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figuring out how to delay a circuit

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  • figuring out how to delay a circuit

    I have finally got all of my stuf to install my labtop in my car but i have a problem. I also have a external HDD going to the radio through the usb port. ive wired it to work off a power inverter and the power inverter is to be controlled by a relay that will be turned on by the acc power of the radio. The acc power is just to signal the relay not to power it. This way i wont have to switch on my power inverter every time i get on the car or accidentally leaving it on. the problem comes from when i start my car the starter requires a bunch of power thus creating a unstable voltage. im looking for something that as soon as i start the car will turn on the power inverter. i was thinking maybe something like a delay relay of sorts id rather do it kinda cheap as well i didnt know if anyone knew how to make one cause id be happy to make it rather than buy it.
    Thanks

  • #2
    I don't think voltage fluctuation is the problem. it sounds like a lack of voltage is the issue. you have your relay triggered by the ACC circuit. the ACC circuit has no power during engine crank.

    ACC has power when the key is in the ACC and RUN positions, and is not powered whe the key is in the START position.

    IGN has power when the key is in the RUN and START positions, and is not powered when the key is in the ACC position.

    sounds like you need your relay trigger to be powered in all 3 key positions. there are two ways you can do it: use 2 parallel relays with separate triggers, or use 1 relay with dual triggers (diode protected).
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    • #3
      ok i havent actually put the system in place but if i understand what your saying if i use the ACC then I wount have power while im cranking right? If so then i could go with a standard relay hooked up through the acc circuit.

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      • #4
        Originally posted by subietuner View Post
        if i understand what your saying if i use the ACC then I wount have power while im cranking right?
        correct

        Originally posted by subietuner View Post
        If so then i could go with a standard relay hooked up through the acc circuit.
        you could, but if you connect it only to the ACC circuit then the relay will turn on as the key passes the ACC and RUN positions, turn off as you start the car, then turn back on when the key returns to the RUN position.

        another issue would be if you're running the inverter with the car off (key in ACC position) then start the car, you'll lose power while the car is starting. you can avoid both issues by using the ACC and IGN lines as relay triggers (using one of the methods I showed you in post # 2

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        • #5
          Alight i get ya now i understand what your saying i was going to run the acc off the stero ACC b/c i have to do the swap this weekend and its easiest. I think ill do the one with the two diodes but what size should i be using of will i have to figure that out my self my invert peaks out at 350 watts if that helps any and im going to be using 30 Amp fuses through the system. Also wheres a good place to get relays??

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          • #6
            use something like this startup / shutdown controller... you can set it to sense the voltage and if it is 13.5 volts or higher it will come on that way it wont turn on the computer till the car is actually running and the alternator is producing the proper power.

            http://store.mp3car.com/Micro_Shutdo..._p/pwr-007.htm

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            • #7
              I got to thinking and i could use a 1 farad capacitor as well in line so there wouldnt be a voltage drop when it is started.

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              • #8
                that would be a total waste to use a cap just for that. a 1F cap costs between $50 and $100 (depending on brand name). if you're going to spend that much money then there are much better ways to go about this...

                the best thing you could do is ditch the inverter idea and buy a DC-DC regulator like the Carnetix P1900. this will replace the bulky and inefficient DC-AC inverter > AC-DC power brick method you're planning now with a smaller and much more efficient DC-DC regulator. the P1900 can directly power a laptop and accessories, it can survive the voltage drop [during engine crank] without any extra devices or extra wiring needed, and it can automatically start/shutdown your laptop with the car.
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                • #9
                  no i was actually going to make a circuit board with a 1 farad or 0.5 farad cap on it maybe smaller id have to do some math on it to make sure but it would deliver a small burst of power when the car is to start but its way to much hassle i can just run a wire off my acc to a toggle switch and ill turn it on after i have started. simple and cheap. i was thinking that somewheres i had seen a cheap way to make a delay switch or something or the sorts.

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                  • #10
                    have a look at 12volt.com ive seen something very similar to this there, or instructables.com have heaps of useful stuff.

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