Hello!
I'm working now on voltage regulators. I'm trying to practice for my exam.
The attached circuit is my 1st problem.
Problem asks to evaluate the voltage at R_Load, and the Zener current and the Power at the Zener. I'm not sure about the Power on the zener because the Pmax is given and it's 0.25 W.
The voltage supply is supposed to be unregulated and the voltage can vary from 18 V up to 22 V.
We learn that we should work for the worst case scenarios and this said we should always find the minimum Zener current which should be 10% of it's maximum current.
Vz = 12 V
P = 0.25 W
and the transistor hfe is 50.
So, for the minimum current I did this:
0.25 = 12*Iz_max
Iz_max = 20.83 mA
Iz_min = 10% of Iz_max = 2.083 mA
Then:
I_R1 = (V1  V2) / R1
I_R1 = (1812) / 470 = 12.77 mA
Then I'm not sure what to do next... I already tried an approach but it is not correct but I'm not sure why...
I have written a nodal equation for the currents at node 2 as:
I_R1 = Iz_min + IB
IB = I_R1  Iz_min
but the IB current I found is too high... 10.69 mA which, assuming transistor working on active zone, will turn into an IE current of 545 mA, ending up in a voltage drop across R_Load of 54.5 V.
I'm working now on voltage regulators. I'm trying to practice for my exam.
The attached circuit is my 1st problem.
Problem asks to evaluate the voltage at R_Load, and the Zener current and the Power at the Zener. I'm not sure about the Power on the zener because the Pmax is given and it's 0.25 W.
The voltage supply is supposed to be unregulated and the voltage can vary from 18 V up to 22 V.
We learn that we should work for the worst case scenarios and this said we should always find the minimum Zener current which should be 10% of it's maximum current.
Vz = 12 V
P = 0.25 W
and the transistor hfe is 50.
So, for the minimum current I did this:
0.25 = 12*Iz_max
Iz_max = 20.83 mA
Iz_min = 10% of Iz_max = 2.083 mA
Then:
I_R1 = (V1  V2) / R1
I_R1 = (1812) / 470 = 12.77 mA
Then I'm not sure what to do next... I already tried an approach but it is not correct but I'm not sure why...
I have written a nodal equation for the currents at node 2 as:
I_R1 = Iz_min + IB
IB = I_R1  Iz_min
but the IB current I found is too high... 10.69 mA which, assuming transistor working on active zone, will turn into an IE current of 545 mA, ending up in a voltage drop across R_Load of 54.5 V.
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