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  • Voltage regulators
  • molgrips
    Free Member

    To connect a USB hard disk to my car stereo I had to add an external power supply. So off I go down to Maplin and buy a 5V regulator in a T220 package. This works fine except it gets really hot. I had to fit a heatsink and bolt it to the outside of the head unit behind the fan to get it to stay cool. Ok I thought, that’s how it has to be except that I’ve realised that all the cigarette lighter chargers for phones and USB devices I’ve used don’t have a cooling problem.

    So the question is, how do voltage regulators work and why does the one I bought on a chip seem to work so much hotter than the ones they put in in-car chargers?

    I think I will cannibalise one of the chargers and replace my dodgy hot thing.

    Mintman
    Free Member

    They use an a combination of inductive and capacitative loads to dampen fluctuations in DC voltage. The reactive power that is part of this ends up being dissipated as heat. So, they’ll all get hot but the better ones will get less hot (well dissipate the heat better) than the cheaper ones.

    There are some nice line diagrams on t’interweb if you’re that interested.

    I’ve got a Lucas 12v regulator in my kitcar and although coupled to a heatsink, doesn’t get as hot as yours seems to be.

    tumnurkoz
    Free Member

    Was it a 7805? should be able to handle the current (1A max) USB should be drawing less than that though. put it in a nice (heatsink) alloy box and ignore 😉

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    IanMunro
    Free Member

    The 7805 regulator you have is a classic linear regulator where the current used by your device is the same as that of the supply coming in.

    So let’s say your USB drive needs 1/2amp of current and runs a 5V, so that’s 1/2 * 5 = 2.5Watts of power. The battery is having 1/2 amp drawn from it too so the total power being used is 1/2 * 12 = 6 watts.

    2.5 of those watts are being used to power the device, the other 3.5 are being used to warm up the regulator.

    Phone chargers use switching regulators that rely on magic and other stuff to work, but the end result is that the battery current drawn doesn’t have to equal the current used by the device, so tend to be far more efficient.

    molgrips
    Free Member

    It’s this I think

    http://www.maplin.co.uk/1a-positive-fixed-voltage-regulators-46475

    The drive has no external power supply so it must be 500mA or less, right? I thought that would be ok but from what Ian says the rest of the energy is being dumped as heat so that makes sense.

    Now the only issue is the stupid head unit doesn’t necessarily recognise the drive if it’s plugged in when the car starts. So I might install some kind of delay before powering it up.

    Thanks for the explanation Ian 🙂 Btw I also had the same problem using the same type of device to charge my GPS from my bike lights at a 24 hour solo.

Viewing 5 posts - 1 through 5 (of 5 total)

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