Low Voltage Enable with Latch

[LukeKnepp]

Here is my complete circuit

Circuit in LTspice -

ENABLE node voltage is green, input is blue, Q2 base-emitter voltage is red.

The overshoot when C1 starts up the circuit causes too much bas-emitter voltage and fries Q2…
I can lessen C1 to reduce overshoot, but then the circuit does not consistently start up…

I can put a Zener diode on the ENABLE node and everything looks good in this simulation, but if I speed it up, the Zener is not fast enough to save Q2...

Any suggestions or is this just not a good circuit for what I’m trying to do?

 

[crutschow]

is this just not a good circuit for what I’m trying to do?

And what are you trying to do?

 

[rjenkinsgb]

Your two-transistor arrangement is pretty much emulating a thyristor - when conduction starts, both device immediately turn on to saturation - or destruction, if the available current is high enough.

Add resistors directly in series with each base to limit the base currents??

 

[LukeKnepp]

And what are you trying to do?

Sorry - I want a latching low voltage shutoff that will start up even with a small ramp in the startup voltage, then not turn back on until power is cycled.

Add resistors directly in series with each base to limit the base currents??

Yes, but I was also wondering about a diode from the Q2 base to emitter...

 

[rjenkinsgb]

OK, so it's the input power via C1 that is reverse-biassing T2 B-E, due to the zener clamp. A reverse diode alone would also change the timing, unless you add a series base resistor before the diode.

To me whole approach is strange.

Why not have the system default to ON, then latch it off after VOUT has been valid for so many milliseconds?
(Though I still do not understand the purpose of the overall thing)?

 

[LukeKnepp]

Okay so here is what I am doing, my goal is very simply, maybe my approach is all wrong...
I want to start up the Enable pin, even if the input voltage is a little slow ramping up.
Then, when the input voltage drops below somewhere in the range of 15-16V, the enable pin gets latched off and does not reset unless the power is cycled...

Why not have the system default to ON, then latch it off after VOUT has been valid for so many milliseconds?

How do I do this? Somehow reverse my latch circuit??

unless you add a series base resistor before the diode.

I will add the base resistor...

 

[Tony Stewart]

Sorry - I want a latching low voltage shutoff that will start up even with a small ramp in the startup voltage, then not turn back on until power is cycled.

Yes, but I was also wondering about a diode from the Q2 base to emitter...

I'm not clear on the specs, but PNP reverse protection on intermittent or slow power up can be protected by moving the 0.1 uF cap.

 

[Tony Stewart]

This requires a more stable reference voltage and not just R ratios.

 

[Tony Stewart]

This works

sim link https://tinyurl.com/27en5d2v

 

[Tony Stewart]

my 1st video