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Another USB standby idea: splice wire and use a relay?..need help

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  • Another USB standby idea: splice wire and use a relay?..need help

    Hi All,

    The saga continues...Situation is: mobo keeps USB ports powered during standby which will drain my battery. I'm determined to use standby in v2 of my carPC.

    So far, my solution has been to cut the USB cable open, cut the +5v and ground lines, and splice these lines into the molex connectors to provide power to them only while the computer is turned on.

    The problem I'm running into: I don't think I have enough power (amps) to rob from these to satisfy all of my devices. I just used 2 USB extension cables to which I made this modification - - spliced into the same Molex connector - and the PC has issues when certain devices hooked up to this are fired up (currently attached is a 7 port hub and a GPS)...This leads me to believe they want more power than they're getting since they work fine when hooked up normally.

    I'm a total beginner as far as pure electronics are concerned, but I just thought of something that I hope will work; I need someone to tell me whether it's possible or not, and if so, what specifics may apply.

    My thought is this: why, instead of stealing power from the PSU Molex connectors (Carnetix 1900, it comes with 2 which I believe are limited to 3A each) and thereby bypassing any and all power that's being supplied via the USB ports natively, don't I try to use the power that's being supplied to those 4 USB ports in some way? It seems like I could do the following:
    1. Splice the USB cables to sever the +5v and ground
    2. Place a relay between the cut wires that is triggered off of a Molex connector
    3. When the PC is powered on, the Molex connection would close the relay and therefore the devices would be getting power from the USB ports.

    My unschooled eletronics mind thinks that this way, I'm not actually stealing (a lot) of power from the Molex connection, but instead, just using it to 'turn on' the power from the USB port; thereby using the USB port power after all.

    Is this thinking anywhere near correct? It would draw the +5V power from the USB port, right? Not the molex?

    If this is right - or in the right neighborhood - what type of relay would I need? I'm thinking of using 2 USB hubs, therefore 2 USB cables I need to do this to, therefore a DPST relay? What about amp / voltage requirements for these?

    Any and all help is usual ;-)

    FYI - As far as the Carnetix: I've ordered the 3rd +5V output which I'll be using to power one of the hubs. The secondary output is currently being used (successfully) to power the first hub and external power to a DVD player. I don't think I can pull anymore juice from these for the USB cable power.

  • #2
    Actually I was considering something very similar, except I was thinking of using a 5v regulator (possibly the Carnetix point-of-load regulator or maybe some sort of homebrew solution) to power the hub, and using the relay to turn the hub on and off. This would be assuming that if I kill power to the hub, the devices connected to the hub would be shut off. I was thinking of using the ACC line from my ignition switch, but it seems that the molex connection might be a better alternative now that I think about it. I'll have to do some experimenting with the equipment that i have available and post up my findings, hopefully in the next few days.

    1997 Chevy S-10, ext. cab
    CarPC Progress: 75% - Functional, but not quite finished.
    Check out my progress.


    • #3
      Originally posted by S4Per View Post
      The saga continues...Situation is: mobo keeps USB ports powered during standby which will drain my battery. I'm determined to use standby in v2 of my carPC.
      Have you measured the current draw of all teh USB devices to see how much power draw there is?
      Also, do you realize that standby will have the computer itself drawing power when it's off, which could also lead to draining of your battery?

      Frankly, if you have a good battery and leave your PC in standby mode for a couple days, it's not going to drain it. UInless you have a buttload of USB devices, I don't think it's going to increase the battery drain so much it will have a significant impact on battery life.

      From a hobbyist standpoint, I understand why you're trying to achieve what you are. It's tinkering and experimentation. It could also yield positive results for the community at large. However, I think that what you're pursuing is really unnecessary. Just my 2.64 cents.
      Have you looked in the FAQ yet?
      How about the Wiki?

      Under normal circumstances, a signature would go here.


      • #4
        Originally posted by S4Per View Post
        I'm determined to use standby in v2 of my carPC.
        Just out of curiosity. Why standby and not hibernate? Wakeup time?


        • #5
          blkwrx - yes, standby time.

          darque - Thanks for the input. I've experimented with this over time and in my case, it has shown to make a big difference in terms of battery drain. I know the mobo still draws power in standby but it is significantly less than the mobo combined with all of my USB devices.

          So, my theory above is correct? If so, what are the guidelines in terms of amps / voltage in relays? i.e. what type of relay - in terms of those variables - do I need?