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Use relay or not for amp when the interface box provides 10amp accessory

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  • Use relay or not for amp when the interface box provides 10amp accessory

    Hi guys,

    I have a TDA7850 which I believe draws less than 1A on average for my setup (as I mentioned earlier in this thread). I heard from many people that the amp should be attached to the constant 12V through a relay. Recently I found this from Metra during a research of wire harness:

    Different from most of the other interface boxes I've seen which only provide only 1amp ACC, Metra claims this one "Provides accessory (12 volt 10 amp)". I wonder whether I can use the ACC directly and save a relay. Of course I will still run a fuse in between, but eliminating a relay and relay holder will free up some space in my double din enclosure, as I originally wanted to put everything except the interface box in the enclosure.

    Apart from the question whether skipping relay would be a good idea. I want to make sure I understand one more thing correctly: looking at the manual of the wire harness:

    Does it mean the red wire provide 10 amp at maximum? I assume the yellow wire is a direct line to the battery like what I have been from other wire harnesses. I have this question because manuals of other wire harnesses, like this one, clearly states the current rating of the ACC wire and the need of relay if a high current ACC line is required. If the red wire of Metra does provide a 10amp 12V it will enable the possibility of saving a relay for my amp?


  • #2
    Trust PAC to show a non-conventionally wired relay - ie, suppression diode line/Cathode to 85 (because their 86 is GND).

    But yes, the AX supplies 10A or whatever power its supply can source.
    Because the others can only supply 1A, you have that drive a relay if more current is required.

    But why not simply use a cheap 15A or 30A (micro) relay? If you want the other features, then of course I understand.


    • #3
      Hi OldSpark,

      Appreciated you taking time to answer my questions.

      Originally posted by OldSpark View Post
      But yes, the AX supplies 10A or whatever power its supply can source.
      Let me confirm I understand this 100% correctly: so the red wire provides 10amp when accessories should be turned on, while the yellow wire is a direct line to the battery? I asked this question because the manual doesn't say which one is the accessory wire. It does say the red wire should hook up to the radio's ignition terminal though. I am just afraid that instead of yellow wire a direct line to the battery, it's essentially the ACC wire which provides 10 amp, that doesn't make much sense to me, but I just want to make sure.

      But why not simply use a cheap 15A or 30A (micro) relay? If you want the other features, then of course I understand.
      Because I want to save some space, and one less component (and thus less points of connections) means one less thing to worry about. Maybe I am just too paranoid, but I want to make myself clear about all the options before making final decision.



      • #4
        I don't know about the colors - I couldn't find any meaningful wiring diagram...
        Ah - now I see, it's for CAN. In that case yes, yellow must be the supply power (hence at least a 10A circuit if it's to supply 10A) and red is the Acc output.

        I don't see how those units can be smaller than a relay. You don't need a relay socket - flying spades would do. And there are smaller 1A to 10A relays, tho micro-DINs are fairly compact & usually have >15A capacity.
        That assumes you can tap into some ACC source to control the relay (maybe radio power).

        My HU relay (86) is off IGN but via a diode. The HU's remote out goes via another diode to the relay (86). Hence once the HU is on, it stays on until I turn it off by its on/off switch. I could use ACC instead, but I never use that.
        I've been meaning to add a switch to enable on at any time, and another to break the remote 'latching' - ie so when the IGN (else ACC) or anytime-on is off, the HU also turns off. (I guess I'd call that the standard automatic mode.)


        • #5
          Won't I need the CAN unit anyway? Without it where can I get a retained ACC source for a relay, if I only make connection to the radio wire harness?


          • #6
            What do you mean "retained" ACC source?

            It depends how your vehicle works.
            Traditional vehicles had IGN power and ACC power (etc).
            My ancient car's buss system only had +12V with each load being told when to turn on via the buss. (There was was no ACC & IGN per se, tho there was +12V redundancy and an 'aux +12V' function.)
            I don't know enough about CANs to know if they still use an ACC power supply.

