Transistor Output Doubt

Hi everyone:

I have a doubt concerning Transistor Outputs on a uPLC. I was thinking of making a wiring like shown in the image I attach.

Wouldn't there be any problem with the wiring in Red colour? I need to run the load on OUT3 by the PLC and by an auxiliary Switch. But when I turn on the switch, I'm applying 24V to the Output, and though the output provides that voltage when on, I don't know if I could damage the PLC or the source with that connection (consider the unfriendly case when both the switch and the output are on).

Thanks.

You will want to check with your specific brand of PLC but generally speaking what you show won't cause a problem. The PLC doesn't actively try to drive the output voltage to 0 when the output is logically off. It simply doesn't supply current. So if some other device attempts to supply current it won't hurt anything.

Keith

Switch and output on is the "friendly" case. Output off and switch on is the one you need to worry about.

Try this instead, use a spdt switch wired so the load is connected to the output OR the +24V. (Output is never connected to the +24V)

Thanks for your tips.

I can't use a spdt instead because I simplified the scheme. The "Switch" is in reality a proximity switch used to detect the rotation of a motor.

I'll check if the PLC (Whitch is actually a Zelio from Telemecanique) forces the output to 0V or if it (hopefully) doesn't.

How about using diodes to isolate the circuits?



-Eric

P.S. Pardon my MS Paint skills...

Eric Nelson said:

How about using diodes to isolate the circuits?



-Eric

P.S. Pardon my MS Paint skills...

Click to expand...

I'm embarassed I didn't thought of that first. Thank's for making me remember I did go to college.

P.S. Your Paint skills are as good (or bad) as mine.

Thanks once again