240v ac to 14 v dc circuit

Discussion in 'The Projects Forum' started by rob-the-viking, Aug 23, 2010.

  1. rob-the-viking

    Thread Starter New Member

    Aug 23, 2010
    7
    1
    Hello one and all. I've just stumbled across this forum and hoped I could tap into the extensive knowledge base on here.

    I'm wanting to build a power supply for my car, so that when I have my laptop plugged into it to run diagnostics and spftware updates (which requires the ignition on for extended periods, i had it on for 45 minutes yesterday and drained the battery!) I can prevent the battery from draining.

    As I understand it, i'd be looking for about 14V dc with a good 30A of current. I know that having current so high means expensive componants, there is an off the shelf product available for about £150, but I figured I'd see if I can build one cheaper.

    I'm an Aircraft Avionics engineer by trade, so not an electronics greenhorn, but I want to do it right first time, not mess it up and end up buying off the shelf anyway! Hence me coming here with cap in hand!

    I figure that I'm looking at a transformer to get the mains (240v ac as I'm in the UK!) down to a 16v ac (i know you should always aim for slightly more V than you actually want to use, and 16 V means 15:1 ratio on the transformer). Then rectify it to get your DC, probably add some protection between the transformer and rectifier circuits, then smooth the DC off and filter it to get a nice clean output.

    The theory sounds fine, but I've been trying to draw the circuit diagram, and can't work out what to do for the best!

    Please help!

    Thanks guys.
     
  2. rob-the-viking

    Thread Starter New Member

    Aug 23, 2010
    7
    1
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,675
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    Are you sure of your current needs? 30A sounds awfully high for anything exceeding 10 minutes. How long does your laptop run on its regular battery?

    John
     
  4. rob-the-viking

    Thread Starter New Member

    Aug 23, 2010
    7
    1
    Sorry about the confusion, i want the power supply to power the car, not the laptop! As I said, diagnostics need to be done with the ignition on so it drains the cars battery rather quickly. Powering the laptop isn't a problem as i've got mains in my garage available.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hello Rob,
    Welcome to AAC!

    The power supply you're looking at really wouldn't be what you would want to use with a vehicle. What you really need is a battery charger.

    Beenthere posted a battery charger in the Projects Collection forum awhile back; that project is here:
    http://forum.allaboutcircuits.com/showthread.php?t=6099

    It is a rather basic charger; it does not have temperature compensation or other sophisticated features such as float maintaining, etc. - basically just a simple bulk charger that is current-limited by the transformer itself.

    I worked up an Excel spreadsheet tool to help people determine % of charge remaining vs battery temp vs battery voltage; it's in the "Tips and Tricks" thread:
    http://forum.allaboutcircuits.com/showpost.php?p=262143&postcount=38

    I suggest that it would be well worth your while to at least review it. Note that battery chemistries and construction even for automotive are quite varied nowadays; AGM are becoming all the rage, and will tolerate much higher charge rates than your typical lead-acid battery will.

    Don't forget that your time is valuable. You can purchase commercial off-the-shelf battery chargers quite economically nowadays; many have advanced features.
     
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  6. rob-the-viking

    Thread Starter New Member

    Aug 23, 2010
    7
    1
    SgtWookie, thanks for taking the time to reply mate. Good to see another serviceman too!

    I will indeed look at the thread you linked me to. As for buying off the shelf, for me this kind of stuff is all about the thrill of the chase if you get my drift! Trying to work stuff out, asking others, then having a go! If you gave me the choice of buying or building something to do a job for the same money, i'd build (or at least have a go!) every time!

    The car itself is 11 years old, and the battery hasn't been changed in the 4 years i've owned her, so none of this new fangled battery rubbish for this call sign!
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    Whenever you think you can build it cheaper, what is your opinion on the chances that will turn out to be true? :)

    Please factor in what your time is worth.

    It just helps to accurately frame the project objectives before you assess it.

    The main reason most people plan out projects and build things instead of buying them is because they want to. They want to because they are trying to test and polish what they have learned and as a hobby.

    It might be true that you can build something cheaper if you are talking only about the component cost, but you also get a bit of what you pay for. You would expect to pay less for an unassembled kit, especially if it comes without the build plans.

    What it really comes down to is will this have entertainment, educational, or other value added for your time.

    After that you might want to review your project.

