Announcement

Collapse
No announcement yet.

usb cutoff switch

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • usb cutoff switch

    I got a small utilitarian speaker for my system for basic feedback. It's connected through usb for power and audio jack for sound. Problem is when computer has NO power (as in 12V feed is cut)...not just off, I get a hum. I'm assuming this is because the ground disconnected by unplugging the computer (I'm doing this in my home right now). I want to know if once I connect the dcdc usb power regulator to the computer if it will do the same when I turn off the car, or is the ground constant and it's just the 12V that's turned off/interupted by the regulator(I plan on connecting the + and - of the battery instead of grounding to the chassis somewhere)?
    Or do I have to get some type of usb cut off switch to cut connection to the computer when it's off?

  • #2
    OK, this is what I'm thinking, need some help because I haven't actually done this before.
    If put a relay on the usb + wire I should be able to cut the power with the added benefit of preventing the thump when the car shuts off. However, if I understand relays properly, it needs constant power to hold a circuit closed. That's fine if I wanted to use the 12V for the relay, but what I wanted to do was use the ignition on the relay. That way when I shut the car down, the speaker shuts down before the dc-dc shuts down the computer. That way I get no thump and no hum. So,

    1/ Is the ignition wire constant power that can hold the relay closed?
    2/ Do I have to use the relay on both + and -, or can it just be +?
    3/ I understand there are many types of relays, could someone point out the right one to use in a situation like this?

    Thanks.

    Comment


    • #3
      1. typically you would use the accessory wire for that-- the wire that turns the radio on/off. this wire should have power when the key is turned to the 'acc' and 'on' postions, but not have power at the 'off' or 'start' positions.

      2. switching the + only is fine.

      3. any standard relay will work. because your only switching one wire, even the most basic relay will meet your needs.-- normally it is referred to as a 'SPST' relay, or Single Pole Single Throw relay--the 'single pole' means that when it gets activated it switches a device on only(double pole relays have a second set of contacts to allow something to stay on until the relay is activated), and the 'single throw' means that it can only activate one wire.
      My OLD 2001 Mitsubishi Eclipse GT:
      "The Project That Never Ended, until it did"


      next project? subaru brz
      carpc undecided

      Comment


      • #4
        I'm a little confused about the pins on the relays. I only need 3, as far as I know, but all the ones I see have 4 and 5.I did find one relay with 3 wires, but seeing that it is the only one I found like that, I don't know if it's special for something and wont serve my needs. Which do I use?

        (5 pin) http://www.amazon.com/HELLA-00779430...6652054&sr=1-9
        (3 wire) http://www.amazon.com/Directed-Elect...d=PRH5W1ENIWF6

        Which should I get? If it's the 5 pin, how do I connect that?

        Comment


        • #5
          pins 85 and 86 are the 'trigger' one gets connected to the positive acc wire i recommended before, and the other one gets connected to the metal car chassis--which is directly connected to the negative terminal of the battery.

          so when power flows through that acc wire, it completes the circuit, making the relay turn on.


          pin 30 connects to your speaker, and pin 87 connects to the motherboard--these are the 'normally open' terminals, meaning they are not connected unless the relay is turned on.
          Last edited by soundman98; 01-15-2012, 03:58 PM.
          My OLD 2001 Mitsubishi Eclipse GT:
          "The Project That Never Ended, until it did"


          next project? subaru brz
          carpc undecided

          Comment


          • #6
            Thanks for your help. I got everything and put it together, it didn't work...as expected. I realised after the fact that the pop would still happen because the speaker is also self powered. So I put the relay on the headphone cable and it seems to function as it should. But I now have another minor issue. Is there a quieter relay, or do all of them have this loud click (loud relative of course)?

