Guidance wanted - 12v-5V regulator

Discussion in 'Mapping & Navigation' started by eRRmmm, Jul 21, 2011.

  1. eRRmmm

    eRRmmm Been here awhile

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    Not sure if this is the right place to post this, but there seems to be other similar threads so here goes.

    I am planning on mounting my HTC Desire on my handlebars in a Ram Aquabox. I have a newly arrived Garmin 60csx for off road tracks, but have Copilot on the Desire for street mapping and voice guidance, finding fuel/accomodation/ATM etc, and also use it for music, net banking, Facebook updates, and all the other smartphone goodies. :D

    An issue with the Desire is battery charging while using the GPS software, many chargers can't keep up. I would like to hardwire the phone via a 5 volt regulator to the switched power from my battery so I know it will be reliable.

    NB The Desire needs to have the two usb data lines shorted, which tells it is connected to a high current power supply, and therefore draws full charge. If they aren't shorted, it thinks it is plugged into a computer and reduces the charge current to protect the usb port.

    Can I just solder a LM7805 voltage regulator onto the end of a Micro-USB cable (after cutting the usb plug off of course), then mount it in an aluminium enclosure under the seat or somewhere (to waterproof it and act as a heatsink)? This should provide a constant 5v at 1 amp, and for a few cents more I could wire 2 of them in parallel to give 5v 2amps, and so forth.

    It just seems too simple? A google search seems to show that I can, but if it is that simple why doesn't everyone do it, since the parts cost is about $10 and it's soooo simple? Did I mention it seems too simple?

    I've looked at a few off the shelf chargers, and the only ones that seem to do what I want are ridiculously expensive. eg http://www.powerlet.com/product/micro-usb-direct-to-battery-harness/485 at about $50. edit This is probably ok value for what you get, just I know it's only a few bucks worth of parts and some basic soldering.

    Any electronics gurus out there care to offer some advice? :ear
    #1
  2. CoDawg

    CoDawg HP c.180

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    I wouldn't use the 7805 alone. You want to have at least some amount of filtering, with a capacitor or two, and maybe an inductor in series too. You sure don't want a voltage spike zapping your expensive electronics!

    There are lots of places that sell inexpensive cigarette lighter voltage regulators. Here's one for $1.99US:
    http://www.dealextreme.com/p/car-cigarette-powered-1000ma-usb-adapter-charger-white-dc-12v-40469
    Just take one of those, break it open, pull the circuit board out, wire it to your 12V, add the appropriate USB cable connected to the 5V output, and short the data lines as you described (only for your particular phone). If you need one for the phone and one for a separate GPS, build two. DO NOT connect two in parallel to get 2 amps (same for the 7805).

    The 60CSx should have the USB data lines unconnected if you're using the regulator to power it. But using the USB connector to power a 60CSx is not so good since the USB connector is fragile. Use the round Garmin 12V connector and skip the regulator in that case.
    #2
  3. eRRmmm

    eRRmmm Been here awhile

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    Thanks CoDawg. Good point about breaking open an existing charger, I've actually done that before for another device, I could probably pull it apart and rewire it for my phone. It was a little in-elegant though, it had leds sticking out, and a usb port I had to remove, and the factory soldering onto the circuit board looks a bit fragile for mounting on a bike. So I'm still interested in making one from scratch, admittedly partly because I can.

    Came across this switchmode 5v regulator which in the data sheet states "typically no external components needed", and includes over current, temperature and under voltage protection... looks good at $9. Any experience with these?

    I'm wiring the Garmin in this weekend using a proper 12v Garmin hardwire cable, completely separate to the phone. That's what prompted me to do a proper job of wiring in the phone too, I've been running it (the phone) in a tankbag with a cig lighter socket which is extremely unreliable.
    #3
  4. SnowMule

    SnowMule [angry moth noises]

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    Those switching regulators work well, low heat and they're pretty reliable. Still may want to throw a 10µF cap from output to ground to clean up the ripple a little more... electrically, the switching regulators are pretty noisy. Not real great for RF or test applications, but for charging a phone they're fine.

    Definitely run the 60CSx off the 4-pin round. No sense in sucking down more current in the regulator than is necessary, and USB connectors are NOT made to take the abuse of an offroad motorcycle.
    #4
  5. eRRmmm

    eRRmmm Been here awhile

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    Just walked back inside after hardwiring the Garmin power cable in. The cable was pretty much exactly the right length, nice. :clap Just had to add a fuse on the positive lead and it's all good. After some deliberation I've wired it straight to the battery - I have a switched relay under the seat, but decided I didn't want the gps turning off everytime I stop for fuel/drink/photo. Hopefully I remember to turn it off before leaving the bike for any length of time...

    Thanks for that advice about the cap. I did some more research at an electronics site this morning and that was the general consensus there too. It was also pointed out that there is no over-voltage protection on the output of the regulator. So if the regulator fails and shorts internally it could put 12 volts on the output, and into my phone. I'm waiting on some advice on a circuit to solve that, I'll put it up here when I work out what it means.

    I'm now thinking it will be easiest to mount the regulator inside the aquabox behind the phone, rather than inline as I had originally planned (since these regulators don't generate much heat). Thinking it through that will make CoDawg's suggestion of using a dismantled car charger easier too, as waterproofing won't be an issue. Will wait until I get the aquabox to make a final decision on that.

    Now I'm going for a ride and see how lost I can get....:lol3
    #5
  6. SnowMule

    SnowMule [angry moth noises]

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    The GPS draws such little power (about 120mA) you'd have to leave it on overnight for it to have much of an effect on the battery.
    #6