Car charger design

executive summary: GO BUY THE CHARGER DESIGNED AND APPROVED FOR YOUR TABLET.

Here's how I got to that decision.

If you had any idea how challenging the car environment was from a transient protection point of view, you'd have asked your question differently.

It's almost impossible to get real specs on cheap after market chargers. It's almost impossible to learn how the internal charging circuitry in your tablet works.

So, what did you pay for your new tablet? How much are you gonna save building/buying a cheapo charger? How much is your time worth? Will your warranty cover any wrong-charger induced failures? Do the math. Are you feeling lucky? If so, EBAY is your tool.

tion_Notes/an9312.pdf

I'm still stuck on the OP's phrase "The AC power is 9VDC @ 2A." Which is it? AC or DC??

But I concur with your advice to the OP. Go buy the car charger (if there is one), or maybe a cheap inverter.

Happy Holidays.

-mpm

differently.http://www.littelfuse.com/data/en/Application_Notes/an9312.pdf

mpm,

you are correct, "if there is one", there is not one available.

The "AC power" comment was to clarify what was needed, but it did not work that way. :-\

Mike,

Yes I do understand your comments.

So, 12V-13.5V DC from a car to 9VDC @ 2A.

Thanks for the App note, I'll look it over carefully.

hamilton

ication_Notes/an9312.pdf

If its just a 1 off I'd probably go linear, not worth all the extra work for a switched mode one. Charging Li-ion requires the right control though, else goto bang.

NT

In some compact devices, the power brick is set to the exact voltage and current for charging LiPo batteries. The portable device only has sensors and an on/off switch; no regulator. A slight deviation can cause malfunctions or serious damage.

I put my camcorder on a benchtop power supply to see what would be needed for a mobile charger. It tests the power to a tolerance of something like +/- 0.05V for 5 seconds before engaging the LiPo but it doesn't detect dangerous voltage fluctuations after that. Being that LiPo batteries belch massive fireballs when they're unhappy, I didn't bother building it.

--
I will not see posts or email from Google because I must filter them as spam

Yeah, but I've never seen this for consumer-oriented phones, tablets, or other computer-type devices -- and the guy said his AC adapter claims it puts out

9VDC, which isn't any standard rechargeable battery voltage.

Actually it's highly unlikely this would happen: You can be confident that anything with a CE or UL mark on it has a battery within it that contains built-in protection circuitry, which is intended to prevent precisely the scenario you describe. (Not that I'd suggest purposely tempting fate...)

---Joel

You'd think so. Back in the day, I built a charger for a Compaq laptop based on the specified voltage on the power brick. Later, after a big black hole appeared in the motherboard accompanied by much smoke, I found a schematic and discovered why. The idiots put a fet switch between the battery and the charging port. They relied on the power brick to limit the charging current. The black hole was where the fet used to be. You can never count on designers to do what you think would be reasonable. They design and the regulatory agencies approve a SYSTEM. "Don't use any power supply other than the one specified" is clearly stated in the manual of every electronic device I've seen. Sometimes THEY REALLY MEAN IT!!!

That is pretty bad!

There are a lot of "universal" laptop power supplies out there these days (both 120/240VAC input as well as 12VDC input)... so perhaps laptop designers have gotten a little smarter over the years?

Agreed, although 99+% of the time it's just a marketing department gimmick to try to get your to buy their particular power supplies. I mean, if you have a proprietary connector on your widget, OK, maybe the designers did do something "weird" -- but for devices with standard USB connectors on them for a manufacturer to claim you should only use their own power supplies is just out-and-out silly.

---Joel

Yep, 90% of the time, statistics are useful. Only problem happens when YOU are the counterexample.

If you have the equipment and know what you're doing, you can significantly improve your chances. The tone of the OP suggested that this is NOT the case here.

Like I said, "feeling lucky?"

I

Well, our Mini-B-tailed wallwart has pins 4 and 5 shorted to tell the LiIon controller that it's a power supply (1A max) rather than USB (.5A max). ;-) A standard USB PS won't damage anything but it'll take a bit longer to charge (not even twice because the current goes down pretty quickly).

I think that's part of the USB standard, though, isn't it? (I seem to recall there was some brewhaha whereby people were ticked that Apple iPads wouldn't charge with non-Apple chargers, but then it turned out that Apple was just after a higher charging current for the iPad and followed the USB charger standard of shorting the data pins together to include a higher-power source...)

That's quite reasonable -- better than devices that just refuse to charge from

500mA sources!

In general USB seems to be pretty robust... although I did have a motherboard once that physically fused the Vbus line that a co-worker inadvetently shorted. He spent the next hour replacing the little SMT fuse on the board... :-)

(I'm always surprised to see engineers who figure a good source of regulated

+12V, +5V, and +3.3V is tapping into their PC's power supply and running a cable out the back... I sure don't want to risk losing whatever it is I'm working on or potentially corrupt the file system if one of those gets shorted and the power supply then shuts down to protect itself!)

Any plans to switch to micro-USB in the next rev. of the design? I'm rather a fan of mini-USB, but from what I've read micro-USB really is more robust mechanically and has the whole charging idea a bit better thought out.

---Joel

It might be, but no one had standard parts to do it. The data pins, IIRC, are

2 and 3. We short 4 and 5 (pin-4 to ground).

We thank LTC for that bit of magic. ;-)

That's a perfect application for a poly-fuse. PS/2 ports even had them.

I've never seen THAT. Sheesh! Buy a separate supply (still a bad solution).

No plans, at least that I'm aware of. We do have an on-going problem chasing Mini-B connector availability. It seems these things are all custom and they then sell the units to anyone. When the primary user goes away (product replacement) the manufacturer withdraws the part from general use. Since we charge at 1A, the candidates are further restricted. We recently had to give up through-hole solder tabs and even locating pins.

Ah, yes, I see -- yeah, that isn't quite standard.

Yeah, someone apparently saved a dime there. :-(

I've actually only known one Real Live Engineer (i.e., worked at a place that could certainly afford decent power supplies, was paid well himself, etc.) who did that (on his work PC!), but several students/hobbyists. I guess they think they're saving the money of buying a separate supply... might be like Joerg, they didn't want to raid the beer kitty. :-)

Ouch... that's kinda unfortunate.

Mini-B connectors seem like they're enough of a commodity you might just be able to make a "life-time buy" (say, 100k parts or something) of the type you prefer from some Chinese supplier and just keep'em in the backroom or something...

---Joel

Jeez, there are probably a million of the things thrown away every year! You don't have to dip into the beer money to go dumpster diving.

We can buy them, as long as we're willing to take what someone else is using.

use, but the things are only $.20 or less. It wouldn't be out of the question. We do buy enough for a couple of years. The guy that runs the purchasing group (a company director) seems to do a pretty good job. He has a good relationship with the disties, too.

Hey, I don't suppose you worked on this product? -->

:-) Just kidding...

Any luck with the updated DisplayPort drivers getting your laptop to work with the additional monitor?

---Joel

Nah, haven't played with it any more. I want to try it on another USB port, too. I was using a laptop port and sometimes they can be goofy. All the dock's ports are full and weird things happen when I move some of them. I hate to change things around when everything is working.

Hmm, I wonder where I put that thing...