Momentary relay?!

[MikeMl]

The capacitor charges through the resistance of the relay coil. The current is high initially, which pulses the relay, and then it dies off with an inverse exponential as the capacitor voltage approaches 12.6V (Classical RC circuit). After the inital current pulse through the relay, the capacitor voltage sits at the battery voltage until the Ign switch is turned off. Current cannot flow through the relay coil while both ends are at 12.6V.

 

[deanwarez]

ah... i understand it alot better now. thank you. x

 

[deanwarez]

ok slight problem with the circuit - and its my fault really for not noticing and specifying in the original post...

the change from accessory position (ignition) to ignition causes a small drop to 0V then back up to 12v! so it charges the relay twice - opening and then closing the screen. i do however have a regulated 12v output that comes from my car-pc when its on and obviously 0v when its off.

so... could you guys re-design it so that it can use the regulated 12v source rather than the annoying flucuating car wiring.

cheers

x

 

[MikeMl]

Look at the wires that come out of the ignition switch. There should be one that stays lit even as the key is moved to START. It sounds like the one you found is the one that powers all of the non-essential stuff which is shut off during cranking. They do this to unload the battery so as much current is available to the starter, and to a lesser extent to protect all of the "stuff" from transients during cranking.

Before suggesting something, I would need to know how much current your switched 12V can supply. A 200 Ohm relay draws 60mA. Can you steal that much? If not, an opto-isolator circuit would be my next try.

 

[deanwarez]

my 12v supply can handle ALOT. its a seperate PSU dedicated for 12v output for externals - and is currently not using anything. so it could handle about 10A i believe!!! im thining that its better to do it from my regulated PSU. x

 

[deanwarez]

bump! ;-) x

 

[MikeMl]

Here is a two relay solution: