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Needing a Short AC Time Delay

01/29/2015 11:56 AM

I bought a couple of solid state timers with the intention of making a variable duty cycle pulse and delay circuit that was supposed to repeat. The concept was supposed to mimic a pair of timers in a PLC program. What I didn't catch was that to reset the timers, I had to remove power (to both timers).

So, after careful consideration, I realized that I could make it work if the output of the delay timer (always longer than the pulse timer) was faster than the motion of the normally closed relay contacts. It works with a large enough relay. Now I have to switch to a smaller relay and I'm wanting to add just a little bit of delay to the coil as power is initially applied. My intuition says that I need a series capacitor and a parallel resistor to do this. The question is what should my target values be.

Everything is running on 120 VAC, 60 Hz. The solid state timer outputs are optically isolated from the inputs and only fire at a zero crossover. If this isn't enough information, I can include the diagram. But I figured that with this description so far, any qualified person would understand what I mean without further detail.

Is it enough information? I'm feeling lazy but if it is too unclear, I can draw it up.

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#1

Re: Needing a short AC time delay

01/29/2015 3:28 PM

You are probably aware of this, but instead of using a capacitor and resistor in your relay ckt, you could use a 555 time to vary the timing with a variable resistor without having to remove power from the circuit. Check the ckts at the following links.

http://www.555-timer-circuits.com/driving-a-relay.html
http://www.555-timer-circuits.com/

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#2

Re: Needing a Short AC Time Delay

01/29/2015 10:05 PM

A series capacitor in the AC circuit will present as a short and therefore offer no impedance to the AC inflow and will not slow the relay actuation, a series resistor may work if your required time delay is somewhere less than one quarter cycle, ie. the rise time of one half cycle. Once energised, the hold current required is less than the pull in current, so you would have to experiment a bit with that

It's nigh on impossible to easily make an AC relay slow actuate if it is not so designed.

This may be what you need.

Alternatively, as has been suggested, building a delay timer circuit (possibly using a 4060 timer IC and triac) may be what you have to do.

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#3

Re: Needing a Short AC Time Delay

01/29/2015 11:14 PM

In India there is a company called SELEC in Mumbai / suburbs of Bombay which makes such timers which can keep on repeatedly doing ON/OFF . Look up link http://www.selec.com/digital-timers

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#4

Re: Needing a Short AC Time Delay

01/30/2015 7:54 AM

Why don't you just use a Repeat Cycle Timer (with self resetting)?? Please see the link below to Digikey's site to buy this. It ranges in cost from ~$18.00 to ~$90.00. Mouser, as well as Newark Electronics sells these as well. Just make sure that you choose the one that says it is self resetting!

http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&keywords=repeat+cycle+timer

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#5

Re: Needing a Short AC Time Delay

01/30/2015 8:11 AM

You could also do a google search for digital timer. I bought digital timer this way which I could also change the time delay to switch the relays that were on the board. I forget who the supplier was but digikey, mouser, avnet, etc will have this type of stuff.

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#6

Re: Needing a Short AC Time Delay

01/30/2015 8:51 AM

Thanks for your comments. Here is the deal. I actually bought 20 of these solid state timers at $60 each. So, I need to use them. The good news is that the output and input are optically isolated. The bad news is that they do not have an inverted output and they require power to be removed to reset. Here is the PLC method:

And below that is the actual circuit used:

The second circuit works as long as the solid state timer is faster than the relay. But if I use a less expensive relay, the race may become too critical. So after thinking about this, it occurred to me that I might delay the relay a little bit with the proposed capacitor. What do you think? Does it have a chance?

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#7
In reply to #6

Re: Needing a Short AC Time Delay

01/30/2015 9:19 AM

I see no way utilizing a capacitor will yield consistent timing results.

Suggest you utilize the output of the second relay for gating the "reset" function on the first TD relay.

If the TD relays you have on hand do not have a "reset" function you can add an "Instant On - Time Delay Off" relay upstream of the first relay to remove the power and reset the circuit.

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#8
In reply to #6

Re: Needing a Short AC Time Delay

01/30/2015 9:30 AM

"Here is the deal. I actually bought 20 of these solid state timers at $60 each. So, I need to use them."

