External stepper drivers control output voltage and current range

Machine: custom build laser cutter, external stepper drivers

Board: LaserBoard

Firmware: Latest Smoothie

Problem/ Question: What is the voltage of the external stepper connection outputs?

I have built my own laser cutting machine and so far everything seems to work per plan. I have motion, homing switches, laser control, etc. Except I need to add automatic height control.

I see that smoothie has babystep information but that’d require two digital inputs, I have only one spare left. I could take a second one but that would take away my future plan of having dual homing of the Y gantry. Moreover, my height signal is analog with something like 0 to 10V, with a starting value that can vary depending on the part being cut.

What I was planning to do is use an existing PLC which is meant to control up to 3 axis, which also has analog inputs. Using one of those axis could send stepping signals to the external drivers. The catch is this PLC is meant to work at 24VDC. The stepper drivers can take 5 to 24VDC. So really I’m left with the interface between the Laserboard and the PLC.

From the design of the wiring it looks like the external stepper outputs are similar to an open collector. If that’s the case then I wouldn’t need much else assuming the transistor or optocoupler can take 24VDC while blocking and 8mA while conducting (inputs have 3kohms impedance).

The external drivers are controlled by 5V common anode with GND switched in by the STEP/DIR/ENA pins. I don’t have the schematic in front of me but I think the 74HC05s on the back of the board drive those and they have open drain outputs. I wouldn’t expect a CMOS part to be able to handle more than 5V nominal on any pin or source/sink more than ~50mA package total.

If it were me I’d build non-inverting gate drivers using BJTs that turn on and off a switching n-channel enhancement fet (or even a jfet, but the choice is largely academic) that will take 24V S-D, connect that open drain to the PLC’s logic inputs and use a common anode voltage of 24V (saying the PLC operates in the same manner as a regular stepper driver).

Ok thanks. I was trying to stay away from throwing other electrical components but it’s better than killing the board.

Yeah, it’s admittedly not an awesome solution but it can be done with as few as three resistors (maybe two if you’re careful about transistor selection) and three transistors per output. Which, over 3 channels with three pins each does make this a bit of a thing, but off the top of my head I can’t think of a better way to do it. I’d probably pick stuff I have a bunch of sitting around like 2N3904/2N3906 for the gate drivers and a 2N7000 for the switching fet. 10k pull-up resistor, 1k base resistor on the 2N3906, 30 ohm gate resistor is probably what i’d start with then tune things from there.

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