Are we too used to doing everything by plc?

The other day I was at a customers that had a fault on his 'none plc' controlled machine.

A very old and none standard relay with a pnuematic delay off timer had failed. Or at least the pnuematic timer part had failed

So here is a quick drawing I have cadd'ed up of the situation



So that when 1K3 was energised with 240V the timing contact immediately opened.
and when the 240v supply was lost the timed contact stayed open
until the time expired on the adjustable dial - then it closed again.
Because this part was not working, the timed contact was making again as soon as the 240 was lost.

All they had spare in stock was some 240V relays and some normal 240V 'delay on' timers.
They said, rather than order new delay off relays and timers could I rig up the same conditions with what they had.

I thought it would not be too hard so set about it with a clean sheet of paper and pen to design it

5 sheets later and 10 re-trys, I couldnt do it with what I had.

I was thinking I was losing my old true relay logic skills.
I told him it was 5 mins work in a plc - but real relays and timers was stumping me.

In the end, I ordered a 240v 'true off timer' and fitted it in parallel with with 1K3

But it has puzzled me since wether I could have done it with what they had

I know some of you just love challenges like this.


any solutions ? using just relays and on delay timers

i think that you need one normal relay to invertion, and the coil of the "delay on" relay, but for the 24 V you have to use his NO contact.

i would use pair of relays and one timer, same way you do it in plc

number of relays can be reduced if output device has contacts
in this example:
X input
T timer
B relays

Excuse the crude drawing.

Still needs an extra relay though

(deleted)

the last contact in my example would have to be NC to match the original timer circuit

Wouldn't this do it?



I think this is what bio meant. Of course, the timer would be energized most of the time. May shorten timer life?

no because the 24v supply has to break as soon as the 240 v signal is present - then stay broken for tx after the 240v signal has gone.

Thats the tricky bit

Something like this then?

Yes you can do it but you need a permanent auxiliary supply of 240V or 24V, drawing below shows it for 24V. I make no apologies for using R for Relay and Red and Black for the 24V wiring.

A 240V relay NO contact energises a second relay that self holds and opens the 24V switched circuit. On loss of the 240V siganl a NC 240V relay contact then puts power to the timer that opens the self hold after a period.

Overall much easier to use a true off delay timer.

Bryan

I missed the significance of 'once the supply is lost'

Nice one bryan, that would certainly work with either permanent supply as well as the on/off signal.

And it sort of proves the topic question - we do rely on plc's sometimes to the detriment of our other skills.

PLC timers don't work very well without power either...

This is a special situation where the energy used to perform the timing isn't even electrical.

There is nothing more you could have done, but being away from the practice of using relay logic probably just made it less obvious than it would have been if you were still used to doing things the old way.

I have had similar experiences, and spent way too much time sorting through hardwired logic that used to be 2nd nature.

Paul