Denford Novamill Mach3 conversion

[JohnHaine]

STOP!!!

The key phrase is written in the thick black box round the speed controller schematic. It says:

WARNING. The electronics is at mains potential do not earth!

This controller, like mine, is a thyristor type with all the control circuitry run direct from the mains. It also needs a dc input from 0 to 10 v on pin 3 relative to the 0 V on the controller to vary the speed. There is no sign of a low pass smoothing filter to convert PWM to the dc control voltage.

To control the speed from the BOB you will need an opto isolator driven from the PWM output, which will safely transfer the PWM to the high voltage environment of the speed controller. Then you need a simple low pass filter to smooth the PWM, to generate the control voltage. The was a circuit on the Mach 3 boards a little while back, I'll try to track it down.

To reiterate, if you connect the 0 V on the BOB to the controller as shown in EddyCurrent's picture, on switching on the mains will be applied through a rectifier to ground. If you're lucky the fuse will blow, but rectifiers blow up faster than fuses! Guess how I know?