I have a project that requires current limiting. The sensing has to be done on the high side and I don't want to order some special chip, so I dreamed up a circuit that just uses jellybean parts.
------+-----Rs-------+------batt, 12v | | \ PN2907 / >| x2 |< |---+----| /| | |\ / 1M \ | | | | gnd | | | +--------------|---- | | to comparator | +---- or op amp | | R1 R2 | | '------+-------' | gnd
the b/e junctions of the transistors will experience a differential voltage Ich*Rs. For a particular DeltaVbe=Ich*Rs, the base currents will exhibit a certain ratio and the collector currents will hew to the same ratio, at least for matching transistors. Choosing R1 and R2 in a ratio equal to the ratio that the collector currents exhibit at target charging voltage will cause equal voltage across the comparator inputs at that moment and trip it.
I haven't tried prototyping this circuit yet, I just came up with the idea this morning, so please criticize it and point out what I may have missed.
This circuit will get bolted somewhere on an internal combustion engine operating outdoors, so temperature will vary to extremes. The transistors' 2mV tempco *should* cancel; but then the circuit operation also relies on the exponential voltage/current relationship, and I haven't found any information about whether the terms of this expression exhibit a temperature dependence. Does anybody know please?