            Comment


            • #7
              i am sure you can locate a quieter version, but i really don't know where you could start looking-- the 'loudness' of relay's isn't usually something that they list in the product specs...

              maybe try to locate models with lower amperage-- the theory is that when the relay can handle less amperage, the contacts that cause all that clicking should also be smaller, hence less mass to make noise..
              My OLD 2001 Mitsubishi Eclipse GT:
              "The Project That Never Ended, until it did"


              next project? subaru brz
              carpc undecided

              Comment


              • #8
                You could buy a (more expensive) solid state relay. This would eliminate the clicking noise.
                "stop with the REINSTALLS, what do you think we got some lame-o installer!!!" - mitchjs
                RevFE
                My Shop

                Comment


                • #9
                  I found this:

                  http://www.amazon.com/Opto-22-Contro...hu-rd_add_1_dp

                  But it doesn't say anything about "SPDT". Will this suit my needs? Or do I need to get a higher amp version? Or something different?

                  Comment


                  • #10
                    it might suit your needs. though this worries me:

                    "Drop out voltage: 1.5 volts at 3 amps"

                    so that means that any source connected to it will loose 1.5v-- if this was connected to the main power wires in the car, when your car is normally running at the typical 14v, by the time the power gets to the device, it will be 12.5v. i don't believe that would be adequate for a usb device.
                    My OLD 2001 Mitsubishi Eclipse GT:
                    "The Project That Never Ended, until it did"


                    next project? subaru brz
                    carpc undecided

                    Comment


                    • #11
                      But I'm no longer using it to power a usb device, I'm now using it to pass through/cut audio. So on 1/2 will be the acc wire, and 3/4 will be the audio wire (let me know if I have 1/2, 3/4 reversed). Wont that work?

                      Comment


                      • #12
                        your connection methodology is correct, but i don't think it will work at all then. most typical audio signals are around 0.5v in amplitude. with the relay dropping 1.5v between the input and the output terminals, i don't think the signal will ever make it out of the relay..
                        My OLD 2001 Mitsubishi Eclipse GT:
                        "The Project That Never Ended, until it did"


                        next project? subaru brz
                        carpc undecided

                        Comment


                        • #13
                          I'm a little lost with that. What does the voltage coming from the battery (source) being dropped by 1.5 have to do with the audio signal passing through the closed circuit? Or am I reading it wrong?

                          Comment


                          • #14
                            sorry, it's the other way around.

                            solid state relays are literally a very large transistor.

                            the easiest way is to first explain how a transistor works-- http://www.satcure-focus.com/tutor/page4.htm

                            i believe this is the most applicable line:
                            Imagine a pool of water near the edge of a table. It rests there with surface tension holding it in place. Now put one tiny drop of water on the table edge and let it touch the pool of water. Suddenly, the pool drains onto the floor as gravity takes over! Your tiny drop provided the catalyst to get it moving. So the base electrons do a similar job for the "pool" of electrons in the emitter - helped by the "gravity suction" of the power supply voltage on the collector.

                            now what lacks in these definitions is how some of the voltage is getting lost to turn the circuit on.


                            and because that amazon link is so hard to read with everything thrown together, here the mfg's site with everything in nice little chart:
                            http://www.opto22.com/site/pr_detail...=4&item=DC60S3

                            the specs show 2 different voltage drop figures. 'forward voltage drop' and 'signal dropout voltage'

                            'forward voltage drop' is referring to the amount of voltage that is 'lost'(voltage can never just disappear-- this 1.5v is converted to heat due to the way the relay is made) going from terminals 3-4--the wires that interrupt the audio circuit.

                            'signal dropout voltage' is referring to the amount of voltage that is being 'lost' from terminals 1-2--the wires that activate the relay.

                            both figures are at or above 1v loss. for a audio application like that, the loss is nearly double of what the expected source voltage will be..
                            My OLD 2001 Mitsubishi Eclipse GT:
                            "The Project That Never Ended, until it did"


                            next project? subaru brz
                            carpc undecided

                            Comment


                            • #15
                              I guess I'm out of luck b/c all the relays by them seem to do the same thing. Is this a common thing with solid state relays, or just their products?
                              Last edited by Champak; 01-23-2012, 01:03 AM.

                              Comment

                              Working...
                              X