The TL431 has a 2.5 reference. So divide the 10V down to 2.5V. That will help you pick the two resistors. R2, R3If I want a trip point of 10V what voltge do I have to set by trimpot? Is it 2.5V or 10V?
When the input to the TL431 is below 2.5 volts the output is almost Vb.Do I really need a base bias zenner diode?
An interesting side note is that those two values of resistors (10.2k and 3.4k) are the only combination of 1% resistors that give an exact 3:1 ratio.Because 7.5V is 3 times 2.5V then the resistor also need to be 3:1.
What about a 3K and a 1K ? - which was what I used.An interesting side note is that those two values of resistors (10.2k and 3.4k) are the only combination of 1% resistors that give an exact 3:1 ratio.
Other values can give close to a 3:1 ratio but not exactly.
3K is a standard value, here's the one I used (I think?):If you can buy a 3.00k, 1% resistor, that's fine, but the standard 1% value is 3.01K.
Most resistor manufacturers follow the E-96 table for their 1% parts. But some also add the E-24 values as well.Okay.
I wasn't previously aware of stock 1% resistor values in other than the standard 1% resistor sequence.
Fairly obviously as transistors require 0.7V to turn ON, and the 10V zener is between base and emitter, it will always have plenty more than 0.7V drive to the base holding it permanently ON. Even when the power is high enough for the zener to conduct, it will only limit the drive voltage to 10V, not remove it.What is disturbing about the circuit that the OP attached, is that it is a well drawn schematic, meaning that this non-functional circuit is published somewhere!
And people will start copying it.
Wondering how come it does not work.