Current limit of PLC input?

[kolyur]

I'm connecting an output from a stepper motor driver to a DC input on my PLC (Micrologix 1500). The control voltage is 24V. As indicated, the current through the driver's optoisolator can't exceed 10 mA.

Originally I was thinking I'd have to put a resistor in series with the signal line (R2). But looking at the input specifications for my PLC, it says the on-state current is 7.3 mA at 24v, and the nominal impedance is 3.3Kohm (which is approximately the resistor value I calculated I'd have to use).

Does this mean that I can connect the signal line to the input without an external resistor? That's my gut feeling but I just thought I'd ask since I really don't want to fry the driver.

 

[keithkyll]

Stepper motor output? Does this mean you are making a connection to the coils of the stepper motor? There is noise caused by back EMF from the motor. This voltage can be quite high. Do not make a direct connection, even with a resistor.
If you want to see if stepper motor windings are active, use a Hall-Effect current sensor.

 

[kolyur]

Sorry I should have been more specific, this output is just the overheat signal from the driver. The motor itself is not involved.

 

[keithkyll]

Inside the PLC is an optoisolator. There is a resistor in series with the LED of the opto. (In addition, there is caps and Zener diodes to protect from spikes). At 24 volts, you say the current on the LED is 7.3 mA. I don't see a need for an additional resistor.
The inputs are designed to take a direct connection to 24DC. Hook it up as you intended.

 

[kolyur]

There is a resistor in series with the LED of the opto.

This is what I suspected but I wasn't sure. Thanks for the tip, I'll go ahead and wire it direct.