Need help on 12V Time Delay PLC Relay Module

[Lyle Simons]

I have a project that requires a time delayed 12V DC supply. The "**broken link removed**" I found on ebay for $3 works perfectly - except for one problem......

When the 12V input to the circuit drops, I want the timing sequence to reset. It does reset if the 12v input is off for ~2 seconds - but if you drop and re-apply the 12v input too soon, the timing sequence doesn't reset. This won't work for my application. I need the timing sequence to reset immediately after the 12v input is off.

I hope I've described this accurately. I'm not very circuit savvy - so please respond appropriately (30 years of application development at IBM speaking here - but not much EE experience).

thanks in advance,
Lyle in NC

 

[cowboybob]

Welcome to ETO, Lyle Simons!

Is your entire circuit a 12VDC power source, the delay device from ebay and some load? And dropping the 12VDC and then re-applying it is sum total of action(s)?

There appears to be a small Electrolytic cap at the input to the board. I suspect it is keeping the delay timer powered up (for ~2s, as it discharges) when the 12VDC power is removed.

While perhaps not the best idea (you'll be removing the Vcc filtering and stabilizing device) I believe you could remove that cap and remove the problematic power-off reset delay.

Are you still at Research Triangle?

 

[Lyle Simons]

Thanks for your quick reply cowbowbob. The only action is 12v from a micro-switch to the delay timer that powers a 12v pneumatic solenoid valve after a predetermined delay. Since this is intended for a mass produced item, I wouldn't want to have to desolder/modify each board. Do all delay timer circuits use a capacitor for the filtering?

Yes, I'm in RTP. I've dodged more layoffs than I can count! Just give me 4 more years till the mortgage is paid off!

 

[cowboybob]

OK.

Googling around, seems there are available delay relay circuits that will reset on power off but they require an additional "delay initiation" trigger. And prices rise rapidly.

Off the top of my head, if you were to use a DPST relay in the timer power circuit, as below:

- at "Timer power" OFF (current screen shot), relay contacts will remove power from the Delay Timer and then immediately short the Delay Timer 12VDC input contacts, instantly removing the charge on the input cap AND stopping the timer circuit. This will allow, essentially, on re-application of power ( irrespective of the duration), a timed delayed output from the timer.

Of course, any power ON condition duration that is less than the selected output delay duration will truncate that delay.

Admittedly, adds expense and complexity but, as a workaround with your existing equipment, I don't at the moment see another option.

 

[Lyle Simons]

I found this article that describes my issue exactly - although it doesn't identify any commercially available circuits - only the schematic: **broken link removed**

Thanks again for your valuable input. So, I presume I could test this revised circuit by manually shorting the timer 12V and ground inputs immediately after switching off the power - correct?

Would it be possible to show me the inputs and outputs using the attached "cartoonish" DPST layout? I'm schematic-challenged.

 

[cowboybob]

Lyle, I inadvertently mislabeled the relay in my schematic. The one I showed is a Single Pole, Double Throw (SPDT), NOT a Double Pole, Single Throw (as you correctly showed in the post above). Please forgive my error.

Below is the correct SPDT relay symbol (and name! ), plus a more graphic example of its wiring scheme. NO and NC descriptions are for a relay at rest, i.e., NO power to the coil. The COM connection is the "pole".

... So, I presume I could test this revised circuit by manually shorting the timer 12V and ground inputs immediately after switching off the power - correct?...

Yes, if your quick.

 

[KeepItSimpleStupid]

Way way back, I designed what might have been a marketable product when there was no Internet or ebay for turn-on delay's for car stereos. Specs were like 0-4 s, 12 V, 1A , short circuit protected, reverse polarity protected via fuse. # wires, in, out and gnd. I used it on my car stereo. Reset was less than about 10 uS providing power in dropped to about 4V. Still have a bunch of unpopulated boards (lead reflow) and the IC I used isn't made any more. Once it times out, the timing cap was shorted too.

I had a head-unit that had a regulated power supply for an antenna amp which was used as the trigger. It did not collapse quickly.
A quick on/off/on/ cycle of the ignition switch could cause it to mess up. If the 12 V dropped to <4V or so it would have been fine.

Tough design, but ultimately seen as too expensive.

Commercial times need about 80-100 mS to reset/recycle. e.g. http://www.ssac.com/files/downloads/30/ERDM.pdf