Add USB connectors to a Miata?

Using a car charger/power-adapter in an NB Miata is starging to annoy me because my fingers hit the adapter when I shift into

5th gear.

I've got several different gadgets (phone, GPS, MP# player, etc.) that can all be powered/charged from a USB port. I'm thinking I could install a 5V regulator and a couple USB ports (power only) somewhere in/on/under the dash. There are all sorts of nifty USB-powered gadgets available like gooseneck LED lights:

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Everything from a disco ball, to heated mittens and a single-can sized fridge:

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And of course a fiber-optic cristmas tree:

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Anyway, has anybody done/seen a dashboard USB connector modification?

Reply to
Grant Edwards
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Check out the forums at mp3car.com -- they do DIY in-dash PC stuff (including excellent dash modifications). I'm sure you'll find inspiration there, if not an outright solution.

Reply to
pkirkovsky

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I have not done this but it would appear that if one were to obtain a common powered USB hub which happens to come with a wall-wart power supply which puts out 12VDC a bit of wiring could be kludged (with a suitably-sized fuse inline, of course) to replace the wall-wart.

Reply to
John McGaw

I'd bet money they all use 5V wall-warts. A 5V wall-wart is the same price as a 12V one, and using a 5V one saves you the cost of putting a DC-DC converter in the hub.

I'll probably use something like a National LM2825 DC-DC converter.

That would be a good option if one were to screw something to the underside of the dash somewhere -- which is probably the easiest option.

At lunch time today I was looking at the 2x3" rectangular panel/plug thats on the side of the transmission tunnel beside the the passenger's knee. I should have left an extra switched

12V wire someplace accessible when I installed the new stero head unit and guages. :/
Reply to
Grant Edwards

For the small amount of current involved a 3-terminal regulator with suitable bypass caps and a heatsink (and possibly a small inductor for noise suppression) could certainly be mounted somewhere under the dash. If only a single USB connection was needed a bit of surgery on a standard USB extension cable removing the female end would do the job. Having a single USB connection rather than four for a standard hub would also bring down the current requirement and simplify the PS even further.

Reply to
John McGaw

Linear would probably be fine at 100mA per port (the limit for un-enumerated USB devices). Two ports would require a linear regulator to dissipate 1.8W. That's on the high end of what a an LM7805 in a TO-220 can do w/o a heatsink, either a TO-3 or a TO-220 with a heatsink should be fine.

The max current for an enumerated USB device is 500mA per port. That's about 4.5W that needs to be dissipated per port using a linear regulator. I was going to try to do two ports, so that would be 9W. That'll require a pretty decent heatsink.

An 80% efficient switching converter will only need to dissipate 1.25W at 1A total current.

I'll have to check on the power requirements of the USB-powered single-can sized fridge... ;)

Reply to
Grant Edwards

Without chasing up a datasheet to be certain, I'm pretty sure that the biggest 7800 series regulator (for this application, you'd need a 7805) is only able to cope with a 100mA draw - And I know from experience that if you're going to get up against that kind of draw for more than a few minutes at a time with a 12 volt input, you're crazy if you don't heat-sink the beast into submission.

USB is specced to supply up to 500mA per port. Anything wanting to charge from a USB port is likely to want at least 250 mA of that, and anything actually considering it as a primary power source is likely to want the whole 500mA the spec promises. Put the draw of the hub itself (Call it 100mA so we've got a number to talk about - I'm sure it varies from hub to hub) into the picture, and multiply by 2, 4, or however many ports on the hub, and you've got a regulator that just isn't going to cope.

Go with the LM2825 converter. More support parts needed, but if memory serves, that one is rated to handle either 1.25 or 1.5 amps - Enough to let a two-port hub feed full USB spec current to both ports, and leave a safe cushion to cover whatever the hub itself wants to draw.

Reply to
Don Bruder

The 7805 has a 1 amp rating and is dirt cheap.

Reply to
Chas Hurst

That's what I get for relying on memory instead of digging up a datasheet, I guess...

Even so, I still wouldn't want to try using it to feed a multi-port USB hub. Not enough "headroom" if both devices decide they want to draw the full load. No clue what the hub itself wants (I have little doubt that it varies from one hub to the next) but I'd be worried about "that much" being "too much".

(As far as that goes, is there even such a beast as a powered hub with fewer than 4 ports? Granted, the concept of "only plug in two items" is simple enough, but...)

Reply to
Don Bruder

It'll need a pretty hefty heatsink and good ventilation: at 1 amp, it'll be dissipating 9W. An 80% efficient switcher will only be dissipating 1.25W.

Reply to
Grant Edwards

I don't want to try to dissipate that much heat in a small space behind my dashboard or inside the transmission tunnel cover.

If I understand the spec correctly, a device can't expect to draw more than 100mA until it's been enumerated. Assuming the cigarette lighter chargers don't actually contain a micro with a USB host controller and protocol stack, a device can't expect to draw more than 100mA from such a converter -- _if_ they're following the USB spec to the letter. I'd just as soon be able to provide the whole 500mA just in case...

Reply to
Grant Edwards

Whatever, there's no need to locate any power supply or voltage regulator inside the dash area.

Reply to
Chas Hurst

Maybe

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Put the cigar lighter adapter anywhere you want out of sight and run 12vdc to it, stick the hub on the dash or console.

-- Larry

Reply to
pltrgyst

Wow, that's expensive. A 5V cigarette lighter adapter is $6 at target (granted, the happen to be on clearance), and you can usually get a 4-port powered hub for about $10-$12. I think I'm going to try to do something that looks a little more "integrated" than sticking on a powered hub.

Reply to
Grant Edwards

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