            But in all cases, if you can access some device that has ACC power available to it, then that's your relay trigger.
            In trad vehicles that was often the fusebox but could also be the radio switched supply or cig socket supply etc.

            Be aware that ACC drops during cranking, but that's and easy fix with 2 diodes and an IGN connection (from EMS or ignition coils etc).


            • #7
              Originally posted by OldSpark View Post
              What do you mean "retained" ACC source?
              The ACC source is retained even after I turn off the engine, until I open the door or the battery goes too low voltage or a certain time (IIRC 10 minutes?) is passed. It's data controlled by the CAN unit and there's no equivalent of the same source directly from the bare radio wire harness. For example the ACC wire of my PAC C2R-VW2 is an controlled output of the CAN unit. A voltmeter will show there's no equivalent from the input side.


              • #8
                It sounds like what HUs etc usually use.
                But if there is no load in your vehicle that uses that functionality and there is no tap for it, then you have no choice nut to use some CAN decoder.

                For door opening dropout I'd use my above described latching relay system with an SPDT relay in the 'feedback' or hold-on loop. IE - door opening energises relay to break the NC feedback and hence turn off the main power relay. And I'd add a low voltage cutout if that was desired.


                • #9
                  In newer vehicles the radio is controlled by the CAN bus and has no true ACC lead. While you can find an ACC power tap somewhere in the vehicle depending on the vehicle the radio harness operates off the CAN bus which is why the radio will run for 10 minutes after the ignition is turned off and your key is removed unless the door is opened. This is the reason for the CAN interface in a Metra or PAC adapter. Depending on the interface they may also have a buzzer installed in them that produces the normal system buzzers for door open etc since that is all done on the CAN bus.

                  As to amplifier turn on/off stuff and ACC lead...

                  Normally in a radio the ACC lead is really only used as a trigger. The majority of the power is drawn through the constant on lead.

                  The remote turn on lead for an amplifier is normally a 12V+ sense line. It should pull very little power and you should be able to run it from just about any sort of source. I am unsure of the actual draw and may depend on the amplifier but I would be very surprised if it was more than just a few micro-amps. Generally the amplifier is only looking for a simple voltage and doesn't pull any real measurable power so using a relay or worrying about how much power draw is really unnecessary. You should let your radio control the amplifier if at all possible to prevent turnon/turnoff pops in your amplifier. Unless you are using your factory deck pretty much any HU will have outputs you can use to drive your amplifiers directly. You REALLY don't need to worry about a relay to turn on your amplifiers unless you have a large number of amplifiers.

                  For a CarPC... There are some exceptions here to the last paragraph.
                  Look at the power supply of the CarPC. Some have a remote turn on built into them which will prevent the turn on/off pop. Otherwise you will want to do one of two things.
                  #1, tap into the 12volt power line of the PC and power the remote turn on from this. Best solution of these two since it will turn on and off with the PC.
                  #2, run a relay off the power source that turns your PC on and off or just wire the power source directly to your AMP's remote turn on. Just make sure it has a small (1amp) fuse inline with it to protect the amplifier and the wiring.

                  With a Car PC if you are using a source to turn the amplifier on other than a dedicated remote turn on and you get a turn on/off pop or get noise into your sound you may want to try one of two types of relays. A regular relay can help isolate the trigger from the amplifier. PC's are known to create noise that can be picked up by an amplifier so it could be the remote turn on lead. You can isolate this by using a relay near the amplifier(s). The relay coil would be powered directly from the trigger source such as the ACC lead or an internal 12volt PC power line. You would then have a wire from the power in line of the amplifier to the relay contact and the other contact to the remote in line of the amplifier. This SHOULD isolate the amplifier remote lead from any potential noise source from the pc. (If it is really bad and this only knocks it down some you may have to use opto isolation but I know my M3-ATX has a remote turn on lead for the amplifiers... )