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Current draw from HU remote wire??

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  • Current draw from HU remote wire??

    Hi

    I've searched but haven't found the answer, so no flaming please!

    Simply wondering if it's ok to power relays from the remote wire from my head unit??

    I know they're commonly used to switch amps etc on and off, but I'd imagine this will be driving a transistor based switch which takes much less current than a relay's coil.

    Many thanks

    Jamie

  • #2
    It's fine to use it to switch the coil, but personally, I would use a separate feed for the power to the relay (30-pin). In addition, you might want to put a diode in-line with the remote wire, to stop any spike going back down the wire to the HU when the coil collapses.

    Panch

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    • #3
      Yes, it will be much safer if you use the diode, because relay is an inductive load which could damage the remote output if it is not well protected. If there is not enough current to close the relay, you could use an appropriate transistor for switching the relay, which would do the trick.

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      • #4
        Thanks - the diode's a sensible idea which I must admit I hadn't thought of.

        Yes I guess I could just set up a transistor and fire 12V at its base, but the simpler I can make this the better!

        Do you suppose there will be enough current to do this? I bought an SPNO relay so that I can switch my screen psu on and off with the head unit.

        Panch - what do you mean by "30 pin" for a separate feed to the relay?

        Thanks again.

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        • #5
          Originally posted by Jamieev
          Thanks - the diode's a sensible idea which I must admit I hadn't thought of.

          Yes I guess I could just set up a transistor and fire 12V at its base, but the simpler I can make this the better!

          Do you suppose there will be enough current to do this? I bought an SPNO relay so that I can switch my screen psu on and off with the head unit.

          Panch - what do you mean by "30 pin" for a separate feed to the relay?

          Thanks again.

          The "30 pin" referes to one of the pins of a standard automotive relay. Pin 30 is the high power source. Basically, and automotive relay is rated at 30 or so amps, pin 30 is where you would connect your high power to provide those 30 amps. You want to use the remote turn on to power the coil, whcih is at pin 85 and 86. I have measured, and also contacted a couple of manufacturers of automotive style relays, and I have found that the current draw of the relay coil is between 100 and 250ma. That is a very small amount actually. Most circuits won't notice an extra amp of draw, so you can power between 4 and 10 relays before you have any issues. You might be able to find how much power that remote turn on wire provides if you have the manual, or perhaps a "google" search for your stereo will provide it. However, do not attempt to use the remote on to power the actual device, your monitor, because monitors generally draw about 2-3 amps, which might be too much.

          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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          • #6
            Wiredwrx, you are mistaking. Remote on wire has maximum output current of a few hundreds of mA. Not only you can't connect 10 relays to it, even one could be too much for remote wire. Everything depends on how much current is needed for a particular relay.

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            • #7
              Originally posted by Slowmo
              Wiredwrx, you are mistaking. Remote on wire has maximum output current of a few hundreds of mA. Not only you can't connect 10 relays to it, even one could be too much for remote wire. Everything depends on how much current is needed for a particular relay.
              That may be truem but not "everything depends on how much current is needed by a particular relay" Actually, it depends ont he relay, on the amp, and ont he HU. I suggest determining the maximum current draw allowed from the HU, and the draw of the amp on the turn on lead, and the actual amperage needed for the relay you intend to use. If it can only power a single relay(A little research turned up that some HU can supply 250ma), you can use that relay to trip a multitude of other relays to accomplish what is needed like tuyrn on the the amp and any other device you want to turn on.

              Also, while I posted in this thread, the information was of a more general nature. Most automotive circuits CAN handle an additional 1 amp with out a problem.

              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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              • #8
                TBH, I think you'll probably be fine switching a couple of relays off a standard HU remote wire, but if you're in any doubt, just switch the one relay off the HU remote, and then use the output from this to switch all the other relays.
                Either that, or go the whole hog and buy a Phoenix Gold DD10..

                Panch

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                • #9
                  Thanks again for the replies. I see what you mean about pin 30 now I looked at my relay

                  I had no intention of powering anything off the remote wire other than the relay, hence the relay being there really. But thanks anyway.

                  I measured the resistance of my coil last night using a multimeter (yes yes I know it's an inductive load and sticking an ohm meter across it is not 100% correct!) Can't remember what the R value was but I worked out roughly 150mA at 12V.

                  Just realised how simple it would be to wire a transistor in for this purpose infront of the relay so I think I'll do that.

                  Thanks again

                  Jamie

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                  • #10
                    A diode is a *must*. I had numerous problems with the relay that powers my rearview camera, until I finally gave in and put a diode (which only took me 5 mins). Incidentally, I am powering the coil from my head unit's 12V, and all is fine. This is the same 12V that triggers my OPUS too.

                    For relay information, and specifically for the diode placement, make sure you read this:
                    http://www.bcae1.com/relays.htm

                    Note that the stripe on the diode must be connected on the positive (+) terminal on the relay's coil circuit. Otherwise, YOU WILL SHORT SOMETHING OUT! Remember, STRIPE WITH (+).
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                    • #11
                      Last night when I read the bit about the diode I thought, yup, cool, put it in series with the remote wire and that'll block the reverse spike. Albeit with 0.7V drop in the forward direction.

                      Then, as luck would have it, my power electronics lecturer was covering DC choppers today and using them to power inductive loads. And mentioned that a diode must be in place as shown in the link above, in parallel with the load rather than in series, to provide a path for the current created by the back EMF of the coil when the switch is disconnected.

                      This does sound much more sensible as we don't get our voltage drop and the inductor is doing its current regulating thing which keeps it happy

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                      • #12
                        All remote wires can support at least 500mA (that's half an amp) of current minimum. Therefore, run a relay without worrying. If you need more relays, have them switched off the first one.

                        FWIW, I've never seen a head unit damaged by using a relay on the antenna/amp trigger, nor have I seen anything caused by not having a diode inline. It's a great theory but isn't required in this situation.
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                        • #13
                          If you say so

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