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Thread: DC-DC Step Up Converter

  1. #1
    Constant Bitrate
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    DC-DC Step Up Converter

    Looking around the forum and trying to decide how to power my laptop. My laptop requires 16v 2.3A in. I have found several inexpensive DC-DC Step Up Converters via eBay. Would a product such as this work for what I need? I realize I will have to have relay set up as well to the actual power on with accessories and off with vehicle RAP (Retained accessory power).

    http://cgi.ebay.com/12V-24V-DC-DC-St...item33643af941


    I appreciate your input!

  2. #2
    Fusion Brain Creator 2k1Toaster's Avatar
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    That will not be sufficient regulation for a laptop. Maybe for a motor or battery or something of similar heft. A laptop does not like huge spikes and large ripple. That thing is a joke.

    Look in the mp3car store, there are ready made products that do exactly what you want with built in auto-on.
    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

  3. #3
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    Quote Originally Posted by 2k1Toaster View Post
    That will not be sufficient regulation for a laptop. Maybe for a motor or battery or something of similar heft. A laptop does not like huge spikes and large ripple. That thing is a joke.

    Look in the mp3car store, there are ready made products that do exactly what you want with built in auto-on.


    I saw those products in the mp3car store. Just a little more than I am willing to spend at the moment. I see that Fujitsu actually makes a product for my model tailored to auto/plane usage. I may opt for one of those.


    Are there any other options out there?

  4. #4
    Fusion Brain Creator 2k1Toaster's Avatar
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    Well if you only want voltage conversion and not "smart" features then you are looking at things like the iGo. I don't know what it is called now or what their rebranded chinese clones are called, but basically it is an adapter system where you just put different tips on it to get various voltages and stuff. I have one from 2005-ish and it works great.
    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

  5. #5
    Raw Wave
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    That dc-dc converter is probably ok but with sufficient (extra) output capacitors.
    I just picked up the May 2011 copy of Silicon Chip which has an adjustable 12V 1000W inverter (dc-dc converter; 15-35VDC output; based on last month's MPPT solar panel controller) intended for laptops etc.

    It's on the MC34063 chip and has three 1000uF 35V low ESR caps on the output for filtering (up to ~7A output current). Those caps could be reduced - its max input is 10A and it has an efficiency of ~80%. For 2.3A, probably one 1000uF low ESR would suffice.
    The way they have it set up, with the converter off, the output is about 1.2V lower than the input. (In thru a 10A fuse, then thru D1, a 0.025Ω resistor, a 100uH inductor, D2 & out where D1 = D2 = MBR20100CT double diodes.)
    Alas I haven't yet found any mention of the input voltage range, but these guys ain't stupid and they have good expertise wrt automotive 12V systems.


    But the linked converter should be ok if its 3A is sufficient - just add (eg) a 1000uF cap if ripple is too high.

    But yes - they have no features like the mp3car converters. But if it's merely to run or charge a laptop...


    FYI - I bought a 12V 45W laptop converter for AUD$22 a few months back... Settable outputs from ~16V to ~24V and the various dc connectors.

  6. #6
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    I just ended up buying the proper DC-DC Converter for the laptop. Now, to Hardwire it instead of using the cigarette adapter. Should just be able to lop it off and use an in-line fuse rated for the fuse inside the adapter(15A) and setup through a relay with 12v switched.

  7. #7
    Raw Wave
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    Yep, but you may not need a fuse as big as 15A.

    But match the fuse to the cable and relay etc, or as low as the dc converter takes.

  8. #8
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    The adapter specs are:

    Voltage :
    12V-13.5V (Input), 15V-17V (Output)
    Power :
    90W

    So,

    90W / 12v = 7.5A Fuse needed?


    Correct?

  9. #9
    Raw Wave
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    Normally I use a Div10 (divide by 10 rule) so 90/10 = 9A => 10A.

    But dc-dc converters usually boost from lower than 12V, and you need to factor in the efficiency (usually 80%) and that, as a rule, fuses should be run at (no more than) about 70-80% of rating...

    Hence the fuse size is 90W/.8/.7 = 160W where you divide that by the minimum voltage - say 10V => 160W/10V = 16A, so 15A was appropriate. (I went from 16A down to 15A because of the /.8/.7 = /.56 "super fat" margin.

    But of your laptop uses 16v 2.3A = 37W...
    Then 37W/.56 = 66W.
    So at 10V that's 66/10 = 6.6A => hence a 10A fuse... or 7.5A if you prefer...


    I'd probably start with a 10A fuse since that is a common size, otherwise 7.5A is ok.

    But I'd probably design the circuit (wiring, relay, fuseholder etc) to handle (say) 16A - 20A.
    Not that it is that critical in this case, but why not design for the converter's max power, and use thicker wires for less voltage drop?

    Sorry if that is too complicated, but I'm trying to show some typical rules and considerations.
    But it isn't worth changing things if you already have the stuff. Your relay is likely to be rated for 30A (assuming it's a common automotive type) but could be 10A or 15A etc.

    Remember 2 things:
    The fuse (near the battery) is to protect the wiring and relay (aka distribution) and should be rated equal to or lower than the lowest rated part. (But it is not to protect the dc-converter - that should be a separate fuse for the converter at the converter end, though no reason it cannot be the fuse at the battery - hence only needing one fuse. The cig plug may have contained a fuse for the converter.)
    And (2), heat will indicate stressed components - eg, a cable that is too warm etc. (But don't burn yourself - especially on fuses that tend to run warm and can be quite hot near their rated current.) It's a check that I sometimes do on occasion - more so at joints and connectors as that can indicate bad contact; cables are usually ok if they were initially ok (unless you add more loads to them).

  10. #10
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    I follow the math and understand your explanation. Well done!


    Yes, the cigarette adapter itself contained a 15A buss fuse. My thoughts were to only have the one fuse. I had planned on using the existing wiring. The IP2 fuse which powered the factory stereo, disc changer and antenna. (None of which will be used any longer) is rated at 10A.

    So if GM did their wiring properly and with the calculations above it should be safe to use the existing wiring. As in 12v switched to activate relay and power source being the factory stereo wiring.

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