Mini-test 12-5v DCDC USB Car Adaptor

[dexters_lab]

Just done a little test on this module for providing USB 5v in-car, before i installed it i wanted to test see how good it was so i thought i'd share

It's a module with a molded USB port for fitting behind the dash etc, its rated at 15watts. 12v in 5v @ 3A output. Branded CPT sourced from ebay / china for £3.74 (GBP) including shipping.

Output regulation looks much better than the other similar devices i have seen. Though i would hazard a guess most of the volt drop is in the cable to the usb connection, at 3 amps output i can feel the cable warming. The module is potted and don't want to disassemble this as i'll be using it as-is. 2 amps is fine for charging my nexus 7 in the car.

output load / output voltage / supply load (@12v)
0ma / 5.01 / 10ma
500ma / 4.95 / 230ma
1000ma / 4.89 / 460ma
1500ma / 4.83 / 690ma
2000ma / 4.77 / 940ma
2500ma / 4.70 / 1180ma
3000ma / 4.64 / 1450ma

[Jon Chandler]

I'm building a iPad/iPhone car charger using a similar switcher module, but mine is just a 12-24v/5v converter and I've added my own circuit board to add the voltage dividers required to make an iPad/iPhone charge at high rate.  The details of what I've determined are here.

At least one of the modules with USB connectors like yours says it will not charge an iPad/iPhone.  Of the ones I looked at, none specifically claimed iPad/iPhone compatibility.

I did a few quick measurements on my module which are shown below.  One important feature I wanted to verify was that at no load, the module draws virtually no current (less than 1ma).

[dexters_lab]

Thanks, FYI i spotted a mistake on my numbers, no load current should have been 10ma not 0.01ma! sorry, although this is read from my cheapo bench psu so is probably not too accurate.

thanks for sharing the resistor values, i could do with checking what these should be to make the Nexus 7 think it's connected to it's AC adapter. i was going to set this up when i got around to installing into the car, i have to chop off the molded usb socket and refit it to the one thats built into the car dashboard already.

[Jon Chandler]

I think that many non-iPhone devices will charge at high rate if a 180 ohm resistor is connected between D+ and D– but I have not tested this out.

I should add that my "lovely" yellow soldermask (not exactly what I had anticipated) were done by Elecrow.  It's another Chinese fab service like iTead and Seeed but they offer v-score panelization for a slight added cost.  I was able to squeeze 8 charger boards into a 100mm x 100mm panel, so I got a total of 80 boards for around $40 delivered.

[dexters_lab]

neat!

looks like the nexus7 just needs the data pins shorting

shame there is no industry standard for the charger sense, do doubt apple have a patent on their method!

[Monkeh]

A 200R resistor between D+ and D- should work for most android devices. I think some of the high charge rate tablets need something else, but I could be wrong.

[Jon Chandler]

I read somewhere that shorting or the low value resistor will work for many devices. Someone (sorry, don't remember where I saw this) was of the opinion that the resistor was a bit safer just in case the charger ever got tied to a live USB connection.

I wonder if the iPad/iPhone resistors AND a 200 ohm resistor between the data lines would make everything happy?  That would be less than 10 mA through the resistor.

[dexters_lab]

Gotta be worth a try

[SeanB]

I was watching a LT video this week where they have a car USB charge module that has cable compensation built in.