so that all appears normal but cannot deliver enough voltage to pull in the power switching relays when called upon to do so the resistor values have been chosen so that with the worst case opposite supply variation and resistor tolerances combined the alarm will occur at some point between the following detection thresholds for example if the 12 v supply drops the alarm is never active as long as the supply is above 11 374 v however the alarm is always active if the supply is below 10 586 v between these two levels the alarm will become active the exact level depending on the actual value of the 5 v supply which is being used as the reference this technique assumes both supplies do not fail at the same time even if the 12 v supply drops to say 4 v the 5 v low dropout regulator will output 3 8 v the incorrect ratios will activate the alarm this power monitor circuit works down to about 1 v 12 v supply below 2 v the alarm relay coil will have released and activated the alarm the shindengen brand dc to dc converter also referred to as the power module has a feature that causes it to shut down permanently until input power is cycled off on manually if it detects an output overcurrent or overvoltage condition there is a concern that this state could be triggered by a transient event such as esd or lightning fields possibly conducted into the bct on the power wiring whatever the mechanism a power module that does not self.