[jcdammeyer]

Hi everyone,

Been a while what with work-work projects but I finally have had an opportunity to get back to doing some work on my never ending CNC project.

Henrik Olsson published a nice application paper on a power dump circuit for handling over voltage conditions generated by a decelerating Servo Motor.

I decided to take that one step further and I've made up a couple of circuit boards that do two things.

1. First board has a relay that when not energized does nothing to a resistor placed in series with the primary voltage going to the Servo Power transformer. This is the soft start board.

2. The second board uses the output of the Servo Power Transformer to signal a small PIC microcontroller that AC power has arrived. After 2 seconds, the PIC turns on the relay on the first board shorting out the resistor and allowing full primary current into the transformer.

The second board also monitors the DC Servo Rail across the filter capacitors. If the voltage goes above a threshold it turns on a FET which places a load dump resistor across the capacitors. After the transient, the PIC waits a bit longer and the turns off the FET.

There's also another relay across this dump resistor so that if the power to the PIC controller vanishes, the relay opens and the resistor is places across the capacitor. This way the power bleeds off the cap fairly quickly without drawing power when it's not needed.

A picture is attached. I'll post the schematic in a day or so after I've done some testing with an actual power supply and UHU Servo drive with motor. I have no idea how well it will all work. I'll post some connection drawings and test results when I have them.

John Dammeyer