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momentary output after ign off


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howie ll 
Pot Metal - Posts: 16,466
Pot Metal spacespace
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Posted: April 15, 2012 at 11:20 AM / IP Logged  
The bloody diode. I've already explained that.
oddity 
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Member spacespace
Joined: May 08, 2011
Location: New York, United States
Posted: April 15, 2012 at 11:31 AM / IP Logged  
In the diagram you supplied the diode is in parallel with the coil how does it stop current from going through both paths?
howie ll 
Pot Metal - Posts: 16,466
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Posted: April 15, 2012 at 11:38 AM / IP Logged  
A valid question if you know nothing about relays but believe me it does. If you're not sure place ANOTHER 1N4004 in line between the cap and the resistor before 85, band away from relay that's another variation, maybe I didn't modify the original enough.
Look now:- 784_constant_to_pulse.bmp
oldspark 
Gold - Posts: 4,913
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Joined: November 03, 2008
Location: Australia
Posted: April 15, 2012 at 8:45 PM / IP Logged  
That's the problem with "physical" diagrams.   
Forgive me if I am pre-coffee confused, but don'tcha hate these Juniors that are right?
Good onya oddity! Thanks for questioning and clearly stating your understanding... er, I mean confusion. momentary output after ign off - Page 2 - Last Post -- posted image.
(I usually draw "circuit" diagrams.)
If drawn showing the relay coil, it is more obvious that without the extra series diode, there is always current flowing thru the coil & 10k resistor (ie, ~12V/10k =1.2mA).
Of course NOW the problem is that the relay will never turn on because of the blocking diode.
Maybe it is a situation where the circuit should be reversed - ie, +12V IGN thru he diode to #86 (#85 to GND) and capacitor (its -ve to GND).
Theory: IGN +12V turns on the relay charges the cap. When off, the diode prevents the cap discharging thru IGN +12V circuits (which is effectively GND). The cap discharged thru the relay coil, thereby holding it on for a while.
The "time constant" tis RC (which is when the RC voltage reaches ~67% of its target, ie, from ~+12V down to GND = 0V in this case) where the "R" is the resistance of the coil.
Hence if ~60R (Ohms), C = t/60, eg, ~0.15F (150,000uF) for ~10 seconds. (Assuming the relay drops out at about 67% down (~4V) which is probably a bit low, but t=RC is still a good approximation.)
The latter also shows why I don't like RC delays direct on relay coils (too high a current (meaning too low a resistance) means a BIG cap).   
Instead, Id rather a transistor "buffer" so the RC handles the transistor's Base current which is far less than the relay coils.
But I'll leave that for later if needed, and assuming I'm correct about that undesired 1.2mA constant leak.
Howie, do you concur?
BTW - the original "reverse biased" diode between 85 & 86 is merely for suppression of the coil's "off" voltage spike.
Meanwhile, coffee and breakfast...
howie ll 
Pot Metal - Posts: 16,466
Pot Metal spacespace
Joined: January 09, 2007
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Posted: April 16, 2012 at 2:51 AM / IP Logged  
Oops red faced he redoes the second diagram, yes I did realise about 3:30 this a.m. but thought the better of getting out of bed momentary output after ign off - Page 2 - Last Post -- posted image.
Revised:- A5B_constant_to_pulse.bmp
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: April 16, 2012 at 5:14 AM / IP Logged  
And I missed that it was to be a pulse output.
I was describing a relay that held on for a period of time...
This is a typical specialized application a differentiator (IGN off) and integrator (time delay), and that can't be done with "passive" components. Well, not easily anyway.
dualsport 
Silver - Posts: 983
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Joined: September 27, 2005
Location: United States
Posted: June 10, 2012 at 10:14 PM / IP Logged  
Drew this up back in 2002, but it does what you're looking for; you'll only need the top half of the circuit. Ignore the bottom.
momentary output after ign off - Page 2 - Last Post -- posted image.
There should be negligible current draw after the pulse is delivered on ignition shutdown.
You can tweak the component values as needed to change the pulse output duration if you want.
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