USB Ni-MH Battery Charger Part 3-The Schematic and Layout

Monday, March 12, 2018

USB Ni-MH Battery Charger Part 3-The Schematic and Layout

I started this project thinking that I would try to fabricate it as I have never tried to fab a board before. But after I soldered on a perf board, laziness kicked in and I think I would just leave the project as it is.

I goofed up big time during my component purchase. I wanted to order BQ2002PN, but because RS Online only sells them in a pack of 5, and I only wanted 3, I opted for another part, with “similar name. I thought it was the same! I bought 3 BQ2002TPN! I didn’t notice that until my controller just wouldn’t switch my current source on. I thought my soldering really sucks so much to the point that such a simple controller circuit wouldn’t work.

I noticed that the current source worked when the controller is not there, so I started poking around the pins and matched the readings with the datasheet. I found some discrepancies. My LED lighted up even though the current source was switched off. I’ve forgotten how did I find out I bought the wrong part, but that’s definitely a facepalm moment.

Anyway, I realised afterwards that BQ2002TPN only comes with temperature increate rate termination, dT/dt. Another facepalm moment, as my resistors were all designed for a dV/dt termination….. Another round of rework and adding in the thermistor, the design finally worked!!! And it worked so well that I could not believe it. It really terminated the charging after it detected a dT/dt of the battery. I have a plan to record and show the voltage waveform, but that can only be done during the weekend…

And for the schematic:

Still need some clean up. I’ve even picked-up FreeCAD to create 3D Model and produced a layout for this board. Well, maybe this project is not going to be fabricated, but the experience will definitely come in handy next time.

The next post would be the end of this project series, it’ll be just a short post to describe how did I choose the resistor value to parallel with the thermistor. Hope you guys have fun reading this. Until next time!