I have recently put together a trivial low-voltage interlock circuit to prevent over-discharge of a 9v battery pack. I use a Potential divider of 1600 / 5600 ohm to produce 4.7v when Vcc is 7.2. I have a reverse biased 4.7V Zener going from the middle of the Potential Divider to the base of an NPN transistor (2n222). The emitter is connected to gnd and the collector is the gnd for a load. Vcc to collector gives me the + and - for my load.
(R3/R5 is the potential divider. R4 is the nominal load. U1 is 1N4732 Zener)
The problem is that emulation (on TinkerCAD) says that Vcc below 7.2v reduces current to the load down to about 1mA. But breadboarding it produces real world behaviour where the load is able to draw 10+mA down to Vcc=4.5
Somehow this 4.7v Zener is not blocking current even when the voltage it is opposing is only 3v.
Presumably I am missing some fundamental point - so if someone can explain to me why a reverse biased Zener triggering an NPN transistor is not a viable low voltage cutoff I would be most grateful. (I realise that it will only kill the majority of the load and the Potential Divider itself will continue to discharge the battery - but at a tiny fraction of the rate.
prevent over-discharge
? How much current compliance is required just prior to that cutoff voltage? What's the current compliance (if different) when the battery is fully charged? \$\endgroup\$