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There is a minimum current requirement in order to use the FET pins on the Micrologix controller of 21mA, and the output voltage of the pins are 24V. The ST-M5045 stepper driver data sheet says in order to use 24 volts as input a 2k resistor must be placed at the input. This will drop the current consumption below the 21mA requirment of the controller. Could a voltage divider placed between the driver and controller handle this problem? Maintain a voltage of 5V at driver input pin and draw 21mA current from the controller.

Micrologix 1400 installation manual (pg 32 bottom table)

stepper driver data sheet (contains diagram of controller and driver)

Optocoupler datasheet

enter image description here image of input circuits to control inputs

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  • \$\begingroup\$ Link to Micrologix user manual and page reference for the FET (note capitals) output specification, please? \$\endgroup\$
    – Transistor
    Commented Jan 11, 2019 at 17:58
  • \$\begingroup\$ I will edit the question to provide the info. \$\endgroup\$
    – seabiscuit
    Commented Jan 12, 2019 at 0:44

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There is a minimum current requirement in order to use the FET pins on the Micrologix controller of 21 mA, ...

20 mA actually, but it is strange. I have never seen a minimum current specification like that before.

... and the output voltage of the pins are 24V.

Since these are almost certainly opto-isolated FET outputs I would expect them to work down to a few volts but the table makes it look as though they are only rated for close to 24 V DC.

The ST-M5045 stepper driver data sheet says in order to use 24 volts as input a 2k resistor must be placed at the input. This will drop the current consumption below the 21 mA requirment of the controller.

Correct. The stepper driver seems to be looking for about 10 mA per input for its opto-isolator LEDs. They would probably be fine at > 20 mA but without a datasheet for the opto-isolators it would be taking a chance.

Could a voltage divider placed between the driver and controller handle this problem? Maintain a voltage of 5 V at driver input pin and draw 21 mA current from the controller.

A simpler solution would be to add a second 2k resistor to ground from each output. That will draw an extra 12 mA and satisfy the output requirement without compromising the stepper input recommendation. If you wanted you could add an LED in series with each resistor.

\$ P = VI = 24 \times 0.012 = 0.288\ \text W \$ so I would use a 0.5 W resistor for both the interface and additional load.

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  • \$\begingroup\$ I am going to edit the question to add the data sheet for the optocoupler inside the driver. I will also provide images of the board as well as a schematic of the control inputs that I pinned out by inspection and multi-meter. \$\endgroup\$
    – seabiscuit
    Commented Jan 12, 2019 at 13:16
  • \$\begingroup\$ Images of PCB are too large \$\endgroup\$
    – seabiscuit
    Commented Jan 12, 2019 at 13:33
  • \$\begingroup\$ The datasheet electrical characteristics are listed at 10 mA and the absolute max table shows 15 mA so 10 mA is your target. My answer remains the same. \$\endgroup\$
    – Transistor
    Commented Jan 12, 2019 at 13:44
  • \$\begingroup\$ I'm too new to give you an up vote, but I would if I could. Thank you. \$\endgroup\$
    – seabiscuit
    Commented Jan 12, 2019 at 13:57
  • \$\begingroup\$ You can accept the answer (tick mark) to indicate that the question is answered to your satisfaction. Thanks. \$\endgroup\$
    – Transistor
    Commented Jan 12, 2019 at 14:12

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