Car 5 VDC for USB outlet

Discussion in 'General Electronics Chat' started by SteveDouglas, Jul 2, 2011.

  1. SteveDouglas

    Thread Starter Active Member

    Apr 30, 2009
    42
    2
    I need to make a 5 VDC supply and connect it to a USB jack in my car. I did use a 7805 in a TO-220 package and installed it in a old cig lighter charger housing and I was able to get a small, finned heatsink on it too. But the voltage differential from 12 to 5 dissipated too much heat, it got very hot after a few minutes but it did work. I won't be using it due to the heat.

    The main use for this is to charge my phone and other uses, my wall charger for this phone says 5 VDC @ 1 Amp so something near that is what I'd like to have. Are there other chips or other ways to accomplish this ?

    PS: please don't tell me to buy an adapter, that's too easy and not what I really want. I'd like to mount the USB jack or jacks in my dash or console and not have anything stuck in the cig lighter socket.

    Thanks for any help,

    Steve
     
  2. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    Somebody's got a tiny switcher in a 7805 TO-220 like package. A better device at a worser price, I think they are $5 each.

    Mounting this in the car should not require miniaturizing it. There should be room for a nice heatsink somewhere.

    Powering a naked 7805 from the auto's 12v system is a recipe for failure. Either add lots of protection around the 7805, or use things like the LM2937 or 40.

    Easy way- Take a $2 cigar lighter USB adapter, take it apart and put the guts in your own box mounting it behind the dash somewhere.
     
  3. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Whatever you use don't forget a good TVS and capacitor on the input to protect against load dumps.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    You can get low wattage 110VAC inverters for cars really cheap these days. Many of them(smaller ones) plug into the cigarette lighter ports.

    Using this will allow you to charge/run ANY device that uses 110VAC wall wart adapters.
     
  5. SteveDouglas

    Thread Starter Active Member

    Apr 30, 2009
    42
    2
    Thanks for the replies. I'd really like to know what they (HTC) use in their 12 volt cig adapter (my phone is an HTC Incredible). Whatever they use doesn't get hot yet supplies enough current.

    I have been hesitant to buy anything like a $2.00 USB adapter as I think it won't last very long and/or supply enough current.

    I do have a couple 12vdc to 110vac units but again, don't want to go that route.

    I guess I could use a 7805 and mount it directly to an unpainted part of the steel chassis for heatsink. But does a rechargable battery in a cell phone only draw as much current as it needs ?

    IIRC, the TO-220 is rated at 1.2 amps. Also the phone does appear to stop charging once it reaches full charge, just don't want to damage the battery or the phone of course.

    One other thing I should mention, I took a Motorola cell phone 12v to 5v cig adapter I have and wired it to a USB jack and it would shut down shortly after starting to charge the batt. so I think that whatever I use needs to be able to supply at least close to 1 amp.
     
  6. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    HTC very likely use a switch mode converter which uses an inductor and a few other components to efficiently step down voltage. They're not difficult to build but you can pick up ones for cheap which will probably work quite well. Often, the MC34063A is found inside these devices - this is a very common controller IC for step-down converters. The chip is capable of 1.5A output current, but allow 1A for safety, although the IC will shut down if it overheats.

    Modern phones want as much as 1A to charge their batteries. Once charging is complete current drops to a low level.
     
  7. SteveDouglas

    Thread Starter Active Member

    Apr 30, 2009
    42
    2
    Tom, thanks very much for that info! I'll check into that.

    Thanks again,

    Steve
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    I don't know about your phone, but mine requires a charger that the phone can recognize. Otherwise, it displays a message saying "Unrecognized Charger" or something to that effect, and refuses to charge.
     
  9. SteveDouglas

    Thread Starter Active Member

    Apr 30, 2009
    42
    2
    Right, I've seen that before on other phones and devices but this one will work with any 5vdc source that I've tried so far. Good point though.
     
  10. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    The LM2940CT-5.0 would meet your requirements. Or better still, the LM2575.
     
  11. SteveDouglas

    Thread Starter Active Member

    Apr 30, 2009
    42
    2
    The LM2575 looks just like what I was looking for!

    Thanks for all the advice and help everyone,

    Steve
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, it would not. The LM2940CT-5.0 is a linear regulator that would dissipate ~9 Watts of power while putting out ~1A @5v. It would be tough to keep it cool.
    The LM2575 is only rated for up to 40v in. The LM2575HV is rated for up to 60v input, which should be used. This IC is rated for the automotive temperature range.
     
  13. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    What can I say, Sgt. You are right again. I did see both the LM2575xx and the LM2575HV. I should have added the final suffix to the part number as it makes a difference in this case.
     
  14. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    USB car chargers are like $2 on overstock.com (plugs into lighter socket and has a USB outlet right on the other end..
    I use one with no problems on my smartphone.
     
Loading...