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### Author Topic: Delay off circuit  (Read 1929 times)

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#### Piccolo

• Newbie
• Posts: 2
##### Delay off circuit
« on: December 02, 2014, 00:11:35 AM »
I have a 12 watt LED, which will be on a 12v battery that I want to make a delay off circuit with. Kind of like a dome light, but this is for a car hauler, and the LED will light up various things: steps, side rails, etc.  I want to wire this up as simple as possible. I have seen dome lights in cars where the light abruptly shuts off after X amount of time. Conversely, some dome lights stay on for X amount of time and begin to fade/dim before shutting off. I don't really care how my LED shuts off, I just want to wire it whichever way is simpler.

I'm not super versed when it comes to electricity, but I have been researching some stuff. Feel free to correct me if I'm wrong about any of this. I know I can use a capacitor to store energy. I know if I exceed the voltage of the capacitor it will explode. I know if I wire the capacitor backwards it will also explode. I know the formula for how much energy is stored in a capacitor. ie. 1/2  CV^2.  Assuming I have a 1000 microfarad (uF) capacitor rated for 25 Volts; The cap should be able to store 0.72 Joules?
.5 (1000X10^-6)(12)^2 =.72 Joules Maybe I should be using 13.8 for voltage in the calculation, but that still puts me at less than one Joule.

My LED draws 12 watts and 1 amp at 12 volts. If I wanted to power that for 20 seconds after the switch is opened I would need 240 Joules (12 watts * 20 sec)? I don't know if i'm looking at the wrong capacitors, or just looking at this wrong in general.

I have various types of automotive relays. Is it possible to have a capacitor power a relay after I open the circuit from a physical switch, with the relay essentially keeping the circuit closed until the capacitor discharges. Meanwhile, the LED would be drawing power from the 12v battery until the relay shuts off.

I apologize in advance if this sounds stupid.

Here is a crude MS Paint schematic of what my circuit currently looks like.

#### Piccolo

• Newbie
• Posts: 2
##### Re: Delay off circuit
« Reply #1 on: December 02, 2014, 04:22:21 AM »
Found this video. https://www.youtube.com/watch?v=gQ37vljpHKs I think i'm on the right track with having the capacitor store energy for the relay. Guess I'll be heading to radio shack for a few other components.