View Full Version : Control Logix OB16D output always On Issue

February 24th, 2006, 11:31 AM
Dear Experts

We are using 1756-OB16 D card.We had put +24 V on Dc+0& 24 V - to Gnd.Now even if output is not On we are getting +24 V at out terminals .What is it?Any help


Allan Barnes
February 24th, 2006, 11:51 AM
Some times on these cards the output will be blown. If it is a new card, it will be covered under warranty. If it is an older installation you are looking at replacing the card to fix the problem. I would look at putting surge suppression on your outputs, if you do not already have it.

February 24th, 2006, 12:19 PM
If these are transistor type output cards then a multi-meter may always "read" the voltage at the output due to leakage.

If the associated LED for that output is ON or the device stays on then it is possible it has shorted ON..ie it is bad.

IF it just shows a voltage reading but the LED is OFF nor is a device being activated, then its just leakage current and nothing to worry about as long as the device does activate when output is ON.

February 24th, 2006, 04:54 PM

Like rsdoran says it may be leakage that a multimeter will read. We use a wiggy if we suspect this. A wiggy uses a coil to measure the voltage applied. Most of them will not read 24v dc, but it will drain it so a multimeter can read it correctly (most times). Good luck.

February 24th, 2006, 05:04 PM
In addition to semiconductor leakage, the OB16D is a diagnostic card. This card applies a micro-ampere current to the output by design to check the continuity of the attached load and sets a status bit if no load is connected to the card. Using a DMM on an unconnected terminal will definitly show a voltage.