Serial DAC Project

I have VFD's driving the spindle motors for my Bridgeport and a couple of lathes. Other than some torque issues at low speed, they work pretty well. In addition to driving spindles, I found small VFD's on eBay cheap enough to use as three-phase converters for coolant pumps. They are proving to be quite handy. This page will contain the information I gather for developing an adapter between EMC and VFD's, or more generally, a DC analog adapter

My VFD's are set up to be controlled by a 0 to 10 Volt analog signal for speed and sinking Forward or Reverse to a Control Common terminal for rotation direction. An overview of my plan so far is shown below.

The DAC is a Linear Technologies LTC1257 12 bit unipolar converter. It has just the features I need, which helps to make it easier to implement than more complex DAC's. The software interface with EMC is an HAL component. It has an input pin for the value to convert to a binary bit stream output pin, plus output pins for Clock and Load*.

Latest Files

Timing Notes
serial_dac.comp
stepper_inch_dac.ini

core_stepper_dac.hal
standard_pinout_dac.hal
serial_dac man page

( Rev. 1 of this page)

The software is now working well enough to test the hardware, which is yet to be built.

The Halscope screen shot below is from a previous revision of the software, but should represent the current software, except I currently run the data out portion of the serial_dac component in the much faster base thread.

(2007/02/01) The board below is just missing the power input diode from the schematic. It's just there to protect the board, if the power is connected backwards. I tested the board with my VFD's and KBIC controller and it works just fine. The input and output remain isolated if you use an isolated power supply (18 to 30 Volt DC at 150 mA or higher wall wart), so it works with the KBIC, but you can't touch any part of the board because it is floating at about 70 Volts AC. The VFD's don't seem to float at high voltage, but it's a good idea to keep things isolated anyway. One VFD could not accept just an analog voltage. It needed a potentiometer across three terminals to work. The DAC can be swapped with a digital potentiometer in this circuit to make it work. I am also looking into a DAC solution. I am also thinking about what can be done to help protect the analog output. Maybe by using a 10 Volt Zener diode to clamp the output to 10 Volts.

(2008/02/07) Now that I have confidence that the basic circuit works, I want to lay out the final board. Which means I have to address all of the details I chose to avoid earlier. Below is the schematic so far. It's getting much more complicated than I want, but to have a professional quality board, I need to address all of the details that come to mind.

The figure below on the right is from gerbv showing my top silk screen. The outline, the dashed line through the middle, diode orientaion dots, and a couple of other features had to be added through creative means because the Eagle CAM did not include them. It seems that Eagle should be able to include these features, I just need to learn more (maybe).

Eagle files here (right click to download): serial_dac-4c.sch , serial_dac-4c.brd


Pages Created by Kirk Wallace kwallace@wallacecompany.com	Copyright © 2006-8 WALLACE COMPANY Rev. Date: 2008/02/08 11:35 PST KW