USB Pin voltages for car charger - Page 1
Forum section: electronics / beginners

author: bobmalaria
author_rank: Newbie
timestamp: July 04, 2014, 02:28:58 pm
content: Hi all,this is my first post, so hello to everyone in the forum.I recently had the problem that a couple of car-phone USB chargers that I bought did not work with my Nexus 4 or the missus Galaxy 3.So I went online and found some information that phone manufacturers/the USB specs allow things like tying the data-pins D-/D+ together, or to give a voltage of 2.3-2.5V on both data-pins to tell the phone it is connected to a wall charger.Today I thought I hack up a little voltage divider for one of that non-working chargers because I assumed that only the VCC and GND pins are connected. I was surprised to find that both datapins are connected through a voltage divider (30k Ohm/ 30k Ohm) to VCC. So I should get 2.5V at D-/D+.Nevertheless the phone does not charge. Does anyone know voltage values for the datapins that work with the Nexus 4?Could the 12V from the car or the switching regulator be an issue? I do not have an oscilloscope, but I thought maybe ripple on the output could prevent the phone from detecting a charger.Yes I know I could now go ahead and find a charger that works out of the box, but there is no fun in doing this...Any thoughts and recommendations are appreciated

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 04, 2014, 02:33:50 pm
content: Welcome to the EEV blog. Great to see new people interested in electronics.So, can we assume that these car chargers work properly to charge/power OTHER gadgets?Can we assume that your phones charge up properly with the original provided chargers?Instead of measuring the chargers that DON'T work, why not measure the ones the DO work and see what is the voltage on the D+ and D- pins?

author: bobmalaria
author_rank: Newbie
timestamp: July 04, 2014, 02:39:20 pm
content: Well, I do not have a single one working for the car with my phone... So it is hard to test working ones.I do have a charger for my car-GPS which works. So the 12V form the car should be ok.The factory wall wart is working fine and so are other mains chargers. The problem is charging in the car.You are right, I could cut the cable of wall wart usb and check the voltages on the pins. So far I did not feel like destroying a cable yet.

author: peter.mitchell
author_rank: Super Contributor
timestamp: July 04, 2014, 03:17:37 pm
content: IIRC nexus 4 likes D+/- shorted together (less than 120 ohms in this specific case), and then left floating (unconnected from anything else)

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 04, 2014, 03:30:07 pm
content: Quote from: bobmalaria on July 04, 2014, 02:39:20 pmBut the question isn't how other CAR chargers work (or not).QuoteTHE question is how that factory wall-wart is handling the D+ and D- pins. Clearly, that is what the phones want to see.QuoteOf course, you don't have to destroy the cord to your phone. Don't you have any old USB devices around that are broken or redundant?USB cables are dirt-cheap, and having one with open leads that you can measure seems like a handy thing to have around if your are doing these kinds of experiments.

author: bobmalaria
author_rank: Newbie
timestamp: July 04, 2014, 07:35:19 pm
content: You are absolutely right. I think I can spare an old cable and measure whether the pins are tied together or have a specific voltage.I think shorted pins will always work, but will also limit the current to USB standard of 500mA if I'm not mistaken. I'll report back what I measure

author: XOIIO
author_rank: Super Contributor
timestamp: July 04, 2014, 07:40:06 pm
content: Once you find out you could make a mini adapter to do this without modifying the charger circuitry.

author: bobmalaria
author_rank: Newbie
timestamp: July 04, 2014, 11:44:51 pm
content: Well, looks like I could not sleep. Maybe because Germany just entered the world cup semisI cut a USB cable and what peter.mitchell wrote was correct. The pins are shorted.I tested this with:Nexus 4 factory charger (rated 5V at 1.2A)Samsung Galaxy 5 factory charger (rated at 5.3V 2A)Garmin PSAIO5R-0500 (generic 5V wall wart that I had laying around)This is somehow a surprise after reading so much about how USB devices negotiate power and especially how the charger signals the device to use a higher power mode.Anyway, this makes it pretty easy as it seams, at least for the devices I'm concerned with. Elsewhere I found that especially for the Iphone you may want to apply a voltage to D+/- (http://electronics.ringwald.ch/?n=Main.IPhoneCarCharger).So I guess coming back to the charger I showed in my first post, I'll probably try to cut the traces or simply solder the D pins together. I may consider putting in a switch to actually be able to either have the dividers working or the pins shorted. That way I would have the most flexible device - just in case an Iphone user travels with me. But this would also make it a bit more bulky. I'll see what I will do.Test in the car will be tomorrow.

author: XOIIO
author_rank: Super Contributor
timestamp: July 05, 2014, 06:45:06 am
content: Quote from: bobmalaria on July 04, 2014, 11:44:51 pmYou can get some very tiny slide switches with two positions that could work to either short it or allow normal operation once you cut a trace though it depends if shorting them would mess up the charger at all, then you would need a switch with four contacts instead of two