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Author Topic: Delay off circuit  (Read 1797 times)

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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.


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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.


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Re: Delay off circuit
« Reply #2 on: December 03, 2014, 22:28:25 PM »
Well, it seems that you do not precisely know what you want to make. The  design you originally want has 2 wires, which means that when the switch is off, it has to have stored enough power to illuminate your LEDs, which is capacitors (or rather supercapacitors) what you want.

The other design is a turn off delay. It uses the capacitor as a timer along with a transistor and a resistor. But this design still requires a power supply connected to power the LEDs.

So, what are your requirements really?