bucks at Wal-Mart/K-mart, or 5 to 10 bucks at a yard sale!
If you buy one new, get the simplest manual charger ... you won't need any of that "automatic" stuff inside anyway. Heavier means more transformer inside. Older chargers are almost always heavier. Most chargers are equipped with a center-tapped secondary and two half-wave diodes. Gut the diodes, ignore (and insulate) the secondary center-tap, attach the two remaining leads to a full-wave bridge such as a Radio Shack 276-1185 ($2.69), run the D.C. output into nice big (10,000mfd+ @ 50DCWV) capacitor, and you should have a nice 34 volt D.C. power supply for somewhere between 10 and forty bucks, depending on your resourcefulness.
Add-ons. On the D.C. positive side, a quick-blow fuse fuse is a good idea, as well as fusing each individual drive supply line. Across the D.C. output, a 0.1 mfd ceramic capacitor for bypass, and a Zener diode to clamp voltage down to less than the 80 volt maximum of the GeckoDrive, and also less than the working voltage of your filter capacitor.
A "forward blocking diode" could also be placed in series with your fuse. You'll loose about a half of a volt, but it will insure that your filter capacitor does not "see" both the voltage coming from the bridge rectifier, and the voltage being "generated" by a decelerating stepper motor, at the same time. If the "Back E.M.F." voltage of the decelerating stepper plus the voltage produced by your bridge rectifier exceeds the working voltage of your filter capacitor, the cap could blow, an event that will surely get your attention.
Other Transformer Sources. Wheeled battery chargers. The new ones, like the 40 amp jobs found on sale for $99, have a transformer about the size of a loaf of pumpernickel. Truck chargers are ideal, as they are made to charge 24 volt batterys. However, you won't know if they are using two half-wave rectifiers or a full-wave bridge untill you "open it up". Those 100 amp "Cracker Box" A.C. stick welders have usable transformers.
AT and ATX style computer power supplies are switching power supplies. They have a very limited ability to store reserve power, as they are designed to respond almost instantly to a computers' power demands. Even so, there is quite a bit of difference between cranking up a cd-rom drive and cranking up a stepper motor. So for all intents and purposses, all of the application caveats noted above for the "battery charger" solution, apply to the "ATX" solution.
AT/ATX supplies are awash on eBay for very cheap, and even new ones can be found for $25 to $40. A 250 watt supply should have 12 volts D.C. at 10 amps coming out of it (230 watt supplies put out 9 amps @+12vdc). Now 12 volts won't "get it", but two or three AT/ATX supplies with their +12vdc leads connected in series, just like a battery, will get it. IF ... the minus (black) leads are NOT common for the +5vdc and the +12vdc, it seems logical that each supply could put out 17 volts, but I haven't had time to play with the idea, and don't have any schematics for these switchers. If you know, write me and I'll put this info here.
For used AT/ATX supplies, insure that there are no ground faults (shorts) between any of the power leads and the case. Always check out each supply before attempting to series them up. Only the Green lead within the five wire power switch embilical should be grounded to the power supply case itself. Figure if it is used and out of service, there may be a reason.
How large of a filter and/or storage capacitor do I need?
First, establish your working voltage, and figure a minumum of a 20 percent margin above your system voltage, to get a reasonable D.C.W.V. (D.C.Working Voltage) for your cap. If you are running at 40 volts, get a 50 volt cap. minimum. To the math.
* C = Capacitance in micro-farads (mfd.)
* V = Volts (in volts :-)
* I = Current in Amperes
C = (80,000 * I)/V
"I have a 50DCWV 7,500mfd cap in the bin; how many amps is it good for?"
I = (C * V)/80,000
I = (7,500 times 40) divided by 80,000 = 3.75 amps
Not enough amps capacity? Well, yes you can "bank" capacitors in parallel, but this is best done with exactly identical capacitors. For example, if you "banked" three of the identical capacitors shown in the above example, you would have 11.25 amps of capacity. Of course, your power supply has to be adequate to keep the capacitors charged up. A single supply will be more energy efficient, unless it is drastically oversized. Wiring and fusing is simplified, and generally makes for a cleaner install. Also, care must be exercised if you use the "daisy chain" method described above with the AT/ATX supplies. |
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