Ah, the Accountant's Ultimatum, "You spent money on this, now use it." Also known as 'When all you have is a hammer, everything looks like a nail."

These sound like some nice timers, and useful in many circumstances, but are they the best solution to this problem? I'll make an educated guess and say 'probably not.' Fro here, it looks like you have three basic options:

  1. Send the timers back for a refund, and order timers that are better suited to this application's needs.
  2. Add these timers to the 'shop stock' for use in a later project, and order timers that are better suited to this application's needs.
  3. Continue trying to add circuitry to make these timers work 'well enough' in this application, spending more time and money on the project to make a square peg fit in a round hole.

Let me close by sharing a little bit of wisdom I've learned: Complexity is Nature's way of saying "You're asking the wrong question."

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#9
In reply to #6

Re: Needing a Short AC Time Delay

01/30/2015 9:37 AM

"The bad news is that they do not have an inverted output and they require power to be removed to reset."

Have you thought of putting the relay on the output of Timer 2, with the relay's Normally Closed output set up to power Timer 1? You'll be adding the Relay's electromechanical delay, and its variance into the timing cycle, but if absolute timing were that critical, I'd assume the project would have had enough of a budget for a full PLC instead of just solid-state timers.

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#11
In reply to #6

Re: Needing a Short AC Time Delay

01/30/2015 11:50 AM

The leg that keeps everything powered up should have been marked as Timer 2 normally open contact.

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#10

Re: Needing a Short AC Time Delay

01/30/2015 11:48 AM

Haven't got any money in the budget. As it turns out, the circuit does work with the recycled relays I had laying around. But there is a critical race that occurs because the output of the second timer has to turn on before the normally closed contacts of the relay can break.

Since I ran out of recycled relays, I had to buy some. Those are the ones I'm worried about. They are smaller and consequently have less mass. Consequently, they might be faster. I don't know yet.

The reason I think that the proposed capacitor should work is that voltage cannot change instantly. To charge up the plates, some current must flow across the dielectric. This is the same reason that current leads voltage in an AC circuit. The proposed resistor is there is to provide a path for the current before the full voltage will be present and available to the coil. I'm only expecting a time delay of about 4 mSec. All I need is enough to make the small relay act as slow as the bigger relay.

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#12
In reply to #10

Re: Needing a Short AC Time Delay

01/30/2015 5:09 PM

The statement that current leads the voltage in a capacitive AC circuit is true, however, this phase relationship occurs in the whole circuit only after initial charging of the capacitor has occurred (at least 4 ms, or one quarter cycle for 60 Hz). At initial switch on, the voltage across the capacitor - and current through the circuit - will rise in phase with the supply voltage, the current will fall as the capacitor voltage approaches that of the supply, but the relay will have actuated by then and the residual slug will hold it.

Expanding on the above - at initial switch on (assume the peak of the sine wave), the supply voltage, relay voltage and circuit current are in phase, the capacitor voltage is roughly 90° retarded (the relay has inductance). The voltage across the capacitor will be zero, the voltage across the relay will be maximum, the current through the capacitor and relay will be maximum and falling to minimum as the capacitor charges and approaches supply voltage. The relay will energise within the first quarter cycle (4ms for a 60 Hz supply) and you will get no extra time delay.

If initial switch on is somewhere other than at the top of the cycle, other effects will occur which will only serve to make the relay actuation time slightly more erratic, but still well within the 4ms. The slug on the relay will hold it in during the crossover.

The resistor that you suggest placing across the relay will serve no useful purpose. If it were of small enough value to shunt sufficient current to slow the energisation of the relay to the rise time of the sinewave, then the relay would never energise.

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#13
In reply to #12

Re: Needing a Short AC Time Delay

02/02/2015 2:52 PM

Ah, Yes! Now I see where I was going wrong! Perhaps I could steal some parallel current from the relay coil briefly while the capacitor was starting to operate.

All these years of steady state calculations has numbed my brain. Then again, I could just be getting old. I used to do transient analysis but now I have to use spell checker to see if I spelled it right! Must be Alzheimer's or Mad Cow disease. That's my story and I'm sticking to it.

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