Relay Logic - Timer Circuit Question 1

[elfman]

This is almost embarrasing that I have to ask this question, but alas, mental block isn't going away.

I have a remote input that turns on a relay, that subsequently turns on a Cycle Timer, On-Off-On-Off, etc. until the remote input shuts off. I am trying to figure out how to get the equipment to run, one last time when the remote signal turns off, if the equipment is isn't already running due to cycling on. I have attached a photo of the pertanant portion of my ladder. The Cycle Maintain relay just keeps things going till the cycle completes. Thanks in advance.

"If your parents didn't have any children, then neither will you." -Benny Hill

 

[jraef]

Without the coil logic for TD170, we can't know what you are doing there.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
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[elfman]

So sorry. TD170 is the last 'step' in the cycle and I use this contact to kill the self latch of CR156 to end the run cycle.

"If your parents didn't have any children, then neither will you." -Benny Hill

 

[waross]

You may need a couple more relays or a second delay relay to seal in and overlap the first time relay.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Hmmm... I don't see the need for that if TD154 is a dual adjust repeat cycle timer. I would still use CR156 as the output interposing relay tied to whatever the load is in order to avoid wearing out the more expensive timer contacts. With a dual adjustable cycle timer TD154 would hold on for the programmed time and then open for the programmed time. But maybe you are using a Repeat Cycle Timer that has only one programmed time? If so, you can simplify this by using a better timer. Example (.pdf). If you don't want to change it, then I guess TD170 is the solution, assuming that the setting of TD170 is more time than that of TD154 because that's the only thing that makes sense. By the way, the convention is to show the timing range next to the coil as you did, but then show the time SETTING next to the contacts; it makes the logic more clear.

If you think about it, watch your Windshield Wipers; they function in the way tou say you want this to work: after you turn them off, they finish one final swipe. You have a really good case here for using one of the new "Smart Relay" type products, which will give you the flexibility to add as many timers and functions as you need for this circuit without having to add hardware. the relay will likely cost you less than what you already show here.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[elfman]

Thank you jraef. I love it when people poke holes in my design without even intending too. I realized something that TD154 may not work the way it was designed. But that is my problem and your use of a dual setting timer appears to be the answer, however...

This will be controlling 4 pieces of equipment that make up a train. TD154 triggers another timer which triggers another timer for a total of 4 additional timers. Yes your programmable relay is a great idea, but I don't know if the customer would buy off on it - they may think it too complicated - don't know. However, the personel that will be running the equipment, well lets just say we have to make these as goof-proof as possible. So I was looking (incorrectly) at TD154 just starting the cycle, but it actually stays on, potentially longer than the equipment train which would immediately restart the cycle - I will put a dual time control in place of the current TD154. Thanks for helping me see that.

Now back to the original question. If I loose power to TD154 (input goes off) during an Off cycle, the equipment train will not run one last time as the timer output is off. I think the easiest option is to add one more timer as a 'One Shot' and have a CR152 NC contact close the 'Start' contact on the additional timer whose normally open contact is in parallel with the TD154 contact on rung 156. This will either, start a final cycle if everything is off, or if the equipment is in the middle of a cycle, the cycle will just end when TD170 times out.

Thanks again for your help. If I wasn't as far through the submittal process as I am on this job, I would reconsider using a programmable relay, but alas, it is time to stick a fork in it. Also, I realy appreciate the suggestion to add the recommended timer setting to the contact. In a lot of places I can't do that as it depends on the customer's process - I don't have much say in that, but in the cases where I do have say, I will certainly start doing it, thanks again.

"If your parents didn't have any children, then neither will you." -Benny Hill

 

[LionelHutz]

The problem with using a CR152 is that TD154 will lose power during the CR152 transition and it might just reset the timer.

You're better off using an off-delay timer for CR152 with the off-delay set to match the timing of TD154. This new timer would have to be set to the same time as the total cycle time of TD154. Even then, it won't work as you ask. Say the stop is given right at about the middle of the TD154 on cycle. This new timer would hold on TD154 and it would complete the off cycle and come back on for about 1/2 of the on cycle before stopping.

You either need to find a flasher relay that will complete the cycle even without the trigger input or you need a relay that gives an inpulse trigger on the high to low transition of TD154 which can be used as a "Stop" for TD154.

A programmable relay or micro PLC with a falling edge trigger would work perfectly for this. A Zelio in FBD mode would do this.