Will this simple shutdown timer work ???

**JPMG** · 10-23-2006, 12:28 AM

I'm trying to add a simple circuit to my system that will HOLD the ACC input line to my M1-ATX line HOT for a period of time after the car supplied ACC line drops to zero. This will prevent some odd boot problems I have had, plus allow some additional time before the M1-ATX kick in to perform the normal shutdown sequence.

I have seen some simple circuits with a relay, transistor, cap and resistor where the cap was charged, then later discharged to keep the transistor powers a short while (and thus keeping the relay coil working).

One I found used a MOSFET transistor

... don;t have access to that

BUT I do have some 2n2222 ones, and I would like to make it work with that.

The circuit below is what I found with the MOSFET .... would this work and how would I have to change it to use a 2n2222 and what cap/resistor values would I need to set for the time.

To me it the circuit digram does not seem right .... I think something is wrong.

[PS: Yes a 555 circuit could also work, but the mini board I am putting this on is almost out of space, this solution would fit on the board]

Any help will be greatly APPRECIATED!

Thanks!
... Mike ...

UPDATE:

Thanks for the info on the circuit, I corrected it and added your time calculations. Now if anyone finds it in a future search, it will be correct.
... M ...

**Sidewalksalvage** · 10-23-2006, 12:34 AM

Well i cant be sure... dont know if your 10 minute time is right.... (i havent looked up the mosfet or had enough electrical engineering classes to no for sure) It looks like it should keep the voltage for X ammount of time....

Can you create a mock up of the circuit? Cuz if so id say go for it, try it out (outside the car on either a batt or ac-dc transformer) and see what happens. (you also may just want to put a testor on the 12v output line and see how long the current lasts before putting the M1 on it incase of weird reaction) Whats the worst that happens? it doesnt work?

good luck!

**fire** · 10-23-2006, 02:58 AM

yes.. that circuit works...
i attemped to use a 555 timer, but it isnt the best to use...

i came up with the same idea in my thread: (circuit diagram posted there)
LINK TO MY THREAD:.. i did mine for a 30 second delay, but thats easily changed

for a 10 min delay, use a 1.2 M ohm, and a 440uF capacitor..

time is given by T (seconds) =R*C*1.1 only for a MOSFET..
where r = resistance (ohm)
c = capacitance (farads)

so for your 10 min one...
roughly....... 1.2*10^6 x 440*10^-6 x 1.1 =
=1.2*440*1.1 = 581 Seconds. = 9.7 Minutes...

im not sure what resistors you can get, but get something close to 1.2 M ohm... 1/4 watt is fine ...

look at my diagram for a way to quickly discharge the cap so it turns off instantly.

dont even look at using a non-MOSFET... they use too much power and you will need MASSIVE caps.

good luck

**JPMG** · 10-23-2006, 05:30 PM

Thanks for the info ... I fixed the drawing and ordered the parts today.

Also last night tried it with a 2n2222 .... yup ... it draws WAY too much current. After adding five 330uf caps in parrallell it only gave me a seond of hold ... I figured that was NOT the way to go!

Thanks!
... Mike ...

**Sidewalksalvage** · 10-23-2006, 05:38 PM

Let us know how it comes out!

**fire** · 10-24-2006, 04:25 AM

hehehhe good work..
well of course, dont rely on my little formula for calculating time.. i stole it off the 555 timer circuits calculations.. its just assuming that the MOSFET hardly draws anypower, like the 555 timer. ..

if its not mosfet, as mentioned,you will need massive caps, and it is just a waste of time.. they probably will overheat and need time to charge...

with the mosfet, my circuit charges basically instantly, ... i can give it power and straight back off again (Vacc)... and it stays on the for the entire 30 seconds..
yours might require 1 second of charging before allowing it to work for 10 mins.... just make sure the 1kohm resistor is enough.. see if it overheats or anything.. it shouldnt since im using very similar cap size..

just check everything out before you put it in the car. test it a few times... once with a long initial Vacc, and again with a momantary touch of Vacc, etc...
i'd hate to be waiting and timing the damn thing, 10 mins each time..

It should work, i dont see a reason why not.
Good luck..

**JPMG** · 10-24-2006, 11:43 AM

Yup .... always test it and tune it on a breadboard first.

Probably hook up an LED load for the test to see it go on/off.
Once I found the parts on-line, the whole circuit will only be a few bucks and not take up that much space, since I'll use a mini PCB relay.

As soon as they get here .. on to make the little board.

Thanks!