time delay relay alternative?

Hi Craigt, We just talked about something similiar for Nick in another thread. The best way to do this would be to use an Op Amp or a 555 timer. You will have to pick a resistance and a capacitor with the RC constant that will meet the 10 second time you have selected. You did not mention the signal level that you are using, If it isn't digital you will need to convert it so that the digital signal can be properly read by the input of the 555. When the capacitor gets sufficient charge the 555 or op amp will turn on and this can trigger your relay. There a many sources for circuits like this available on the web. Here is just one example.

http://www.talkingelectronics.com/projects/50%20-%20555%20Circuits/50%20-%20555%20Circuits.html

John

Hi Craigt;

I had some extra time today so I bread boarded a circuit to delay the actuation of a relay for 10 seconds from the time power is applied to the circuit. Here are a couple pictures and a schematic.

If you click on the images they will enlarge. I had to actually use 2 resistors on the breadboard to get the required 80K to make the unit trigger at 10 Seconds.

John

Hi John,

Wow, the amount of effort you have put into helping me is completely unexpected.  Thank you.

I did read the question from Nick123 before posting, but thought that my application was slightly different and I did not want to divert the focus in his thread.

Do you know of any pre-packaged 555 applications for common delay lengths?  I'm looking for something that can be used in an industrial setting, so enclosed akin to a typical plug-in relay.  I'll be using a 24 VDC signal.

John, John, John.  Let's put a CL resistor in the base lead of the Q.  Otherwise, you are shorting the output stage of the 555 to .6V.  Survivable for a 'C555, but nasty.

The 'thresh' terminal can be grounded.  Thresh is a comparator set at 2/3 Vcc for turning off the F/F.  We never intend to turn off the F/F, in this design as John proposes it.  If this circuit works, which I take on faith, Trig dominates Thresh, which is just a peculiarity of the F/F, not that it matters.

The time delay here is, presuming the cap is initially discharged, the wait for RC to reach (neglecting latencies) 1/3 VCC .  Another idea would be to trigger the 555 on power up using a small RC, than use a big RC to reset the thing via Thresh.  This would give us a crossing-point at 2/3 Vcc for more bang-for-buck out of our timing components, and relieving us from worrying about our device triggering on pwr-up, because we intend for it to.

RC time constant / voltage calcultor

Hi Craigt,

The 555 is just a chip and the delay is selectable by the choice of R1 and C1 in my schematic. There are no plug in modules at a reasonable price though I am sure there are some multi function 24 V time delay relays available for industrial applications. The 24 volt input that you mention would require us to put some sort of simple voltage regulator on the 555 as it is limited to 15VDC max. It does, at this point, sound like your application would be fairly easy to adjust to but if you have no experience you may want to get an engineer or a technician to help with the design. Any ideas or design ideas that I put out on this thread are free for you to use but If you are going to use them for any critical process you must get verification from an engineer that they adequate for what you want to do and will work in your industrial setting. Anyone who is going to put a design together will need all the parameters of the application so they know the voltage going in and how big a load you are controlling as well as any other important aspects. You will also have to decide how you want the system to respond to different inputs. For example the circuit that I have drawn will respond to an 11 second input pulse with 1 second of relay actuation. Is this what you are looking for? There are often many solutions for a particular problem but the resolution will start with a good picture of your operational parameters and a clear description of your goals. Keep us posted how you are progressing.

John

Don is most certainly correct and thankfully sees the flaws in my circuits and corrects me. We should add a resistor between the #3 pin output of the 555 and the base of the transistor to limit the current draw on the 555. I put a 1K into my test circuit and it worked the same way. Since this is a design that I got from the 555 circuit book listed above I am not sure what Don means by wanting the #6 Thresh pin tied to ground. I will have to experiment with this tonight to see what that does. The current analysis is that the circuit works as drawn but the insertion of a 1K resistor between the #3 pin of the 555 and the base of the transistor is a very good idea.

John

If the reset pin isn't asserted on 555, a signal below 1/3 Vcc on Trig sets the F/F and a signal greater than 2/3 Vcc on Thresh resets it.  It seems Trig dominates Thresh.  If this doesn't make sense, read about set/reset dominated flip-flops on google.  The combination of two comparators, a three-resistor string and a flip-flop is a very common occurrence in analog, the 555 is a great idea!  John is a great spirit and always accepts my chiding with good humor.  I try to twit him, but it never avails.

http://www.ti.com/lit/ds/symlink/lm111-n.pdf

The 311 comparator works up to 36V, if we use it, we won't need the VR, not that it a bad idea.  Look at example #50.  The output stage of the comparator is open-collector NPN.  So they are using it to drive a PNP output transistor, forming a http://en.wikipedia.org/wiki/Sziklai_pair.

The input stage works to slam the output stage based on the differential input - the offset voltage.  Gain is very high, but there is a small zone of metastability.  To repeat, if the + input is slightly higher than the - input the output slams high, if the - input is slightly higher than the + input it slams low.  Input range is a consideration with comparators.

We can make a string of a resistor (the 'ballast') and a backwards Zener and hook them across the supply to provide a reference, or we can just use a 2R string.  This would go to one input of our comparator, the other input would be attached to the tap of the timing RC across the supply.  When C charges past the threshold-we switch.  We might hook up a very large resistor (small conductor) from the output of the comparator to the + input (for example) to keep our circuit from chattering.  This is a form of a 'Schmidt trigger,' which removes the metastability.

You can get an Omron H3Y-2 H3Y  timer relay (DC24V 5A, 2-60s) for about $4.65

Hi Doug, I checked my suppliers and they are all listing this relay for about $100.00. Can you share a source with us?

John

I just did a quick search on ebay - there were lots of suppliers.