    16V at 30Amps and how do you plan to connect this to your car?

    Did you consider how you were going to match this to your car safely in order to avoid blowing up the ECU and the battery?

    How did you you arrive at the value of 30Amps? Put your laptop and other equipment on mains power, not powering them through the car. Just your ignition switch left on causing your car draining in 45 Minutes is possibly a sign of a problem. Did you have the car stereo turned on (loud?), dome lights on, or other accessories like the fan motors? Electric fuel pumps and other parts that run on the car electrics, and are on with the ignition switch would drain the battery but 45 minutes is fairly quick.

    Looking at your suggested purchased power supply I would have to say that it is not intended or suitable for this use. What you want is a battery charger. I don't think you need 30 Amps either. I still doubt that you need 30Amps unless you are running high current accessories.

    At worst case a low amp battery charger should allow you to work for another half hour to 2 hours, then you might have to turn off the ignition and take a break. Go have a cup of tea, and make a few phone calls, while the charger brings the battery back up. Come back in half an hour or so from break, and the battery might be getting near full again (it would naturally regain some voltage even without a charger on it).
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    I understand the "thrill of the chase" ;)

    You might find this thread helpful, too:
    http://forum.allaboutcircuits.com/showthread.php?t=41757

    I posted a circuit for a low-level SLA charger w/temp compensation in there. While the charge rate is not sufficient for your needs, some of the same concepts could be used in your charger; just scaled up.

    I've done a good bit with avionics myself. Been fascinated with aircraft since I was a toddler.
     
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  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    :D
    There were no replies when I started to respond, but I Was distracted by something else, and came back to finish my response later. It turns out it was all covered.
     
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  10. rob-the-viking

    Thread Starter New Member

    Aug 23, 2010
    7
    1
    Gents, thanks very much for the replies. I've just realised how I can make the software run much faster (he said not having tried it yet!) so this might not be needed as yet.

    @ Potato Pudding, you're right mate, doing it yourself usually ends up costing the same if not more in the long run mate! But I just can't help myself! The engineer in me has cravings, I'm behind a desk at the moment, teaching people to fix planes, I miss spannering like you wouldn't believe though!

    @ SgtWookie Gratuitous plane pics coming up mate....

    I work/teach on these

    [​IMG]

    And love the sound of these

    [​IMG]


    Hope pics won't upset the mods!
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    If the battery hasn't seen any charge or maintenance in 4 years it's probably shot.

    I do know exactly what you're trying to do though, I've got a full diagnostics program on my laptop as well.
     
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  12. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Nope, a Lancaster will never upset the forums. I've heard single Merlins in a P-51, but never four together. I'm in F-16 and A-10 country, with the occasional B-2.
     
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  13. rob-the-viking

    Thread Starter New Member

    Aug 23, 2010
    7
    1
    That's great to hear! As for the sound, there is nothing quite like it. I'll try and find a link to a video clip i shot of her flying over the taxiway at not many feet. Glorious sound.
     
  14. rob-the-viking

    Thread Starter New Member

    Aug 23, 2010
    7
    1
    Just a tad off topic, but just for you beenthere

    Lanc Clip

    If this isn't one of the greatest noises in the world, then I'll eat my hat! Turn the volume up before you play it by the way!

    Apologies for the shot of my feet and the taxyway at the end by the way, I was a little overwhelmed....

    This Lanc 65 years old and stunning IMHO, seeing a Squadron of them coming at you must have been enough to strike fear into the hardest of Germans!
     
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  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Nice photos! Love the sound of the Lanc.

    My step-Grandad flew one of these during WWII with VMF-215:

    [​IMG]
    Shot down 18 Zeros and 2 Betty bombers with 'em.

    Here's one of my 'birds from 1976

    [​IMG]

    The commanding officers from all of the fighter squadrons based at MCAS Beaufort, SC in the 1978 to 1980 time frame:

    [​IMG]

    Worked on this prototype a number of years back:
    [​IMG]

    It's been interesting. ;)
     
  16. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Great clip. Al you need is one of a Beaufighter with the Bristol engines for perfection. Always wondered what a sleeve valve engine sounds like.
     
  17. soda

    Active Member

    Dec 7, 2008
    174
    13
    sgtwookie

    Thanks for all the beautiful pic's you add. I saved them all
     
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