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  • Noob Question about relay...

    I'm almost done designing my carputer. I'm using an inverter setup, despite the popular choice of a dc-dc power supply. I want the inverter to only be powered when the car is on, as it will be in the trunk and out of the way to switch on and off.

    If I'm correct, I can leave the power switch of the inverter in the ON position, wire a relay into the power line of the inverter. The relay would then be wired to the ACC line or the ignition wire. The acc line would be easier I'm guessing. If this is all correct, my question is, what size/type of relay will I need? Do I need a 12v relay capable of handling the size of my inverter (planning on 400w as of now). That being said, I need a 12v relay, capable of handling 64A (according to http://www.the12volt.com/wiring/recwirsz.asp) if I'm understanding this correctly?
    Carputer Design - %95
    Parts Ordered - %10
    Parts Installed - %0
    Tested - %0

  • #2
    I don't think I'd plug the relay up to the acc because your acc turns on when you turn your key to the acc position and turns off well cracking the car (maybe there are relays for this), but I think the easiest would just be plugging it into the ignition signal and add a switch. You can order relays online or you can go to the part store tell them the volts and amps you want, so say my computers gonna be running 20amps max then I'd get a 12v relay that can handle over 20amps. The reason is because when your cracking your car the voltage drops.

    links to look at:
    FAQ
    Wire Diagram I'm working on for my carpc i'm doing

    oh also because your running an inverter (personally I wouldn't) your gonna have to figure out a way for its power not to be cut when you turn the car off or else it will be like unplugging your computer everytime without shutting it down. The DC-DC PSUs actually shutdown your computer and all your devices for you when you turn off your car.
    2002 Honda Civic Ex --Worklog

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    • #3
      You can solve this problem one of two ways:

      1. Use a DC-DC power supply.

      2. Use a timed relay for the inverter. Because the relay will signal the inverter to start as you move the key from acc to "start", the inverter will momentarily signal the computer to start, then cut power to it. Using a timed relay set to, say, 3 seconds will prevent the signal to the inverter until after the car is started and running.

      A 64 amp relay will be BIG, by the way. Most of the circuits in your house are 15 amps at 120 volts. That is a honking big relay. Therefore, what you probably want to do is to substitute the relay (or wire it in parallel) for the on/off switch of the inverter. If it is a quality inverter, the switch only acts as a signal to the circuit to start the inverter rather than running the entire 64 amps through the switch.

      You'll be able to tell if this is the case by looking at the wires and the switch itself. They would be REALLY THICK GAUGE if they carry the entire load. I'd be surprised if that were the case.

      As cla1067 mentioned, that solves only half of the problem. The other half is shutdown. You can simply cut power and get away with it for awhile....or not. If it is Windows, eventually you will have problems. I understand that Linux is a bit better about this but it is still not a good idea to shut any OS down without warning.

      You can see why people use a DC-DC power supply. They seem pricey until you add up all the stuff they do for you.
      Originally posted by ghettocruzer
      I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
      Want to:
      -Find out about the new iBug iPad install?
      -Find out about carPC's in just 5 minutes? View the Car PC 101 video

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      • #4
        I planned to wire the computers power button into my center console so that as I pull up to my destination, I press the button, and shut it down. (I have to lock up my head unit anyway, which would gives me extra time to let it shut down before cutting the power).

        I guess I'm staying away from a DC-DC power supplies is because I don't have much knowledge of them. I mean, I've read all the FAQ I can without my eyes bleeding and I'm not grasping something here. I've been building computers for 5 years and obviously never used one of these. I'm going to be using a normal setup in a case and I dont get where the DC-DC power supply goes or how it hooks up to the computer. I understand how the supply hooks up to the car though.
        Carputer Design - %95
        Parts Ordered - %10
        Parts Installed - %0
        Tested - %0

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        • #5
          Hey bugbyte whats the cheapest way to get a timed relay? It only needs to handle 5A or more and only needs to last 1 second or longer. I want to hook a hot wire and ignition signal wire to a 3 position switch and then hook the switch to the realy. The problem is is that if I switch the switch then the power is going to be lost for however many miliseconds. I can either go with a timed relay or two switches and a normal relay, but I think I'd rather just go with one switch and the timed relay.
          2002 Honda Civic Ex --Worklog

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          • #6
            Originally posted by TimS2k View Post
            I guess I'm staying away from a DC-DC power supplies is because I don't have much knowledge of them. I mean, I've read all the FAQ I can without my eyes bleeding and I'm not grasping something here. I've been building computers for 5 years and obviously never used one of these. I'm going to be using a normal setup in a case and I dont get where the DC-DC power supply goes or how it hooks up to the computer. I understand how the supply hooks up to the car though.
            Basically the input wires hook up just like a head unit. A ground wire, one to a always live Battery wire and one to the ignition circuit, that's it.

