On/Off Delay Timers

Good luck with your apprenticeship. You should find it most rewarding.

In terms of industrial controls, which given your trade I will assume you are learning about, you have named the two common delay timers. Perhaps you should be thinking of them not so much as what the input state is but what change the input state doing. To put that another way, you should assume these are edge triggered devices. For an example if the input changes from off to on or 0 to 1 or false to true it is a rising edge. An "on delay" timer will start counting then change state, NC will open and NO will close. If the input changes from on to off or 1 to 0 or true to false the "off delay" timer will start counting, once it has met its timing criteria it will change state.

The on-delay uses the timer function built into a time delay relay just as you noted. The simplest of off-delay would be a pnuematic timer that times the release of air and then changes states of the contacts. If power is lost to the realy the contacts will remain closed or open until the delay time is met.

If you use a plc you have better control of both time delay on and time delay off. the plc must always be powered, however the advantages are enormous.

Good Luck

"when the coil is de-energised, the contacts will return to their initial position after a pre selected time delay"

Yes, this is an accurate description of the "standard" off-delay. Good job.

Yes you've got it! The only thing I would add is that an off delay timer (and i'm talking about an actual piece of hardware as opposed to a PLC timer) will require a seperate supply to maintain its control function in addition to its coil input.

Bob

You are correct about the on-delay timers. The contacts change state after the set time has passed after the coil is energized. As for the off-delay, the contacts change state immediatly when the coil is energized. After the coil is denergized is when it starts to time. After the preselected time expires the contacts revert to their original position.

Basically: On-delay=power on; time; change of state; power off & change of state

Off-delay=poweron & change of state; power off; time; change of state

Guest poster-your descriptions are correct.

If I may add: Timers have their contacts shown on electrical schematics with unique symbols for the contact function. An off delay contact will usually be drawn differently than an on delayed. It is also common to have a timer relay that contains both timed and non timed contacts. And there are plenty of electrical off delay timers that do NOT require separate power to function.

"And there are plenty of electrical off delay timers that do NOT require separate power to function."

I am a little puzzled by this. I have not run across any off-delay relays that can maintain an energized state without some sort of power input. Can you elaborate on how these work?

http://www.ab.com/industrialcontrols/products/relays_timers_and_temp_controllers/industrial_relays_and_timers/pdf/700-p.pdf#Page=8

If the above link works- You will see adder decks that 'stack' on a standard (in this case 700P) industrial relay providing a timed on or off contact function.

I have never taken one apart so I cannot elabortate further on how they work other than stating they work "pneumatically" thus no extra electrical power required.

"pneumatically" = air pressure = power

This particular add-on on page 237 of this bulletin brings up another question. If these time delay modules are pneumatically powered, how reliable can they be. With pneumatics there is a problem with restrictions to control air flow in that the air flow will change over time due to dirty compressed air. This is a particularly prevalent problem south of the border in factories that don't employ air cleaners and dryers in their systems. On the other hand, maybe this is some sort of enclosed system that is not subject to this problem? Can anyone enlighten me about this? Also, when you remove power to a standard relay, the contacts open. The add-on module's sole purpose is to continue to provide power to the coil of the relay. Thus, the relay is indeed powered during the time delay period.

Along the same line as providing power with pneumatics, I suppose one could provide power with a timer circuit utilizing an RC circuit and a solid state relay. There still has to be power, either from the capacitor or some other source to keep the "contacts" closed. I wonder if this would work if you were trying to provide a connection to ground for a period of time after the removal of all power?

Replying to your 3 previous posts:

"pneumatically" = air pressure = power

so what?- I said electrical power. Are you suggesting that these require some sort of external air compressor to work? That is silly

"The add-on module's sole purpose is to continue to provide power to the coil of the relay Thus, the relay is indeed powered during the time delay period."

Wrong- they are simply delayed contacts- Look at the schematics. You do not re-wire the coil. Just add these to the top of an existing relay and you will have delayed contacts for your use. Coil stays wired as it was prior to the addition of the adder deck, and all of the 'instant' contacts still function as before the addition.

As far as dirty air/ reliability these specs are probably in the document- I would suspect (but not assume) that the vendor has done their homework and these work just fine- when properly applied. I have seen so called "solid state" timers fail due to electrical noise, surges and brown outs. - Reliability is relative and this conversation is becoming ridiculous- I stand by my original post as accurate- after all there was electricity prior to electronics, and the need for timing goes back farther than both.

I appologize if my queries came across as demeaning or offensive in any way. I was merely trying to understand how you could have a time delay as far as opening the contacts are concerned without some sort of power being applied.

As you can see, the add on possesses contacts that would have to be wired in

series wih the coil on the relay in order to perpetuate either an on or an off delay.

D1X to D1Y would be normally open while D2X to D2Y would be normally closed.

Am I not understanding this correctly?

I have been wrong many times on this web site, it is all part of the fun. But on this issue I guess I am not making myself clear- 1 last try:

Go back to my original link and look at the first page- (Pg 228) There is a picture of the main relay 700P. (it is a normal relay with 'instant' contacts) In the center of the face of the relay is a square with a slotted screw. This square is a 'plunger' it moves with the contacts. If the relay coil is energized the relay contacts all change state and the plunger moves 1/4 to 1/2 of an inch into the relay. When the coil is de-enregized the plunger returns to the flush position (as shown in picture).

Now-The add on deck mounts on top of the 700P and is actuated by the plunger movement. The actuation does happen instantly with the 700p coil energization, however the pneumatics within the adder can delay the adder contacts from changing state. Only the adder deck contacts are delayed, all other contacs on the relay as well as the plunger function instantly.

To re-state: All of the 700P relay contacts function the exact same with or without the adder deck installed. Only the 2 adder contacts are delayed. The adder has no coil it only has contacts, and pneumatics, and cannot function without the 700P relay 'plunging' into it.

I apologize for my frustration- I hope this helps to clear it up. It appears as though no one else is going to try and explain it, or at least take a side. I cannot think of another way to explain it, so the next step would be to buy one and see for yourself. - If I am wrong please send me the bill

The bill will not be necessary. I am sorry for my confusion. It makes sense now. I had just never seen anything like this before. Thank you for your response.

Regards,

Keywalker