"First Up" Hard Wired Circuit

inst_tech · Jun 1, 2012

Hello,

Was wondering if anyone could improve on my "first up" circuit. I'm sure there is a way but cant quite get it!

What I've got is 4 switches pulling in a contactor. I wish to find which switch drops out first. I've done a program in statment list but was trying to do it hard wired as well.

The attachment shows what I have. First cicuit is the original. The rest is the wiring I need to capture the switch dropping out.

Is it possible to do it with just 4 relays rather than 8? I must be missing something!

cheers

bce123 · Jun 1, 2012

think about latching r1,2,3,4 and using not r1,2,3,4 in series with the switches

CheezyMane13 · Jun 1, 2012

By the schematic you drew up they would all drop out at the same time. You need to latch the relays. Might even want to try using both NO and NC contacts.

OkiePC · Jun 1, 2012

His circuit would work, if I understand correctly, the original circuit is shown not in the normal state. So, he wants to know which switch caused C1 to drop out first. I have done something similar years ago with what was termed a tattle tale relay arrangement.

I think there's a thread on the subject here in the archives somewhere...
http://www.plctalk.net/qanda/showthread.php?t=35449&page=2

inst_tech · Jun 1, 2012

BCE123:- i cant really modify the existing circuit to much as its controlling a turbine starting up. Wiring relays 1-4 in would just be a temporary measure.

Cheezy:- have simulated this program using PLC SIM and it works fine (unless i made a mistake doing schematic as i just copied the program)

inst_tech · Jun 1, 2012

Thanks OkiePC. You are correct in saying the circuit is shown in the "turbine stopped" condition.

rootboy · Jun 1, 2012

I've gotta agree with Cheezy, when you lose Switch1 you lose all of your relays. Then it just becomes a relay race to see which one latches in first.

It's one thing to test it out on a sim, it's another to have to deal with the built-in latency of a mechanical relay.

Eric Nelson · Jun 2, 2012

Just thinking out loud...

Assuming the circuit is DC, what if you wired an optocoupler ACROSS each switch contact. When the switch is closed, the optocoupler is shorted, so it is off (and C1 turns on). Something like this...

            SW1       SW2       SW3       SW4

(+) <---+---]/[---+---]/[---+---]/[---+---]/[---+--------(C1)---> (-)

        |         |         |         |         |

        |         |         |         |         |

        +--(R1)---+--(R2)---+--(R3)---+--(R4)---+



         --] [--   --] [--   --] [--   --] [--

When a switch is opened, enough current flows through the coil of C1 for the optocoupler to turn on, yet not enough for C1 to stay energized.

(+) <---+---]/[---+---]/[---+---] [---+---]/[---+--------(C1)---> (-)

        |         |         |         |         |

        |         |         |         |         |

        +--(R1)---+--(R2)---+--(R3)---+--(R4)---+



         --] [--   --] [--   --]/[--   --] [--

Each optocoupler (R1 thru R4 in my 'schematic') would drive the latching relays in inst tech's drawing (using a NO contact rather than the NC one he used). When one switch is opened, ONLY the optocoupler associated with that switch would turn on, latching the appropriate relay, which immediately disables the rest.

Like I said, just thinking out loud...

-Eric

rootboy · Jun 2, 2012

That would work. Nice.

inst_tech · Jun 2, 2012

Thanks for the comments and thanks Eric for the idea! I'll have to look into that a bit more and maybe get some optocouplers to try it out. Unfortunetly I'm on an oil platform and optocouplers are a bit difficult to get hold off. I doubt I'll even manage to get relays.... or spare wire come to think off it!!
cheers

OkiePC · Jun 2, 2012

It would be best if you could wire a separate contact from each switch to a PLC (or relay arrangement) so that it is completely electrically isolated from the C1 control string. Then have the PLC handle the detection/alarming.

jdbrandt · Jun 2, 2012

Easy;
Install 0.016 amp fuses across each normally closed contact.

When a contact opens, the fuse will blow as all the power goes through it.

inst_tech · Jun 3, 2012

Ah now that suggestion is a possibility with the bits of kit available out here! Nice one.

KalleOlsen · Jun 3, 2012

So you don't have 4 spare inputs on a PLC or a smart relay a la LOGO! laying in a drawer somewhere?

Kalle

inst_tech · Jun 4, 2012

I'm afraid not. The spare kit available on the platform is basically relays, contactors and even that is a struggle to get hold off. Only one PLC on the whole place and thats an S5. I doubt a PLC would be quick enough as I've seen that method used in the past (onshore). They hooked up a dedicated siemens 315 looking at 8 contacts with 1 network of code which would pick up the faulty contact. Wasnt quick enough. They then wired up 8 relays and they caught it first time. Unfortunetly I didnt take a note of the wiring they used, doh!

"First Up" Hard Wired Circuit

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