Info on Power Supplies

There are a couple of threads in which questions about power supplies have been raised. We will try to provide a little info here:

1. We don't want to discourage anyone from trying out surplus power supplies. Supplies in the range of 12-35 volts DC with adequage current capabilities should work fine for the motor supply side of the driver system (the yellow and black driver wires). You will, of course, need to keep the control side hooked up to the 5v being provided by the box. 'Adequate' is related to the number of driver boards you are going to hook up to the new supply. On a standard ShopBot, one driver could draw up to 4A, though it is unlikely that this would happen for very long because we shut the drivers down to a standby current if they are stopped. You don't need to run all the drivers from your new supply, just the ones you want the improved performance from. But if you were running them all, and had 5, then 18-20A would be a nice safe capability for the supply. In practice, you can get along with less than that. On a tool with just 3 driver boards (the original cable-drive ShopBots), the 9.5A of the PC power supply seems to do fine.

2. The reason you may be interested in another power supply is that a higher voltage supply could improve your higer speed performance. Stepper motors have less and less power, the faster they are stepped. This is largely because as inductive loads it takes them time to reach there full power (current draw). The faster they are stepped, the less time each coil has to power (load) up. As an inductive load, if you hit the motor coil with a higher voltage, it will get to power (load up) more quickly. Thus with a higher voltage power supply, you are capable of stepping at a higher speed and still having power [the driver chip comes in here, because when the motor loads up at the higher voltage, it must be controlled in order to not suck even more current ... the humming you here from your motors at rest is the driver chip chopping the power to the motor so that it does not draw more current than it should ... somewhat analogous to a light dimmer]. So, this means that in the best of all worlds, a little more voltage will get you a little more speed. In theory, changing power supplies will not have much effect on the power you have a low speed (say .5in/sec), because here, even at low voltages, the motor has adquate time to power-up on each step.

3. A 12 volt bench power supply (like one you might buy at Radio Shack) typically puts out 13.8 volts DC. The power supply in a PC, though rated at 12v, often only puts out about 11 volts under a load. Thus, just switching to an external 12v power supply can often improve top end speed a bit. The 3volt increase will translate into a 10-20% increase in the speed you'll be able to jog at. For some, this may be usefull.

4. The speed increase on an original cable-drive tool will be limited because the motors on the X-axis run from a single driver. When we have created hot-rod cable drive tools for users, we have added a second driver for the X-axis. Without the driver, a cable-drive tool will probably jog about 2.5in/sec on a 24v supply, with the additional driver, it will do over 3in/sec.[The new R&P tools come with higher voltage supplies, 4+ drivers, etc...; they deliver over 3in/sec].

5. There are some attractive power supplies available as over-stock or surplus items. For us, we need supplies that are available in volume over time so that they can be reliably supplied with tools. Thus we offer two options:
a. 12v(13.8)DC 25A peak/20A continuous supply @ $175
b. 28DC 25A peak/18A continuous @ $350
(We've had a lot of experience with both of these supplies and have found them quite robust and reliable; the 12 is a 'switching supply'; the 28v is a 'linear' supply).

6. You can sometimes get away with putting 12V supplies in series; -but- we don't recommend it. Many 'switching' supplies and some 'linears' will not work when configured in this manner.

7. If you decide to add an additional power supply, you will improve the interaction, life, and performance of the supply by making sure your driver boards have a smoothing capacitor(s) on them. ShopBots shipped in the last year have these capacitors, earlier tools do not (you can check by looking at your driver board for a blue electrolytic capacitor(s) that is mounted where the yellow and black supply wire are connected to the board, or on very recent tools, soldered under each driver board). Use something like a single 440uf electrolytic capacitor (35-50v); or 1 100uf electrolytic capacitor on each driver. Attach them at the blue power terminal. Note that the capacitors are polarized and must be attached with the negative wire in the negtative power terminal.

8. If your tool is an early ShopBot, serial number less than ....0000118 (purchased before 8/97). You need to do one additional thing before hooking up the external power supply. You will notice that the colored ribbon cable leads to your driver boards soldered into your driver boards, as opposed to being attached with a blue header plug. If they are soldered -- you'll notice that there is no connection made into the ground input in the little batch of holes. So what we need to do is just run 1 jumper wire from the negative terminal of the blue power input on the first driver board (i.e., at the beginning of the 12v or 24v daisy chain) to any of the ground screws on the controllers terminal board (the green terminal board up above; there are several ground available). This will bring your older tool up to speed.