Why design a voltage regulator?
Now, here is the question that is related to my own experience.
In college, I got an assignment to make a mobile charger using basic rectifiers and filters.
so, when the charger was almost ready, we found out that the output we were getting was still not stable and constant.
And we were strictly told not to use any regulator IC in the whole assignment.
So, in search of a basic regulator design I learned, how to design a voltage regulator in just 3 basic steps.
Basic circuit diagram to refer:
As you can observe in the above circuit, input voltage Vin is kept zero volts. But it’s not zero, it’s unknown.
It’s upto you that how much input voltage you will supply to the regulator circuit.
Basic information of components are:
1. Rs is the source resistor.
2. R1 is the current supply resistor for the base terminal & zener diode D1.
3. Q1 is the transistor (don’t refer to the model name, it’s taken for random.)
4. D1 is the voltage regulating zener diode.
5. R2 is the output resistance, from where output will be taken for further supply of regulated voltage.
steps for designing an E-F voltage regulator:
There are 3 basic steps to design an emitter-follower voltage regulator.
Those steps are as follows:
1. Selection of zener diode.
2. Selection of current limiting resistor R1.
3. Selection of transistor Q1 (on power dissipation criteria.)
1. Selection of zener diode (D1)
first apply kifchoff’s voltage law in the right side loop or the output loop of the circuit.
from there you will get the equation of relationship between output voltage Vo, base-emitter voltage V(BE) & Vz means voltage across zener diode.
As we can see in the equation (1), Vo is upto us to decide. Because output will decided by us.
And V(BE) = 0.7 v (standard cut-off voltage).
so, as you get Vz, you can search for the same rated zener in the data sheets & choose.
2. Selection of current limiting resistor "R1".
First of all,
Why to use current limiting resistor R1 ?
Because, for the transistor Q1 to work it takes some amount of current from base terminal to stay as an short circuit.
So, as the R1 branch provides the requires current to both, zener diode & base terminal.
but, the base ternimal takes maximum potential difference V(B max) & the zener gets the remaining Vz(min).
Now, what’s the relationship ?
Sometimes, the input you will get willl not constant or fluctuating.
So, we have to take Vin(min) as the input , but in the path the source resistor Rs eats up some potential difference.
It can be denoted by V’in(max) & the equation of relation is given in above image as equation (3).
Here we assumed current i(l) is flowing in input loop, so we got this relation via KVL.
So, as we get V’in(min) just put it in the equation (2), & get the value of R1.
3. selection of transistor Q1
It’s the most important step in designing the whole regulator.
Because transistor is the one that regulates voltage with zener diode.
So, we will check the power dissipation handling capacity of the transistor & then, choose the one from the datasheet.
As, you can see from the above equation (5), P(D max) is max power dissipated form the Q1.
power dissipated = voltage.current
therefore, Ic(max) is the current flowing through the transistor Q1 & Vce(max) is potential difference between collector & emitter terminal.
but, Vce(max) is unknown, we can calculate it by applying KVL in the input to output branch, as given in equation (6).
As you get the power dissipation limit, always choose transistor that have more power dissipation factor than the recommended one (for safety).
1. No provision exist for varying the output voltage since, it is almost equal to Vz of zener diode.
2. with increase in room temperature the values of V(BE) & Vz tend to decrease, thus the output voltage Vo can’t be maintaned.
I know that the analysis we did above is therotical & when you will implement it in practical circuit the values will vary.
So, there always assume the worst cases and assume or select the values that are above the calculated or suggested one.
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