Relay Help

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georgestolz said:
To be honest, I'm not quite sure what that means exactly. :)
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Ladder diagram example

image005.jpg
 
Bob's example is a good one. Each "leg" of that ladder, for your relay issue would be hot on the left, the the neutral running down the right. If you ever get into PLC programming, the "ladder logic" of the programming follows a similar theme.
 
I used to see pneumatic relays on submersible well pumps, as a backspin timer. If the power went off, and then back on, it would not restart the pump until the pneumatic timer timed out, as a start delay. When the pump shuts off the water goes back down the column and spins the motor backwards-often there is no check valve. If the motor starts with it going backwards the shaft will break.

There are now electronic timers that do the same thing and easer to set up.
But if they still want a pneumatic look at Sq D and AB.
 
I do see a mistake in the EE's diagram. He says he wants a NC contact on the timer, but his writing and the diagram both show he really wants a NO contact that will only close for 15 second after the LMO switch is closed. The R-prime contacts bypass the contactor after that time.
 
The relay I had found in the opening post was an "on-delay" timer, which means (if we assume he intended to write "N.O.")

  • LMOS is pressed
  • Contacts remain open
  • Timer counts down to 15
  • contacts close
  • Contacts remain closed until timer is de-energized

I suppose the difference in part numbers could mean the relay specified is an "off delay", which would energize immediately and then open after 15 seconds, huh....? :mad:
 
georgestolz said:
The relay I had found in the opening post was an "on-delay" timer, which means (if we assume he intended to write "N.O.")

  • LMOS is pressed
  • Contacts remain open
  • Timer counts down to 15
  • contacts close
  • Contacts remain closed until timer is de-energized

I suppose the difference in part numbers could mean the relay specified is an "off delay", which would energize immediately and then open after 15 seconds, huh.... :mad:
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I don't think the contacts remain closed. I think he means for them to close for 15 seconds, then open. Otherwise, there's no reason for the R-prime contact.

Something more like this:
-LMOS is depressed
-Timer coil is energized
-timer contacts close and 15 second countdown starts
-if door switches are satisfied, relay coil R is energized
-Relay contact R-prime closes
-timer expires 15 second countdown
-Timer contacts open
-Contact R-prime stays in circuit, jumpering around the timer contact, keeping relay R energized.

In this arrangement, the LMOS switch has to be maintained, otherwise you'd need to stand there and hold the switch forever to keep control power. You could work around that by paralleling another set of contacts from relay R around the LMOS switch, but you'd need to open a gate someplace to dump the circuit. That's probably why he specs a maintained switch.
 
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I finally got the relays in for my stoplight, X-Ray In-Use Lights, and so forth, so I've been updating that diagram as I've had time today. I discovered the EE did have a minor goof on the stoplight, but I'm not going to mention it (aside from here).

The red light on the stoplight comes on when either the X-Ray or the "beam" (I assume it's the linear accelerator beam) comes on - but there are seperate signs on either side of the stoplight outside the door that come on to say it's either the X-Ray or the beam. As I was doodling I realized that both the signs would come on at once outside the door, and that I was a wire or two short going out to the door.

All things being equal, I do respect the amount of thought it takes to make this sort of thing happen, I enjoy thinking about it (once the frustration subsides)> :)
 
georgestolz said:
Would a pneumatically-delayed timer be better than an electronic one?
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I think we would have to decide what 'better' is for each and every job.

IMO a pneumatic timer has a high degree of reliability and has the added benefit of not requiring any control power to run it.

I don't think any pneumatic timer has been taken out by a lightning strike or other line spikes. :cool:

Pneumatic timers are still used in every fire sprinkler flow switch I connect. The air timer delays the action of the contacts and typically have times of 0-60 seconds. No power needed for the timer and darn little to go wrong.
 
georgestolz said:
So it uses the circuit to be switched as the trigger to run?
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The ones I have seen have no 'trigger' terminal like an electronic timer has.

A pneumatic timer needs no power 'to time', with an electronic timer it needs power to time.
 
But, power being applied to the line side of the contacts causes it to begin timing?

What's the mechanism pushing the air out?

(Edit to add: How many licks does it take to get to the center of a tootsie pop?) :D
 
georgestolz said:
::Scratching head::

So, the ones you have seen, what tells them to start and stop timing, and start over? :-?
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They have been simply relays, power the coil on and the contacts switch immediately, cut the power and the air delay holds them closed for a set time.

But that is probably just the difference between a off delay and an on delay.

I notice your link seems to be a set of accessory contacts for another contactor so I would not expect a 'trigger' I would expect you power the coil of the contactor this mounts to, once the contactor closes this set of auxiliary contacts has delayed switch over.
 
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