constant current load

[bmihura]

I'd like to use my NI card to measure discharge profiles of AA batteries, so I'm looking for the simplest way to build a constant current load using cheap, discrete components.

I'd like to get a constant 500 mA +/- 1 mA through the battery as it discharges from 1.5 to 1.0 VDC while measuring its voltage with my NI card. I envision a transistor and a resistance I can tweak.

This is really a circuit design question. Are there other good online forums for this?

[Steve_B]

Hi bmihura,

You may want to look into using a current mirror and its derivatives as a possible configuration. You would have to look into the components but a current mirror could be a fairly simple option. You would need to be careful that your battery voltage range will work with your selected transistors.

Wikipedia - Current mirror
http://en.wikipedia.org/wiki/Current_mirror

[Wayne.C]

Greetings bmihura,

My first thought would be to build some sort of a self regulating current source circuit.  Similar to what is used in resistance measuring devices.  Simple instrumentation circuit monitors voltage across a resistance in the current path and provides feedback to drive the pass device.  Difference is that you only have 1V source not the typical 20VDC source.  The trick will be finding a pass device that has a very low impedance when conducting.