A couple questions about car charger hacks...

[chconnor]

Hi there -

I'm hacking together a few things for my new (old) car: I want to plug in to the cigarette lighter jack, drop the voltage to 5V, and have two jacks in parallel coming from that to charge my cell phone (5V, 1A is the rating on the wall-wart charger, conveniently) and/or my USB devices (also 5V of course, using the standard USB charger hack at: http://forum.hackedgadgets.com/viewtopic.php?p=3152 ).

- First question is about the voltage drop: my understanding is that you can use a DC-DC voltage converter or use a voltage-dropping resistor (after testing the car's actual voltage.)

The resistor is simpler, it would seem, but I'm curious as to the pros/cons of using a voltage converter? It'd be nice (maybe just psychologically) to not be wasting the burned off energy all the time.

- Also, the variance from 12V that cars actually have: is that per battery or per car or both?

- Any issue about the nature of the power coming from the car? Not sure what would happen; noisy or unpredictable or spikes or dropouts in voltage/current, undervoltages, or whatever else might hurt the devices?

Any info greatly appreciated.

Thanks!
-Casey

 

[Externet]

Hi.
A 78T05; or LM323K; or LT1083CK-5 voltage regulator with a good heat sink may be your simpler approach.
Use no resistors for that task.
Miguel

 

[Diver300]

A resistor is really bad for dropping the voltage. Unless you know the load current exactly, your resistor value will be wrong. Also the load voltage will depend on the supply voltage.

You cellphone will take a variable current so whatever resistor you chose, it will be wrong most of the time.

The best thing to do is go out and buy two car cellphone chargers. You can get those for about $5 if you avoid the posh cellphone shops.

Buy a splitter to give you more cigar lighter sockets, and plug the cellphone in. Cut the plug (that would plug into the cellphone) off the other one and solder it onto a USB socket.

Even cheap cellphone chargers use a switching regulator, so they don't get hot and the output voltage is regulated. On all the ones that I have looked at the exact output voltage is set by two resistors. If you change one of them you can adjust the output voltage.

The IC that controls the switching regulator almost always has a feedback voltage of 1.22 volts. There is an input to the IC, and if that input gets over 1.22 volts the regulator stops, and it starts again when the voltage falls below 1.22V. The two resistors form a potential divider that reduce the output voltage (maybe 5V) to 1.22V. If the output voltage is too big, the feedback voltage will be too big as well, so the regulator stops.

If you increase or decrease one of these resistors a bit, you can adjust the output voltage. You shouldn't make it much bigger that it was, as the output capacitor may only be rated to 6V if they have been cheap.

 

[SMUGangsta]

Or if you have kids, steal the freebie ones that have the USB ports on the back of the cigar lighter plug. I have 5 or 6 here that i have aquired. Im sure they came free with the new packs of sylus's (sylii??) for the kids nintendo things, which the kids always loose, im sure the pack was about £3 from the computer games shop and had 3x Stylus, a carry case for games/SD cards?? a wallet for the console and the charger thing.

 

[Willbe]

- Also, the variance from 12V that cars actually have

Max 15.5v if your regulator is working, negative several hundred volts if you have a load dump transient, normally a low of 13.3v with the engine running at faster than idle speed, 12.76v for a fully charged battery with no surface charge, a few mV of AC on the DC because of your alternator rectifying 3-φ
AC (more mV of AC if your battery is failing).

 

[chconnor]

Ok...

Wow, always a good idea to check here first, I've learned that much.

Thanks, all.

So, Diver300, it sounds like what you're recommending is roughly the packaged/consumer version of what Externet suggested?

So that raises a few more questions (it always goes like this, doesn't it!)

Diver300:

- perhaps I'll get one charger, and just splice into the leads (parallel) and wire them to the USB jack, rather than getting a splitter and a second charger? Or perhaps it will draw too much on the single charger/regulator to charge two devices off it at the same time?

- on the issue with the two resistors and adjusting the voltage: I think you were just sharing that info for the sake of my learning, right?, or is it something you expect I may need to adjust? Since both my cell phone and USB devices ask for 5 volts, I was assuming that I'd find a charger/regulator (or two) that outputs 5 volts, and all would be well... ?

- the simple USB-connection schematic I'm planning on (image here: https://img171.imageshack.us/my.php?image=usb5me0.jpg ) shows a 10k resistor from the +5 line to the center two pins; I assume there's no problem with such a setup in combination with what I'm/we're proposing... (supposedly some USB devices expect a small voltage on those pins in order to wake up and charge...)

SMUGangsta - no kids, but thanks for the tip!

Willbe - Ok, good to know. Car power = crazy.

Thanks again everyone,
-C

 

[chconnor]

One more thing...

Oh, and I'm curious about the "auto-sensing" features of products like this:

**broken link removed**

...wondering how that works and if it would interfere with charging non-cell phone devices...

Thanks so much,
-Casey

 

[Diver300]

- on the issue with the two resistors and adjusting the voltage: I think you were just sharing that info for the sake of my learning, right?, or is it something you expect I may need to adjust? Since both my cell phone and USB devices ask for 5 volts, I was assuming that I'd find a charger/regulator (or two) that outputs 5 volts, and all would be well... ?

Yes, but it is possible that the cellphone charger voltage isn't quite right.

- the simple USB-connection schematic I'm planning on (image here: ImageShack - Hosting :: usb5me0.jpg ) shows a 10k resistor from the +5 line to the center two pins; I assume there's no problem with such a setup in combination with what I'm/we're proposing... (supposedly some USB devices expect a small voltage on those pins in order to wake up and charge...)

No idea. Sorry

 

[chconnor]

ok

Cool, thanks. So if I get a charger for my phone specifically, and test the voltage at 5V, I'm good to go...

Thanks for the help,
-c

 

[Willbe]

I squared T

Let's say you want to get rid of your negative load dump transient and filter the DC power from your car's electrical system.

Using the datasheet for a 1N4006 diode the surge rating is 30A rms for 8.3 mS which gives an amps-squared-seconds rating of 30^2 (.0083) = 7.5 .

From the 1N4006 datasheet the incremental resistance of this diode with 10 to 20 A flowing is R = 14mΩ, so if you connect this diode to 14.4v with a cap to ground the instantaneous current for a very short time is 1030A.

For the diode not to be damaged by this surge, 7.5 needs to equal (1030^2)/5RC, if my derivation is correct.
1030 squared is 1.1 mega-amps, so the max C usable with this diode and this DC voltage is 7.5/(1.1MA x 5 x .014) = ~100uF.

If the (I^2)T rating of the weakest fuse upstream of this circuit in your car is more than that of this diode circuit the diode will blow before the fuse.

If you need more filtering you need to add series resistance or, better yet, a series choke.