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<a href="http://en.wikipedia.org/wiki/Current_mirror" target="_blank">http://en.wikipedia.org/wiki/Current_mirror</a>

[centerbolt]

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