No announcement yet.

Using a relay to auto-power on HDD and power inverter

  • Filter
  • Time
  • Show
Clear All
new posts

  • Using a relay to auto-power on HDD and power inverter

    Hello again everyone,

    For the past year I've been using a power inverter to power my laptop and it's worked fine. I also have my external 500g hard drive hard wired into my jeep liberty. I am getting tired of having to flip the switch into the ON position for each of these devices each time I power on my car though, so I picked up a few relays to help me out.

    The problem is though that I am not entirely sure how to connect them.

    I currently have:

    Which is the RL3040, a 12VDC 30/40A from the install bay.

    I've search around for quite a bit but can't seem to come up with a clear answer as to how to connect it.

    What I believe at the moment is that I need an ACC wire connected to the relay, along with the power line from the actual switch on the inverter, is this correct so far? And if so, which number corresponds to what? (Ex. 80 = ground)

    The closest thing I could find to a diagram as to what I am doing is here (it is not mine):

    except that I don't want that switch on the left hand side of the diagram. I just want the inverter to power on when I turn the jeep on.



  • #2
    That's a standard bosch relay.

    Either coil -> ground
    Either coil -> ACC
    Common -> battery
    N/O -> Inverter


    • #3
      Curiousity, thanks for the quick response. Sorry, I've been busy traveling and tied up with work for the past few days.

      So I need:

      12v line

      all running to the relay, then finally the n/o goes to the inverter, but specifically where on the inverter? am I popping off the on/off toggle switch and soldering the wire there?


      • #4
        I assumed you would put the relay inline with the main power going to the inverter. Did you want to switch it on without acc as well? That may be a little harder to explain.


        • #5
          Hi Starter.
          I'd suggest ignore the upper RHS balloon (console on/off, inverter etc) - just put fuse in its place (in the red +12V to #87; ignore the battery ground).
          Then #30 goes to your load (inverter, PC etc).
          (Normally the supply would go to #30 and the load connected to #87 - ie, R's about as drawn - but that won't matter is 87a isn't used. 87a should be the NC = Normally Closed contact - ie, #87a connects to #30 when the relay is off/de-energised.)

          #86 goes to IGN or ACC instead of the switch. But keep in mind that ACC turns off when cranking whereas IGN is on during cranking. (Hence I recommend IGN unless the load is too high (probably not) or cranking causes brownouts or transients that may damage the inverter).

          FYI - 85 & 86 are the relays "coil" or solenoid that actuate the main contacts (30, 87, 87a). These are interchangeable unless the relay has an inbuilt diode (snubber, quenching etc diode to prevent coil induced transients) in which case #86 is +12V & #85 is 0V/ground.
          By convention, relays are usually shown with #85 as gnd (or the more negative) and #86 at +12V (or the more +ve). That's just in case a diode-protected relay is used. (BTW - I hate them. I prefer to add my own external diode and used 85 & 85 interchangeably.)

          Sorry for the above confustigation, but hopefully is "redraws" your diagram as well as adding some eventually useful information.
          The original diagram is somewhat vague and ambiguous (like why have a switch in the RHS balloon, and why the inverter - is the relay output AC power??).

          An alternate method you may consider that overcomes cranking issues is a relay that is on whilst the engine is charging.
          If you have a charge-lamp in your dash, it should be simple.
          The only problem then is when the engine stalls, the relay opens. (So to with IGN off, but that happens anyhow.)
          But there too there is a simple trick that can be used to keep the relay closed until you push a button. (It's still the same relay, but with 1 or 2 extra push-buttons, and maybe 1 or 2 diodes.)

          I have posted such diagrams somewhere. I think here on mp3car, but otherwise maybe on
          the12volt also have similar connection regimes (though I seem to be one of the few to use charging-control).


          • #6
            Curiousity and Spark,

            Thank you very much guys. The instructions were very detailed. I'll take a deeper look at it when I get home to get a better understanding of it all.

            As far as that diagram goes, it's not mine - I just grabbed it off the internet, but it was the closest thing I could find for what I was looking for.

            What is an upper RHS balloon?

            By #30 going to the load (inverter/pc) what does that mean exactly? The wire is connected/soldered to....the switch on the inverter?

            My goal is to have the inverter turn on whenever the ignition is turned on in the car.


