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Thread: How powerful of a relay do I need....

  1. #1
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    How powerful of a relay do I need....

    So I've got some questions about Relays. I went to radioshack to ask but no one there new the answers, so I figured I'd come here and ask.

    I'm going to have a carnetix regulator sending power to my laptop and screen. I have the third 5v output option for this unit also. (haven't built anything yet, just in the planning/purchasing stages).
    I'll have the acc line connected to a relay that will control the carnetix. The carnetix will shut down the laptop when it sees that the car battery is getting too low. I assume the carnetix will not only shut down the laptop power line, but also the power line going to the screen, and the extra 5v line I'll be adding. So on to my question....

    Can I use that extra 5v line to control relays for my amps? Basically, I'll have the remote line coming from my headunit connected to the amp so they turn on like usual. But, I want the 4 gauge power wire for the amp to run through a relay controled by the carnetix 5v line so that the amps will only be on when the carnetix is on.

    I want to do this so the carnetix can turn EVERY thing off when it sees the battery is too low. It'll turn off the laptop, screen, and disconnect the power going to the amps.

    So will this work? and if so, what kind of relay will i need to control multiple amps? Also, what specs for the relay do I need to look for....not sure how powerful this relay will need to be since it will be controlling the 4 gauge wire that powers the amps.

  2. #2
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    Let me add a bit more here that I just read about:

    amp = watt/volts

    So, I would need at least a 125a relay because 1500w(size of my amplifier)/12volts = 125a.
    Now, looking for a relay this large is pretty easy, but they seem to be very power hungry using up to 2.9watts or more just to power the coil in the relay. So, if I were to use the 5volt line coming out of the carnetix, to power the coil used in this relay, would that be too much for the carnetix? I'd need at least two of these relays (one for each amp) so that'd be about 6 watts coming from the 5volt line. Any input on this?

  3. #3
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    You know...I think I just answered my own question.

    If i just use that 5volt power line from the carnetix to controls relays that operate the remote line going to the amps (instead of operating the actual power lines), that will turn the amps on with the carnetix and off when it turns off. Then I wouldnt need those huge relays. Man...I like to make things much harder than they really are apparently

  4. #4
    Maximum Bitrate JimmyFitz's Avatar
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    Quote Originally Posted by JonnieBlaze View Post
    You know...I think I just answered my own question.

    If i just use that 5volt power line from the carnetix to controls relays that operate the remote line going to the amps (instead of operating the actual power lines), that will turn the amps on with the carnetix and off when it turns off. Then I wouldnt need those huge relays. Man...I like to make things much harder than they really are apparently
    You have the right idea about using the remote line on the amp(s) except that the voltage will need to be 12V (most likely) in order to trigger them on. You can get that from the Carnetix too. You don't even need relays, that is what the remote input does.
    ~Jimmy

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    You could just use the same power line going to the screen, DLYON (delay on) to trigger the relay for your remote line instead of having to add the extra 5v for $25. That way it can still (de)activate when your computer is told to (de)activate, and when the voltage drops and the power supply shuts itself off it will turn that off as well.
    Unless you wanted to spend an extra 25 bucks for the sake of spending it, I accept cash checks or money orders.
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  6. #6
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    Well I think all audio amps turn on wire require 12V, and not 5V.

    I think you might be able to use the same 12V off the secondary rail of the PSU to feed your turn on signal to your audio amps.

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    haha...geez...where the heck was my mind. I completely forgot about the 12v rail on the carnetix. That would be easy enough. After I used the 12v power lines for the two amps, would it still have enough to power the screen as well?

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    The audio amps are feed via a fairly large cable from the battery to the amps. The remote turn on, doesn't require a large amount of current, so yeah I think you should have plenty of juice to power all that.

    The carnetix also have a delayed 12V wire that can be used for this purpose as well. I think it would require a standard small 30/40a bosch style relay.

    Here's a little sketch on how it would go..


  9. #9
    Maximum Bitrate JimmyFitz's Avatar
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    Quote Originally Posted by JonnieBlaze View Post
    haha...geez...where the heck was my mind. I completely forgot about the 12v rail on the carnetix. That would be easy enough. After I used the 12v power lines for the two amps, would it still have enough to power the screen as well?
    Your screen (7"?) will likely use about 1A at 12V. The remote turn ons will be practically nothing compared to that. You don't need any relays.
    ~Jimmy

  10. #10
    Who am I? HiJackZX1's Avatar
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    I dont know what kind of soundcard you will have, but if its USB you will get the dreaded thump of doom when starting up the PC, because the AMPS will turn on before the soundcard does. I am not sure if this happens with Onboard soundcards. I know some PSUs have a delay, but on my OPUS 320 its 20 seconds which is not enough time from cold boot (I have alot of devices that have to be started). I decided to go with a program called AMPCTRL that you connect the AMPS remote wire to pin number 4 on your serial port. Once the program sees that something is producing a stream of sound it turns on the amps.
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