I’ve been working on the USB charger project in force over the weekend and I’ve made some good progress. It’s not completely prototyped, but all the parts that I need (except one, but it’s almost a passive part) have been made to show signs of life.
But here’s the thing… sometimes you need to try something only to have it fail. Then debug it and make it better the next time around.
Take one of the simplest parts of the entire project: the current sense resistor.
All it is is 0.01Ω resistor placed in series with the power supply to the USB. That tiny resistor will drop a tiny amount of voltage that will be sensed (hence: current sense resistor) to determine how much current (ergo: power) is being drawn.
So, for the example of 5V (USB supply voltage) and 1A (iPhone charging) you’ll get a 0.01V across the resistor. It’s not a lot, but it’s enough that once it’s amplified it’s perfect for the D/A convertor.
The first one I built was pretty janky.
It had male connectors on the top which is annoying to use and it seemed to have too much wire.
I had an idea — just slap on another pin and viola:
(And also make it double-wide)
It looks neater and it makes breadboarding easier.
Except it gives the wrong result.
At the current-sense it was showing 0.015V.
That’s when I realized that this was with a 1A load.
The addition of the connection back to the breadboard was adding another 0.005Ω in series with the already in-series resistor causing an addition voltage drop.
So I made up another another one… this was a combo of both the first and second ones…
That fixed things right up.
Basically I wired it up with a Kelvin connection and it was all good.
Since there’s almost no current going into the sense amp there’s virtually no voltage drop in the sense direction.
I guess it’s something that I had to spend a few bucks on for the parts to learn that lesson C’est la vie.
And don’t get me started on the resistor I blew. :-/