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Thread: Using another 5v source for high powered USB devices

  1. #1
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    Using another 5v source for high powered USB devices

    Is it safe to use a seperate 12v to USB adapter (cig lighter type etc) to boost the power for higher powered USB devices like external dvd drives that have a USB Y connector? I could possibly hack open the USB cables going from the board and put a small diode in there so as to not backfeed between the onboard 5v and the other supplied 5v.

    One half of the Y (data end) into device and the other end of the Y to the external power source, then the other end to the pc?

    Im using an M4ATX power supply with an AMD x2 dual core and JW mini itx board, which after ordering, found out AMD cpu's run off 5v not 12v, which would use 12a of the available 15a on the 5v rail on the m4.

  2. #2
    Fusion Brain Creator 2k1Toaster's Avatar
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    Adding a diode drops the voltage by about 0.7v, and it isnt needed anyways. If the PC is run from the car, it is all commonly grounded.
    Fusion Brain Version 6 Released!
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  3. #3
    FLAC
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    Problem with a cigarette lighter USB adapter is that most of them can only do 1 amp max @ 5v. If you have a 7 port USB hub you should get something that can do atleast 3 amps (ideally 3.5).

  4. #4
    Vendor - Qube colin's Avatar
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    Look for the thread about the MP3Car USB hub when it was announced. A few people (myself included) didn't like the price tag, so we built our own. A free enclosure sample (OKW or PacTec) and a cheap USB hub (eBay) and a 3A 12v-to-5v switching converter came to under $15 and it looks pretty cool.

  5. #5
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    I ended up going out and getting a few lm7805 regs, a few caps and a sheet of project board and making my own 5v reg. I've made up a couple of 1a and a 3a version. Just gotta find that old 586 cpu heatsink I saw floating around here somewhere the other day to hack up to sink the regs.

    The 1a version running off a 12v jumper pack @ 12.181v with no load shows 4.997v, and the 3a version shows 5.002v. Will have to test them @ 14.4v tomorrow. It's too damn cold to go outside to the car and test it under charge tonight.

    Then I've gotta figure out how to mount the case in the car. Its not a dedicated carpc case, so there are no mountings to screw it down by. The case has a clear window in the side so im gonna have to find some LED's to make it look purdy too might have to incorporate it into a false wall behind the back seat somehow.

    Hopefully I'll have it all installed by the weekend after this one coming when I pick up the car from my girlfriends place.

  6. #6
    Vendor - Qube colin's Avatar
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    For the record, under load the 7805s are going to be WASTING all that energy as heat. Very very very very inefficient. Go buy a switching regulator.

  7. #7
    Fusion Brain Creator 2k1Toaster's Avatar
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    +1

    Better use a good heatsink for those babies...
    Fusion Brain Version 6 Released!
    1.9in x 2.9in -- 47mm x 73mm
    30 Digital Outputs -- Directly drive a relay
    15 Analogue Inputs -- Read sensors like temperature, light, distance, acceleration, and more
    Buy now in the MP3Car.com Store

  8. #8
    Raw Wave
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    FYI - I've been buying 12-24V phone charges for up to $5 each...
    They all use the 8-pin MC34063 converter usually as a straight Step–Down Converter but some add the External NPN (or PNP saturated) switch to boost output current.
    (Refer to MC34063 data sheets for circuits. The only addition is the indicator LED across the output. The PCB is contained in the fused cigar-plug.)

    The MC34063 itself is good for 1.5A though it is often short-circuit limited to less than that. Add external transistors for higher currents.

    Not that I have yet studied the MC34063 in detail - I've just bought 10-20 $1.60-$4 chargers, tested their voltages and checked their circuits.... (Some are 5V (ie, 5.00 +/- 0.05V) but most are in the 5-6V range.)
    I figured a cheap charger (whether $3 or the full ~$20) with an added $2 MOSFET (for 50A or more) was cheaper than stupid $300 20A dc-dc battery chargers... (long story - it involves a mate on local forum...)
    Maybe one day I'll get back to it..... (or am I confusing MC34063 with the MC33363 or uA3525 or whatever I was using for CDI - ie, easy to drive MOSFETS for higher currents...??)

    BTW - efficiencies are typically way above 80% unlike 317s & 7805s etc which are under 40% (typically 35% in a running vehicle).

    Food for thought?

  9. #9
    What can I say? I like serial. Curiosity's Avatar
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    Ba careful. You should cut off the USB side source if not using diodes, which yeah you don't want to use. You want 5.0V. Most regulators use the output as the sense for regulating, so one or both regulators can be damaged when one isn't on or they have a different voltage. Also many USB hubs have both 5V sources connected directly together.

  10. #10
    FLAC
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    So basically what you are saying is...rather than finding a single regulator that can handle a high current, you can just use multiple smaller ones in parallel isolated with a diode at each regulator instead?

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