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Thread: DCDC-USB and power wiring.

  1. #1
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    DCDC-USB and power wiring.

    With it being one of the cheapest PSU's available to me, and because I need regulated 12V output into a jack, i'm looking at the dcdc-usb PSU.

    Looking up the wiring, it appears to have 2 Vouts and 2GND's, so I could potentially run my monitor off this supply aswell. Add in an amplifier wiring kit for 10 off eBay (from a UK seller, 5m 4awg cable, 0.9m 4awg cable, in-line fuse holder (60A) and mounting screws/connectors), and my psu will come out at about 66 (72 if I go for the psu enclosure). My pc is rated at 12V 4A so I imagine 60A would give plenty of juice and expandibility.

    4 awg from the battery (fused in the engine bay) to my DCDC-USB in the boot, grounded to the chassis in the boot. I'm guessing the IGN wire won't have any current draw off it (?), so would a single core cable in similar diameter to one of the internal wires of a UK 13A mains flex suffice for that? (1.25mm2). The cable to the pc would be fairly short, no more than half a meter, so would use some old ATX PSU wire and a barrel jack, but from the PSU to the monitor (most likely a Lilliput LCD) i'm unsure as to what cable I would need to use for that to run the length of my car.

    I'd also want to run an amp as i'd be replacing the head unit. I have one - its labelled as a 1000w amp, but output is 350w rms.... 83.3A if you use the first figure, 29.2A if you use the second - i'd only be running 2 (stock) speakers off it! Confused much!

    Does this all sound good so far?

  2. #2
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    kind of surprised that no one else tackled this yet...


    the easiest way to figure out what size wire you need is to look at the fuse size of the device.

    the dc-dc usb has a 10A fuse on it which means that is the maximum that it will ever draw.

    so according to my handy-dandy power chart:


    you require a 10gauge wire from the battery for only the dc-dc usb for a approximate 5 meter wire length.


    for the amp, i recommend looking at the fuse that it has installed in it, and use that for power wire sizes, as the wattage ratings of different amplifiers can vary widely.


    where it might get more complex is when you want to run multiple devices off the same wire. for this, simply add the fuse ratings together, and use that to determine what size wire you need.


    of note-- always fuse the wire for whatever the max is-- so if you were to only run the dc-dc usb off a power wire, you would install a 10A fuse by the battery as well.

  3. #3
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    Quote Originally Posted by soundman98 View Post
    kind of surprised that no one else tackled this yet...


    the easiest way to figure out what size wire you need is to look at the fuse size of the device.

    the dc-dc usb has a 10A fuse on it which means that is the maximum that it will ever draw.

    you require a 10gauge wire from the battery for only the dc-dc usb for a approximate 5 meter wire length.


    for the amp, i recommend looking at the fuse that it has installed in it, and use that for power wire sizes, as the wattage ratings of different amplifiers can vary widely.


    where it might get more complex is when you want to run multiple devices off the same wire. for this, simply add the fuse ratings together, and use that to determine what size wire you need.


    of note-- always fuse the wire for whatever the max is-- so if you were to only run the dc-dc usb off a power wire, you would install a 10A fuse by the battery as well.
    I did think about this, but as it's going to require me to cut through a rubber grommet to route the cable into the cabin i want to get it done with some expansion available - wire isn't too cheap and to be honest 10 for 6m of wire and fuse holder/connections seems to be pretty good - running so little off such a big cable shouldn't have any problems should it (basic high school physics!)? Amp wise, I don't think my car will manage a full size amp extra - it's only a little 1200cc thing! Been looking at USB amps and mini amps to see whats around. Just need to get a screen to go with it now!

  4. #4
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    Quote Originally Posted by sparx View Post
    ...ts labelled as a 1000w amp, but output is 350w rms.... 83.3A if you use the first figure, 29.2A if you use the second...
    Ignore other Wattage ratings - only RMS counts. (That is your "power" requirement allowing for inefficiency, eg, 350W RMS out /0.8 (assuming 80% efficiency) = 440W input, or 32A @ 13.8V or 37A @ 12V or 44A at 10V (depending on how the unit is rated & designed) - hence a 50A or 60A fuse based on loading the fuse at no more than 70-80% of its rating (32A/.7 = 45A => 50A fuse; 44A/.7 = 62A = 60A (not that I'd expect you to be running full output at 10V; a 50A fuse means 50A x 10V = 500W @ 10V => 500 x 0.8 (efficiency) => 400W output - more than the amp's rated 350W, so 50A is probably a suitable fuse).



    Also, if the dc-dc usb has a 10A fuse....
    Quote Originally Posted by soundman98 View Post
    ...you would install a 10A fuse by the battery as well.
    That in not incorrect, and it may be common advice, but I would probably install a different sized fuse.
    If it hits 10A, why stress BOTH fuses?

    I might hence use an smaller fuse (5A? 8A?) if the that fuse is cheaper or easier to replace than the dc-dc usb fuse.

    Usually though, because I would use a cable rated above 10A - whether for a lower voltage drop, or for future expansion - say 15A or 25A, I might then run the usual 15A or 20A-25A fuse to protect the cable. (Nothing wrong with a fuse that handles the load but is way below the cable rating.)
    In the latter case, with a 20A or 100A cable and a 20A fuse, the dc-dc usb's 10A fuse blowing is unlikely to thermally stress the larger 20A fuse.
    (Not that fuses can eventually "fatigue and blow" due to successive high temps etc.)
    And since it is usually not practical to fuse below the target's internal fuse (ie, the dc-dc usb's 10A), the heavier cabling with heavier fusing is usually what I'd do.

    It can be frustrating if using 2 equal fuses if one blows (eg, if both are 10A). You replace one only to have the other blow soon after. Or to have to replace both simultaneously - though that is rarely done thru necessity - its usually thru experience or preventative replacement/maintenance.

  5. #5
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    Cheers for all the advice.

    It looks like (from the spec sheet), that my car has a 70A alternator (as it doesn't have air conditioning), so I doubt (Going off 30A+10A=40A) i'm going to be able to use my amp alongside my pc - been looking at usb amps which would probably do the job (I'm not an audiophile, and I won't be fitting any new speakers, i'll just use the stock ones).

    Trying to get a wiring and component diagram setup so I can quickly see what I need to do and what I need to buy - i'm assuming that the Ign/Acc wire to the DCDC-USB will only need to be big enough gauge to prevent too much voltage drop from the front of the car (unless I can find an ign/acc wire in the boot (trunk) somewhere!)
    Last edited by sparx; 11-20-2011 at 03:10 PM.

  6. #6
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    Are usb amps as powerfull as your 350W amp? They are usually limited to ~4W total, but yes, usb-poweres amps will save a lot of power...


    350W is only ~35A which the 70A might handle depending on what else you run, and the actual output profile (eg, 10 minutes idling hence max 45A ouptput, the 50 minutes at sufficient RPM for 70A, etc.)
    The vehicle might take 20A leaving up 50A for the amp, etc.


    And I doubt you'll be running at full 350W output. At half of full volume, it's only using 50W (I think...?).

    And not that the altenators has to supply the peak power - the battery makes up for shortfalls.
    But the battery should be (nearly) fully charged at the end of the trip.
    That's where the "what is it your want?" comes in. If you want full voltage all the time (eg, 13.8-14.4V), then the alternator has to supply your peak demand - ie, lights, wipers, pc, & audio te whatever output. Otherwise it's the battery-alternator (and load and RPM) combination.


    Though theoretical design is one thing, the best practice is a dash mounted voltmeter that indicates the state of the charging system, and battery state when not being charged (assuming it has rested after charging - eg, the next morning before starting).

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