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Tight space, time delay requirements - need some equipment suggestions

Sparky1949 (Petroleum) (OP)
11 Apr 11 14:01
Warning, I know very little about PLCs! THIS IS A LONG POST IN AN ATTEMPT TO FOLLOW THE GUIDELINES


(I suspect there is an answer here somewhere, and I have found some great leads by searching it, but the site is sooooooo big.)

I am looking for a recommendation for a timer or PLC/PLR.

This equipment will be installed in a NEMA 4X box with a bunch of bare cards. There is NOT a lot of space for relays and wires. Perhaps 1ft^2 or 30cm^2. That is why a minimum number of parts is a goal. Not to mention that the fewer parts, the lower the labor costs and failure rate due to connections.

The requirement is simple and can easily (sort of) be done with relays.

The jpg is a ladder diagram of the relays currently installed. http://files.engineering.com/getfile.aspx?folder=a40749f6-5e5f-4ac4-b702-450ece5021b8&file=TIMEDELAYRELAY.JPG
  
Legend:
signal = 120VAC from a contact closure in another device. Since it is a simple contact closure it does not have to be 120VAC, that is simply how it is wired before adding this equipment.

Operation:

There are six "signals".

There are two different devices that are controlled by R1. Four signals control one and two signals control the other.
The devices are relays.

There are also six solenoid valves that open when the time after the relays are closed has elapsed. One valve is for each signal.

The signals only arrive one at a time. They will arrive more than ten seconds and up to hours apart.

The relay 1 activates instantly. The timer delays the activation of the solenoid valve for a fixed amount each time (about 4 seconds +/- 1 or so, but needs to be field adjustable by hand or laptop at the device).

Each circuit may remain active for several minutes but it is unlikely that they will all be active at the same time for more than five minutes. But it is possible that they might all run concurrently several times a year. Again, they will never start at the same moment. They will only remain active for fifteen minutes max and usually less. The maximum usage now is <30/day.

When the signal goes away the relays open. If power is lost all relays open.

Specifications:

All voltages are 120VAC
The relay is <1A
The sv is rated 5A inductive (a fluke shows ~1A when it is pulled closed).
The cabinet is occupied by another controller with bare cards. The outputs of this controller provide the six inputs. The outputs are currently wired to 120VAC but could be 12VDC (on that controller now) or another voltage via a new supply.
The cabinet is a NEMA 4X SS in direct sun. Calculations show that no cooling is needed if the maximum permissible temperature is 90C.

While space is a limitation, I have done it with 12 ice cube relays and six timers. It looked a mess with them in various locations around the circuit boards but it is working for now.
Six of the relays were because the timers could not handle the valve inductive current.

I think that there are time delay relays with separate instant contacts that would do the job too, but I don't know part numbers. There is the problem of space and wiring with them too.

I am thinking about using a Click with high current relays and RC surge suppressors on the solenoid coils.

Thanks in advance for your wisdom and time.  
Skogsgurra (Electrical)
11 Apr 11 14:36
Any little PLC will do the job beautifully. Even programmable relays like the Siemens LOGO! will work. You may have to add some I/O to the basic unit, but that will not cost you much space or money.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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