Output LEDs partially lit when off

[Joel Moore]

I've got an Anaheim Automation 23MD series motor/drive attached to a T100MD1616+ here. I purchased the sinking version (23MD106S-00-00-00) of the Anaheim motor

I connected the direction and clock inputs to outputs 1 and 5, respectively, on the T100 PLC and when I power everything up those two output LEDs on the PLC are "partially" lit even though the outputs are off.  I can clearly see that the direction LED toggles between bright and dim as I toggle the output.

Everything seems to work fine but I was wondering if I should be worried about this.  Is this normal?  What's causing it?

[support]

If the NPN output of the PLC is connected to a load whose high side voltage is lower than the PLC's power supply voltage, then the output LED will be turned on partially by the reverse current flowing out of the terminal even though the PLC output is OFF (floating). This should not pose any problem. If you connect a diode in series to the load then this problem can be eliminated.

[Joel Moore]

The diode fixes the "problem" but I guess I don't understand why this would happen when the load and the PLC are connected to the same power supply.  But I guess that question is difficult to answer without knowing how the load is designed.

[support]

There is probably some leakage current flowing into the load from the PLC output even when it is off.  Connecting the diode blocks that reverse current flow and therefore the LED will not light up. I don't the partially lit LED will be of any negative effect, although I guess it sure doesn't feel good to see the output LED partially lit when the load is supposed to be off.