"Connecting a spindle" shared ground query

[ww1g16]

Hey,

I'm just looking for some clarification on something mentioned on the following page.

https://docs.duet3d.com/User_manual/Machine_configuration/Configuration_CNC

Under the "Connecting a spindle" section, it says the following:

The ones with the square blue PCB have a design flaw (uses an opto isolator, but the input and output grounds are connected together) and should be avoided because (a) they are more difficult to drive from some Duets, and (b) sharing a direct ground connection between a Duet and a high powered brushless motor controller is a bad idea.

When wiring the suggested green board (e.g. this), you have a PWM input (DIN+ and DIN-), power input (12-30V, GND1) and an analog output (A0, GND2). In my particular scenario, I've connected the PWM-to-analog module to my 6XD via the VFD header, with this wiring

PWM-to-analog-module.DIN-      to     6XD.VFD-header.GND
PWM-to-analog-module.DIN+     to     6XD.VFD-header.VFD

PWM-to-analog-module.GND2    to     VFD.GND
PWM-to-analog-module.A0          to     VFD.VI1

The VFD I'm using is the YL620-A (a Huanyang 2.2kW clone), which happens to have a 13V output which I think I can use for the power input (12V-30V and GND1) to the PWM-to-analog board. But when it comes to keeping things as isolated from the Duet as possible, is it better to power the PWM-to-analog board via the VFD, or via the 24V DC power supply used for the Duet?

e.g. option 1

PWM-to-analog-module.12-30V      to     VFD.13V
PWM-to-analog-module.GND1        to     VFD.GND

e.g. option 2

PWM-to-analog-module.12-30V      to     DUET_24V_POWER_SUPPLY.24V
PWM-to-analog-module.GND1        to     DUET_24V_POWER_SUPPLY.GND

I think the answer would depend on whether GND1 and GND2 are connected on these modules, and whether either/both of them is isolated from DIN-.

From the quoted section, do I interpret that as DIN- is isolated from both GND1 and GND2 on these green boards, and GND1 and GND2 are not necessarily isolated from each other? In which case, it'd be best to power the PWM-to-analog module from the VFD's 13V and GND.

Alternatively, does the quoted section mean that both the PWM input and Power input have a common ground (DIN- and GND1), which itself is isolated from GND2?

Appreciate any input!

Regards,

Will

[dc42]

@ww1g16 said in "Connecting a spindle" shared ground query:

But when it comes to keeping things as isolated from the Duet as possible, is it better to power the PWM-to-analog board via the VFD, or via the 24V DC power supply used for the Duet?

Power it from the VFD. It's the PWM input of the converter that is opto isolated. The 0-10V output and power input have a common ground.

[ww1g16]

@dc42 Great, thank you!