What's wrong with this scheme? I expected to be able to set the output voltage value modifying the values for R4 and R5 (I can replace them with a potentiometer), and why do I have negative voltage at the zener diode when it should be 6.2V (it's still negative if I remove the negative feedback)?
Before you ask:
ZTX696B - medium power high gain NPN transistor
ZTX869 - medium power high current NPN transistor
I thought that the output should be 9V + 1.4V , so 6.2V x (R4+R5)/(R5)=10.4
that means (R4+R5)/(R5)=1.67
But I don't get the expected voltage...
Also, I changed the op amp model, the new one is unity gain stable (I don't know if it helps).
Third version: (It works but I don't understand why, and the output is not 9V for any input higher than 12V)
For example if Input=16.00V then Output=15.00V
the op amp is "rail-to-rail"
- "rail-to-rail" op amp, "unity gain stable", supports up to 16V
- changing zener diode value has no effect, same for R4 and R5
- output should be less or equal to 9V
what's wrong now?
does the op amp AOL value matters?
5th version: (The final one)
I wonder if the "improvements" are good...
I used a PNP transistor for short-circuit protection (How can I find the value for Rsc? and when does Q4 enter in saturation?), Rsc Is actually a fuse...
A schottky diode for protection in case you power a motor
A capacitor at the input (I don't know what is it for)
Q1 has Icmax = 5A
Q4 has Icmax = 4.5A