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Powering carpc with SPDT relay

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  • Powering carpc with SPDT relay

    Hi guys,

    I have purchased most hardware I think I will require for a carpc install but have a question regarding powering and relays.

    I will be useing a P4 dual core on a zotal 9300 board with a m4 ATX power supply. I will also be running my 7 inch lilly via the m4atx on the 12v rail.

    as for paripherals: Just the usual, gps mouse, bluetooth, ultimate dab radio and possible scan tool obd etc.

    As my pc case (like many other's) will be under the front seat, I will also be extending pc power/led/reset switch to dash.

    I am not sure if this rationale is correct but I want to use a switch to fire up a SPDT relay to hard off the pc during extreme circumstances were ALL batt. power is required (or to kill the pc during extended peiods of car down time). The reason for this is because I know the M4 atx even with pc shut down can still have current being drawn to it.

    And the reason I want ot use an SPDT rather than SPST is because terminal 87a and 30 (common) is available - which means under 'normal' condition were I want the pc to have normal power control, this terminal is used. However, should I want to completely kill the pc, I would only then activate the relay switch - which completely removes the pc from car circuit.

    Also, I have done a rough calculation and it seems a 40amp relap should be sufficient for the pc.

    So how am I going so far? Is it advisable to wire the relay in this fashion? or is there a 'better' way to ensure pc does not drain car batt.


  • #2
    the off-state of the power supply will be less then 500mA, probably closer to 50-100mA. your engine computer will probably drain more sitting for long periods of time then the carpc. i leave my car in the garage for 2-4 weeks at a time, and my dsatx, and 2 amps have not drained the battery enough to prevent starting.

    the relay should work fine in the way you want it to.
    My OLD 2001 Mitsubishi Eclipse GT:
    "The Project That Never Ended, until it did"

    next project? subaru brz
    carpc undecided


    • #3

      In that case, i will keep it nice ans simple.

      thanks Soundman


      • #4
        Normally car-off drains are under 50mA, and even that can be taxing for some batteries (over a week).

        EMS (engine computers) are off when the IGN is off, and memory functions (radios, EMS etc) are usually nan-Amps - ie, well under 1mA. (Though some alternator "Sense" wires can be 12-15mA - eg, Bosch from the 1980s to 1990s.)

        Normally a PC would be powered through some SPST relay to turn it on.
        To use a relay to turn it off (ie, pull it off an SPDT relay's NC (Normally Closed) contacts) requires power - typically 60-250mA. Then when the battery flattens, the relay de-energises and reconnects the load or PC. That is not what you want!


        • #5
          i see your point, however the problem is the startup time of the psu.

          i considered recommending a spst relay that would be triggered by the acc signal, though the problem is that nearly all psu's have a delay before they output power. so that could prove to be unreliable at starting up the pc.
          My OLD 2001 Mitsubishi Eclipse GT:
          "The Project That Never Ended, until it did"

          next project? subaru brz
          carpc undecided


          • #6
            Maybe my previously posted latching relay which uses and ordinary SPST relay, namely:

            ...though I prefer its circuit diagram below (so "how it works" can be understood easier):

            ... noting that the feedback Off-switch isn't shown in the circuit diagram albeit described in its blurb (ie, the feedback link keeps relay energised until broken, or 12V POWER is removed from the circuit).

            The above do or should exist on mp3car but I took them from because of a more recent post.
            But mp3car should also have diagrams where I substitute a low voltage cutout - aka battery protector (eg, the ~$20 MW728) - as the feedback switch to prevent battery drainage if the PC is forgotten and left powered.

            Note that the latching relay's +12V inputs can be the same or different. IE - the power is probably its own fused battery constant supply, but the +12V trigger or ON source can be any +12V signal (and the momentary-ON switch omitted, or included in series, or parallel, etc).

            Maybe it's about time I updated with an "all in one" diagram if not already posted? If I know exactly what you want so I can omit optional on (or off) triggers, switches, signals, bypasses etc, it should be less confusing? (Let me know if interested. I can then liaise to achieve the final circuit.)
            Last edited by OldSpark; 08-22-2011, 11:27 PM. Reason: spelink.. (includes -> included)


            • #7
              I kinda did the same thing. I used two small Radio Shack SPDT 12VDC relays. I have an old M1-ATX, and I did not want the PC to power on as soon as I turned on the key. Many times I just need to move the car, and there is no reason to power on the PC. My 93 Honda Civic does not have a sunroof, but since I have the dash out of an Si, I have a non-used sunroof switch. Checking out the switch, I found that there is essentially 3 Normally Open switches in it. Using the relays and the sunroof open/close buttons I built a latching circuit using the Normally open switch contacts. I made it so that the key on powers the whole latching circuit, that way if I either shut off the key, or press the "off" switch, the PC starts to shut down. My latch works great, my problem is the M1-ATX. The M1-ATX is supposed to essentially push the power button on the PC, but it does not. I've scoped the signal and found that it's almost completely shorting out the power switch. The voltage across the switch goes down to one volt for about 200 milli-seconds during the supposed "button press" from the M1-ATX. That's not good enough to turn on my ITOX G5M100. I called Mini-Box, but as the M1-ATX is old, they don't support it in the states, support is in the UK. I sent an email request for help. The response I got is that they want me to switch it to traditional PSU mode and see if that turns on the PC. So until I get a chance to try it, I've found that if I manually hibernate the PC, then shut off my latching circuit, the PC hibernates and then the Power Supply does a "Hard-Off". The next time I latch the circuit, the PC comes out of hibernation. This works, but it would be nice if shutting off the key or shutting off my latch circuit automatically hibernated the PC. I'm getting the feeling that I'm going to need a new PSU.