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I will purchase a Spindle Brushless Motor Driver to drive my Lathe Spindle, and I need to know if I´ll be able to connect an output signal from the Driver to my CNC BreakoutBoard (BOB), which is a GECKO G-540 G540 Manual, and they don´t share common grounds.

As this is a Chinese Driver, It lacks detailed documentation, so there´s no datasheet yet, only an user manual.

The Chinese driver is powered from 220VAC and internally it has 5V logic to it´s motor controls. (The bruhless motor is powered by 310VDC internally)

But I need to read this alarm signal.

What I know from the lack-of-info-manual is that the alarm signal outputs 5VDC when in normal use, and 0V when in Fault State. I´ll attach 3 pages of the manual, as jpgs, that have details about how it works:Page 3 Page 4 enter image description here

So, as I don´t have all the info needed yet, I though of this opto-isolated circuit (Based in a help I had with user @transistor in another question) to interpret the signal over my CNC controller, without the risk to fry it.Opto Diagram

I just don´t know if I could risk damaging the driver by connecting the Alarm signal to ground with a 330R resistor, and if it would work or not.

EDIT: The Chinese dealer sent me this partial schematics of the part where the ALM (alarm) signal is created and sent: enter image description here

So although it has some kind of internal isolation (there´s an optocoupler inside) would it be safer for me to use another optocoupler between ALM (5V or 0V) and GND1, as I drew on my diagram, to completely separate both devices (driver and BOB)?

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2 Answers 2

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The output will be relative to the Control Signal ground (GND1). This is probably the same ground you have for all the other outputs from the BOB.

You can check this by using a multimeter to measure between the alarm output and GND1, you should see approximately 0V or 5V depending on the alarm state.

Once verified by a multimeter, you should have no issues directly connecting this signal to the BOB using the 330ohm resistor as outlined. Most BOB boards have this resistor built-in and the opto-isolated may or may not transition with two resistors in series (660 ohm).

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  • \$\begingroup\$ I figured the circuit would be ALM signal and GND1. But as the brushless driver is powered from 220V AC Outlet, and the BOB (Gecko G540) is powered from a separate DC Switching 48V DC Supply. So, could I join the GND from the BOB and the GND1 from the Driver? \$\endgroup\$
    – Rodrigo
    Commented Mar 28, 2020 at 1:51
  • \$\begingroup\$ Yes, if you are paranoid, put a 100K resistor between the two grounds and measure the voltage across the resistor (with all power supplies connected and everything powered up). If the ground is isolated (which it should be), you will measure 0V across the resistor. \$\endgroup\$
    – Dean Franks
    Commented Mar 28, 2020 at 5:12
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Optocoupler doesn't provide any help if the other signals aren't galvanically isolated. You have to make all the signals the same way - all isolated or not isolated. It depends how the driver and the electronics are made, if they have isolation already embedded.

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