So I am trying to created a current source that will be used to drain a battery at specific time intervals for a specified duration.
simulate this circuit – Schematic created using CircuitLab
Here is the circuit that I am attempting to use, pulses from the square wave source will trigger the transistor allowing the current to flow and thus draining the battery.
This needs to be implemented three times for my design but the first two (which are 10uA and 10mA sources) work fine and do not vary much with changes in the battery voltage, however I am struggling a bit with my final current source which needs to draw 100mA+ from the battery.
The issue is not with getting the correct current, but in fact getting the current to remain constant with changes in the battery voltage, that is the voltage at the collector of the transistor.
I was advised that I could possibly need a current buffer before the base pin of the transistor and I implemented this but it still does not yield a steady and constant current source!
This is the circuit for the 'current buffer', not really sure what it is but that is what I was told to do:
As you can see all it is a pnp ahead of the previously show circuit. Again my issue is not with this, I just need a method of keeping the current very constant, ie. No significant change until the battery voltage has dropped < 1V or something like that!
Any help would be greatly appreciated!
Best Answer
Try this:
simulate this circuit – Schematic created using CircuitLab
This circuit is capable of ~1% accuracy with good layout. If the battery is removed, the op-amp will current limit into the base, which should not hurt it. If you don't like that, or if you use another op-amp that doesn't current limit as well, add a small base resistor such as 100 ohms.