3.5.1.4 Overcurrent Detection

Another implemented protection mechanism was an overcurrent detector. This works in the same way as the power failure detector mentioned earlier, but with an analog comparator monitoring the output of the current sense amplifier with respect to a set point in a DAC. The set value is dependent on the measured gain of the op amp with an additional signal margin.

Unlike most of the errors implemented, the overcurrent error is not cleared automatically when the condition no longer exists. This was done intentionally to prevent users from attempting to restart a failed-short Peltier or other electrical failure.

One flaw with the linear topology for the Peltier element is a brief current surge when the Peltier element starts up. This surge can trip the protector prematurely. To reduce the detector’s sensitivity to the start-up surge, this protection circuit is polled as part of the periodic self-check, rather than being interrupt driven. This allows the start-up current surge to pass, rather than triggering an immediate shutdown. An overcurrent event that lasts longer than the polling cycle will still trigger the circuit.