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#### nnvkct

##### Newbie level 6
Here is the circuit:

And I'm stuck on following questions:

- If P3.4 =0 P3.5 =0, or P3.4 =1 P3.5 =1, or P3.4 =1 P3.5 =0, or P3.4 =0 P3.5 =1, what's the value of VDD and VPP.
- What's the role of zener 5.6V and zener 12V on this circuit.

(of course, I tried to stimulate this circuit by Electronics Worldbench but I'm confused its result. Please help me to understand how does it run.)

#### artem

roughly if VDD is more that 12.6 V , Vpp can take based on P3.4 and 3.5 :
- 0V VP3.4 = ~ 1V , VP3.5 irrelevant
- 5V VP3.4 = 0 V, VP3.5 more than ~1 V
- 12 V VP3.4 = 0 VP3.5 = 0

### nnvkct

Points: 2

#### gszczesz

##### Full Member level 2
Assuming Vdd > 12.6V, this circuit is a voltage regulator.

P3.4 is the shutdown. If P3.4 is high, then the base of Q1 is brought down to 0.

P3.5 is the voltage select pin: 12V on the output if P3.5=0, 5V on the output if P3.5>1.

The 12V zener sets the base voltage of the output follower (Q1) to be 12V, but you need an extra Vbe for biasing. Therefore D3 is added to take out the extra Vbe.

The 5.6V Zener already includes an extra 0.6V, so it does not need an extra Vbe.

In summary, this is a 5V or 12V output regulator, with P3.5 the select pin, P3.4 the shutdown pin.

Greg

Points: 2