I have a question regarding a circuit I've been working on for some time. The circuit is based around existing supercapacitor protection modules and uses a TL431 (actually a TLV431 lower power version) precision Zener device. Basically, the circuit works, but I wanted to get some feedback about resistor values, and whether or not my NPN or PNP transistors are doomed to fail.
In summary, the circuit uses the TLV431 and it's reference and sensing resistor divider to set a trip point (somewhere around 2.65 V as I have it at the moment) the TLV431 then starts conducting, pulling current (this is where I get foggy) from the base of the PNP transistor (BC557) turning it on (hopefully in saturation). When it turns on the NPN gets (an unknown amount of?) base current, and turns on sinking the charge current (with a maximum of?) through a resistor and 'around' the capacitor cell.
Eventually I'd like to have this circuit in series with my series capacitor strings so that any stage in the stack never sees more than it's 2.7 V (2.65) so long as I stay under the dissipation capabilities of this circuit on the input. The current modules that I bought from eBay only seem to limit on supplies up to about 500 mA. I'd like to get this up to 2 - 3 or 4 amps. Let me know what you think - the specific question is about R3 value, R4 value, R5 value, and if a resistor is required between the PNP and NPN base. Thanks!
I've seen versions of this circuit that omit the PNP base resistor, but I've added one because without it I think the TLV431 would burn and fail - perhaps someone could also clarify this aspect of the circuit? If anyone else is building protection circuitry around the TLV431/TL431 for 2.7V supercapacitors I'd really appreciate you chiming in!