            On the output side, their are various wires at different voltages designed to give you the range of voltages you need for your PC. So assuming its for a desktop board, you would plug it straight to the computer and eliminate the built in 120V power supply in the computer. It also sends a "smart" signal to start up the PC, ignore low voltages during engine cranking and provides a signal for shutting down.

            These PS really are the best way to go. If you need help selecting or wiring one, there are plently of people here that can help.
            My 2007 Ford F350 Work Log located HERE

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            • #7
              Originally posted by TimS2k View Post
              I planned to wire the computers power button into my center console so that as I pull up to my destination, I press the button, and shut it down. (I have to lock up my head unit anyway, which would gives me extra time to let it shut down before cutting the power).

              I guess I'm staying away from a DC-DC power supplies is because I don't have much knowledge of them. I mean, I've read all the FAQ I can without my eyes bleeding and I'm not grasping something here. I've been building computers for 5 years and obviously never used one of these. I'm going to be using a normal setup in a case and I dont get where the DC-DC power supply goes or how it hooks up to the computer. I understand how the supply hooks up to the car though.
              You're making it too complicated.

              1. You know where the ATX PSU inside the computer case is? Unplug it from the motherboard, remove it from the case and store it inside the house.
              2. Get one of the ATX compliant DC-DC PSU's and hook it up to the car.
              3. Plug the ATX cabling into the motherboard and the ACPI switch.
              4. Start the car and enjoy.

              If your PC is over 250 watts, though, you may be stuck with an inverter install.
              Originally posted by ghettocruzer
              I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
              Want to:
              -Find out about the new iBug iPad install?
              -Find out about carPC's in just 5 minutes? View the Car PC 101 video

              Comment


              • #8
                Originally posted by cla1067 View Post
                Hey bugbyte whats the cheapest way to get a timed relay? It only needs to handle 5A or more and only needs to last 1 second or longer. I want to hook a hot wire and ignition signal wire to a 3 position switch and then hook the switch to the realy. The problem is is that if I switch the switch then the power is going to be lost for however many miliseconds. I can either go with a timed relay or two switches and a normal relay, but I think I'd rather just go with one switch and the timed relay.
                Google up "Automotive time delay relay" and you'll find plenty. [Edit: here's an mp3car thread that discusses the issue]

                There are also lots of diy kits out there that you can build your own. That's what I did.
                Originally posted by ghettocruzer
                I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
                Want to:
                -Find out about the new iBug iPad install?
                -Find out about carPC's in just 5 minutes? View the Car PC 101 video

                Comment


                • #9
                  not really sure how to make my own, but thinking a capacitor and relay would work since it doesn't have to be exact. Anyways this is what I found and don't know if there's anything better/cheaper:
                  PAC TR7

                  ELK-960
                  CK1614
                  Multifunction Relay Module
                  Pulse Timer with Spdt Relay

                  wouldn't a capacitor like this work and a normal relay?
                  2002 Honda Civic Ex --Worklog

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                  • #10
                    Originally posted by cla1067 View Post
                    wouldn't a capacitor like this work and a normal relay?
                    Yep = for about 7.8 milli-seconds

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                    • #11
                      I really should have started a new thread :/ I just thought it would help me and the original poster some.

                      Anyways can someone tell me if I got a 15 or 16 volt capacitor what size I'd need in order to make it last a tleast 1 second to run a relay like this or one similar? Even a rough estimate would get me started.
                      2002 Honda Civic Ex --Worklog

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                      • #12
                        ok so a dc-dc power supply (one thats ATX compliant) will hook up to my computer just like a normal power supply? Also, what is this ACPI switch you speak of?
                        Carputer Design - %95
                        Parts Ordered - %10
                        Parts Installed - %0
                        Tested - %0

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                        • #13
                          a DC-DC PSU is almost exactly like a standard ATX PSU except instead of running a power cord to a 120vac wall plug, your running two wires into (battery and acc signal) the PSU and one out (ground). hooks up the computer just like a normal PSU

                          and this is ACPI

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                          • #14
                            Originally posted by cla1067 View Post
                            ...if I got a 15 or 16 volt capacitor what size I'd need in order to make it last a tleast 1 second
                            I'd suggest a 25V relay for 12V automotive use.

                            T = RC where R = Ohms (probably ~160 Ohms for those relays) and C = capacitance Farads, & T = time to change ~2/3rds of voltage - ie, from 12V to ~4V which is probably where a 12V relay will release (drop out voltage).

                            Hence for 1 sec, C = 1/R = 1/160 = 0.00625F = 6,250uF => 10,000uF 25V.

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                            • #15
                              I sent you a pm because now im confused lol.
                              2002 Honda Civic Ex --Worklog

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