Author Topic: Output LEDs partially lit when off  (Read 10662 times)

Joel Moore

  • Full Member
  • Posts: 128
    • View Profile
Output LEDs partially lit when off
« on: May 02, 2004, 02:56:38 PM »
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

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 3174
    • View Profile
    • Internet Programmable PLCs
Re:Output LEDs partially lit when off
« Reply #1 on: May 04, 2004, 12:46:48 PM »
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.
Email: support@triplc.com
Tel: 1-877-TRI-PLCS

Joel Moore

  • Full Member
  • Posts: 128
    • View Profile
Re:Output LEDs partially lit when off
« Reply #2 on: May 16, 2004, 04:05:29 PM »
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.
« Last Edit: May 16, 2004, 05:54:11 PM by Joel Moore »

support

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 3174
    • View Profile
    • Internet Programmable PLCs
Re:Output LEDs partially lit when off
« Reply #3 on: May 16, 2004, 06:31:27 PM »
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.
Email: support@triplc.com
Tel: 1-877-TRI-PLCS