A Case Of Fault Finding

Posted by under Electronics, on 9 November 2011 @ 11:52pm.

One of my current projects is building a 5v USB regulator for charging mobile phones. I’ll be using this when away camping so myself and friends can charge their phones whenever they like at full speed. My previous solution was a cheap cigarette socket version which only supplied 1A between two sockets. This custom version will provide 1A to each socket, of which I’ll have a total of 4.

Now from the beginning I thought “this is going to be easy!”. After all it’s just a regulator, some capacitors and some resistors. I connected up a nice basic circuit with an LM7805 5v 1.5A regulator. It was solid as a rock on 5v and could easily supply 1A. 1A is more than enough to charge most phones, and my mains charger only outputs 0.7A. However, try as I might, I could not get the phone to accept a decent charge.

It would start off at 0.5A, then fall to 0.1A and often drop off completely. I was getting really frustrated. I’d tried dozens of techniques I’d found on the web for connecting the data pins with different value resistors etc. None of them worked! I even tried another phone to no avail. I thought my circuit was to blame and I wanted to know what I was doing wrong. Subsequently I hacked open my genuine blackberry charger to see what on earth was going on inside. It turns out nothing special was going on inside.

So I wasted a perfectly good blackberry charger for nothing (not that I ever used it but that’s not the point). By this point I’d spent over 8 hours trying to get this working and I even went to the trouble of posting on a forum and e-mailing a guy on youtube who I though may have the answer. He turned around and said he doesn’t answer personal project questions like that – fair comment. Being a knowledgeable guy he probably gets too many of them anyway. He told me to post on his forum so I did. It was at that point a friend of mine suggested something…

It’s the most stupid thing ever, but he suggested trying another USB cable. So I did…

Straight away I was getting 1.07A charge rate into the phone, rock solid. In 1 hour it was fully charged and didn’t flicker or fault once the whole time. I cursed like I have never cursed before because it was such a stupid thing not to check for in the first place.

I haven’t looked into why the cable I was using wasn’t working properly but it would explain why I had so many issues using it to sync to the PC. I always thought it was the phones fault but it has to have been the cable. It all makes sense now.

So what’s the moral of the story? Always check everything, even the stupid things like USB cables. You can’t easily diagnose a cable so just swap it out and rule it out as a possible cause. My guess is there is a small break in the cable causing intermittent problems, or there is a bad connection on one or more contacts. Without testing it I can’t be sure but suffice to say the cable is now deemed out of action.