# how to analyze discrete voltage regulators

#### justtrying

Joined Mar 9, 2011
429
At the circuit attached, I cannot figure out how to get the currents necessary to determine the minimum load value for which the circuit still regulates. I have calculated the output as 9.15V. I believe that Q2 should remain off as long as the current is not too high, i.e it turns on when voltage drop across R2 is 0.7V to prevent Q1 from damage. If I have to calculate when it turns on... I don't think I can

Thanks for any help. Would be nice if it was quick

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#### Jony130

Joined Feb 17, 2009
5,156
For this circuit the minimum load current is equal 0A
Simply becaues R5 must provide enough minimum current Zener diode work in Zener region.
Vo = (Vz + Vbe) * (1 + R4/R3) = (4.7V + 0.7V) * (1 + 1.18K/1.7K) = 9.1V
And the max load current is equal to.
Io_max ≈ Vbe2/R2

#### justtrying

Joined Mar 9, 2011
429
for max, that's because that is the current required to turn the switch on. Right? I think i get it now.

#### Jony130

Joined Feb 17, 2009
5,156
Yes, Q2 should act as a overcurrent protection.
For the normal load condition Q2 must be in cut-off.