Internet PLC Forum

General => Technical support => Topic started by: Ken on September 30, 2002, 11:42:47 AM

Title: power on when no supply power
Post by: Ken on September 30, 2002, 11:42:47 AM

My situation is this.  When there is no power to the plc but I do have loads connected to terminals 1-8 which are of course hot the plc acts irratic yet does clear up when power is connected to the plc.  My concern is that it closes the ciruit under a no power situation as the loads mentioned are powered independently.  Is this normal?

Thanks,
Ken

Title: Re: power on when no supply power
Post by: support on October 01, 2002, 02:58:19 PM

The PLC outputs are either MOSFET or Darlington  transistor and these can conduct partially if their gates are at some "floating voltage) when no power is applied to the PLC

So if you must handle such a situation then it is better to use a relay to isolate the output. . The leakage current from the partially-on condition is normally not enough to turn on an electro-mechanical relay so that can ensure that your other circuit is not affected.