            • #7
              Originally posted by Curiosity View Post
              That's a standard bosch relay.

              Either coil -> ground
              Either coil -> ACC
              Common -> battery
              N/O -> Inverter
              Curiosity, have you recommended one of your relays? Just to explain, Curiosity builds Solid State relays. i have a DQP which I wanted to switch to a certain mode when the car was turned on. His relay solved all those issues. I am sure one of his solutions can be adapted to your setup.
              Nirwana Project, the Android/Win 7 hybrid system!

              1X Ainol Novo Flame Tab
              4X MK808b
              3x Perixx Touchpads
              3x 7 inch Screens
              1X 7 inch motorized Screen
              1x Win 7 PC


              • #8
                Oh do you know how many times I could suggest those per week? I don't know if I'm allowed to though. But the timer part is what I try to help with, plus I don't know how much current is going through the switch. The small timers don't halde that much. That's a good idea though.

                The relay could be connected coil to ground and ACC. Then the common and N/O to the two sides of the switch inside the inverter. I'll draw up a diagram if you like Startingline? You could also put a switch in series or parallel depending on if you want to manually switch it on without ACC or off with ACC like the top pic.


                • #9
                  Originally posted by Startingline13 View Post
                  What is an upper RHS balloon?
                  By #30 going to the load (inverter/pc) what does that mean exactly?
                  The balloon means the text box with "on/off button or game console....".

                  #30 to the inverter/pc means:
                  The battery +12V goes through a fuse to #87.
                  #30 goes to whatever 12V load you want to turn on.
                  - IE: when +12V is applied to #86, #30 connects to #87.
                  (A fuse should replace that "on/off button" balloon.)


                  • #10
                    Thank you gentlemen, I think I'm starting to get the idea behind it now. A digram or picture would be greatly appreciated. I'm going to try to throw something together later as well when I get a chance. I've had a busy couple of weeks so I haven't had an opportunity to work on this yet, just trying to acquire the information before I start throwing the relay in there.

                    A toggle switch (like what I have in my arm rest for my to turn the laptop on/off) would be useful as well.

                    Where can I find your solid state relays?

                    And this is a bit off topic, but I looked at your link for the Xport and I think that is what I could be looking for as another solution for a problem. Often when I restart the car, Centrafuse will not pick up the com port for the GPS dongle automatically, and I use GPS gate. Will that software solve that problem?


                    • #11
                      OK, here's a quickie. This will control power to the inverter 3 with options. Switch to right - only with key on, switch middle stays off and switch left always on.

                      To use just an on/off toggle as in the original, connect it to the right 2 wires only.
                      Attached Files


                      • #12
                        Wow, that basically cleared everything up for me and made it 100x easier.

                        My only question is the blue 86: what's that?


                        • #13
                          Ha ha! The blue is the "centre pole" - the one that gets flipped from battery to ignition (and through off if it's a 3-position).

                          So hence the blue connects the ACC/IGN or Batt+ +12V to the relay's actuation coil (86) whose other side is grounded (85). (Hence when 12V across 86 & 85, the coil energises and "pulls in" the relay contacts - ie, connects 30 to 87 - hence powering the inverter (and any other 12V thing you want powered - a 30A relay should handle at least a 300W load - ie 300W inverter.

                          Alas I dislike those "wiring diagrams" - I prefer the "circuit diagram". But that's because I have no problem translating the circuit into real connections - and I prefer to see the circuit's operation.
                          But both have their uses. It's just that nerds don't need the wiring stuff - except when not nerdy enough, or too tired, or checking why the circuit fails or smokes!

                          I might eat my nerds and post an equivalent circuit later.... Then the operation should be fairly obvious....

                          EDIT#1 - for Curi below - yep, ain't it a ship! I hate passing ships in the night.
                          About time I beat you at something though!


                          • #14
                            You don't have to use blue. I'm just using different colors for fun. 85 and 86 are connected to a coil. When electicity flows through, it creates a magnetic field and closes the contacts on 30 to 87. So one side of the coil is grounded, and the other side is connected to a switch, then to power (or ACC in this case). Think if the blue and green as connected when the switch is on.

                            Edit: Ah, beat me to it!


                            • #15
                              I think the simple diagrams are much easier for the non-electronics guys. Schematics might make it harder for them to visualize it as in what wire goes to what. Keep It Simple.