MAX712 NiMH Battery Charger
After I found out that my expensive MAHA MH-C777PLUS charger did not work with NiMH batteries, I decided to try out the MAX712. It could supposedly charge 1 to 16 cells. What I needed was a portable charger that could charge 2, 4, or 6 cells at a reasonable rate.
The circuit is pretty much taken from the MAX712's spec sheet, with the three pole three position switch added so I could select 2, 4, or 6 cells. The pass transistor Q1 must be well heat sinked, since the current is around 2/3 amp (determined by R7/Rsense). The timeout is set to maximum, to accomodate newer high-capacity cells.
The power supply requires a 12VDC at 1 Amp supply. I found an amazingly small surplus wall wart from some old Dell equipment rated 13V 1A, which is pretty good, but it must be a switching supply, since it accepts 100 to 240 Volts in at either 50 or 60 Hz, making it ideal for travel. (Dell p/n 3317D.)
In a project like this it is important to minimize the resistance of the circuitry that carries the current to the batteries. In particular look for a good quality switch with low resistance contacts, and good battery holders. I tried 3 different battery holders that all looked the same, but only one had low resistance contacts. Unfortunately the MAX712 is measuring the voltage at pin 2, not at the battery terminal.
The following photos show the project. The gadget on the right is the thermistor glued to a small magnet.
Having attempted to build other projects that use thermistors, I know that this is the builder's achilles heel because no one has the right thermistor in their parts box, and obtaining the specified part can be difficult to impossible. The MAX712 spec sheet specifies a 10K ohm (at 25 degrees C) NTC thermistor, and suggests an Alpha Sensors p/n 14A1002 part. This part is not available to hobbyists, forcing a search for a substitute. Beside the "10K @ 25C" specification, the key rating is the "beta", which determines how the resistance changes with temperature. This part has a beta 0/50 of 3889. Using DigiKey's excellent parts search engine yielded a Honeywell bead thermistor p/n 192-103LET-A01, DigiKey p/n 480-3158-ND, which is in stock with a unit price of around C$4.90. The beta is close at 3887 and should work fine. Other close parts are also available if you do your own search. Because of this project I added a thermistor dialog to my HamCalc program.