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Help with Constant to Momentary circuit (for power switch) please.

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  • Help with Constant to Momentary circuit (for power switch) please.

    I'm trying to build a circuit to turn on my laptop when the car is on. I got the diagram from here.



    This is how I built it using components from Radioshack.

    The problem is it doesn't work consistently at all. Like it works for a couple times then stops working (no pulse sent to laptop). Did I do anything wrong?

    (The red wire from the wire to the power switch is connected to 30, btw).

    Thanks.
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  • #2
    The problem could be the duration of the pulse.
    Try using this formular and subtitude component if needed.
    Time (seconds) = Resistance (ohm) X Capacitance (farad)
    I am not 100% sure if that formular work for your case, but you can try.
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    • #3
      The the12volt.com site says that circuit will give about 1/2 second of continuity between 30 and 87. Is that correct according to your formula?

      So if I want to get 1 full second, I need a bigger capacitor, right? How about 2200 µF?
      iGMon - Integration between iGuidance and Road Runner
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      • #4
        perhaps reducing the resistance would also increase time and allow you to keep the circuit as small as possible.
        http://neonpc.blogspot.com
        [||||||||||||--------] 60% done

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        • #5
          Actually, taking a second look, the resistor in the diagram is in parallel with the capacitor so it wouldn't matter at all, would it?

          The job of the 10K Ohm resistor is just to bleed off the charge in the capacitor when power is removed so that next time, when power is applied, there will be current through the capacitor again (until it's charged up).

          Am I correct?
          iGMon - Integration between iGuidance and Road Runner
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          • #6
            Yeah, i think you are right. I'm a bit confused though. Due to the presence of the resistor, couldn't current always flow through this thing? I'm no electronics expert as you can see.
            http://neonpc.blogspot.com
            [||||||||||||--------] 60% done

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            • #7
              The resistor is so big that there's no current through it. (Otherwise, the power switch would always be shorted despite the capacitor there).
              iGMon - Integration between iGuidance and Road Runner
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              • #8
                *delete*
                http://neonpc.blogspot.com
                [||||||||||||--------] 60% done

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                • #9
                  Originally posted by b8bboi
                  Actually, taking a second look, the resistor in the diagram is in parallel with the capacitor so it wouldn't matter at all, would it?

                  The job of the 10K Ohm resistor is just to bleed off the charge in the capacitor when power is removed so that next time, when power is applied, there will be current through the capacitor again (until it's charged up).

                  Am I correct?
                  Actually not. The resister is there to delay the discharge of the capacitor. The higher the resister, the slower it will discharge, in essence, keeping the relay energized. The other problem with increasing the value of the capactiro or resister, to increase the relay stays energized, is that it will take longer for the capacitor to charge. This may not be an issue in your setup, like if an extra second before the computer starts doesn't make a difference, then that would be the way to do it. It would probablly be best to increase both slightly.

                  Michael
                  ...I love the French language...especially to curse with...Nom de Dieu de putain de bordel de merde de saloperies de connards d'enculés de ta mère. You see, it's like wiping your *** with silk, I love it.

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                  • #10


                    and buy a soldering iron
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                    • #11
                      Wiredwrx, thanks. I'll try using a 2200uF cap and see if that would help.
                      iGMon - Integration between iGuidance and Road Runner
                      CFX - Skin for Road Runner

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                      • #12
                        Originally posted by b8bboi
                        I'm trying to build a circuit to turn on my laptop when the car is on. I got the diagram from here.
                        ...
                        The problem is it doesn't work consistently at all. Like it works for a couple times then stops working (no pulse sent to laptop). ...
                        Thanks.
                        The purpose of the resistor is to discharge the cap when the circuit is de-energized. Assuming the cap is fully discharged when you start, it has 0V across it. When you first turn on, the relay coil resistance is in series with the cap/resistor. Since the cap was at 0V then all of the 12V is applied to the relay coil which turns it on. The cap starts charging and its voltage increases up to a point which is near 12V. As this happens, the voltage across the relay coil decreases by the same amount down to near 0. When it gets low enough the relay's magentic field gets so weak it lets go of the contact arm. The circuit remains in that state until the 12V source is removed. Then the cap discharges through the resistor in parallel with it. The larger the cap and/or the resistor value, the longer it takes to discharge. If you turn on the 12V source again too soon, the cap starts charging again but the relay won't turn on if the cap still has high enough voltage across it. Changing the cap to a higher value will give a longer on pulse but it will also require a longer "recovery time" between turn off and restart. A smaller resistor will shorten the recovery time, but too small a resistor will limit the voltage the cap charges up to and at some point (depending on the relay coil resistance) you would never get to the point where the relay turns off. Keep the resistance at least 10 times higher than the relay coil resistance to avoid that.

                        Hopefully understanding what the parts do will help you arrive at the timing you desire.
                        ~Jimmy

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