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KBIC SCR Motor Controller Notes

I have had little experience with SCR motor controllers, so in order to learn more, I bought a popular controller that is used with Sherline mills and lathes for spindle speed control. Along with information on how the controller works, I would like to present information on modifications that would enhance using this controller with EMC. A full featured PWM controller could be purchased for about the same price as one of these SCR controllers, but I find that most people tend to modify what they have rather than replace, so even though most people needing features beyond the standard KBIC would be better off replacing it, there may be some value in the modifications presented here.

One modification might be adapting a digital potentiometer such as the Analog Devices AD5290. This would be very similar to my Serial DAC project ( here ). This would allow EMC to control spindle speed using 12 bits of information through three parallel port pins.

It would also be nice to have EMC control the direction. Two more parallel port pins could be used for Forward and Reverse. I haven't studied the implications of braking, and reversing under power yet.

Another solution is here - DigiSpeed . EMC could be configured to output a PWM signal to one of the parallel ports, which would then connect to the Digispeed DC-03. Some of the Digispeeds have a serial communication option which could be used with EMC.

The main characteristic of this controller is -- it converts 115VAC to 120Hz PWM at ~100VDC, which is actually the original AC with the negative portions of the AC "folded" up, to make a positive only "AC". The zero crossing portions of the AC input are carried over to the DC as zero touching trenches. An SCR will turn on if its gate is turned on, and will stay on until the input voltage goes to zero. Since the input voltage goes to zero every 120th of a second and the SCR gates are pulse triggered, the SCR's are triggered and reset every 120th of a second. The amount of time between an SCR being triggered and reset determines the pulse width and thus, the amount of power sent to the motor.

Motor Direction Control

The KBIC has no facility to reverse motor direction. KB Electronics does offer add-ons to do this, braking and load protection. Usually these and other add-ons are mechanical relay based, which is okay, if your into those sorts of things. I wanted a solid-state solution. The schematic below is my first attempt at a direction controller. EMC will need to provide a Forward and Reverse signal while making sure that both are never on at the same time. (Don't try this circuit at home, I have neither built nor tested it, so it might work, ... or it might blow up and burn your house down.)

(Update 2007/02/01) Please, disregard the schematic that was here previously. It suffered from "block copy disease". I just copied the upper opto-coupler circuits without thinking how they would work on the lower FET circuits. I have tried this circuit below and it actually seems to work.

(more to come)

Basic Power Section Schematic

Basic Speed Input Schematic

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Rev. Date: 2008/02/01 16:57 PST KW