Tricky simple controller relay circuit

[Pete.S.]

how about this circuit?

It's good but it's not really needed in industrial PLCs.

The circuit will work the same but what you have done is put a flyback diode over the relay coil. It will dissipate the current surge that the coil produces when you turn off the relay. So it protects the digital output on the PLC.

A flyback diode is standard practice in electronic design but not in industrial PLCs so I assume that the outputs of a PLC can handle things like this.

 

[jawolthuis]

A flyback diode is standard practice in electronic design but not in industrial PLCs so I assume that the outputs of a PLC can handle things like this.

Adding surge suppression IS standard practice with PLC's.
I have seen many problems caused by this exact issue; of people assuming that suppression is not needed.
Always suppress every coil.

 

[John Morris]

wire the relay with a second coil wire off the output ( poor man's latching )

Use the plc to break the neutral line. that will drop the coil when program is complete.

Or I'm complete off the mark. In which case, see comments below

 

[Pete.S.]

Adding surge suppression IS standard practice with PLC's.
I have seen many problems caused by this exact issue; of people assuming that suppression is not needed.
Always suppress every coil.

Let's just say that my experience is not the same as yours.

Since I'm curious by nature I just disassembled an old Siemens PLC (S7-200). I see that there are diodes on every single output - which is what I suspected.
So there is really no need to add additional suppression diodes for regular relays.

 

[Sergei Troizky]

How about this?