# Feedback and error amplification regulated power supply

Discussion in 'The Projects Forum' started by ghks, Jun 27, 2015.

1. ### ghks Thread Starter New Member

Jun 27, 2015
3
0
Hello, I would like to design the voltage regulator below, with R4 connected to the Zener diode, but I can't seem to find the equations for the dimensioning of the components of this configuration in particular. Would anyone lend a hand? Thanks in advance. (Sorry for not being more specific, English isn't my first language)

2. ### #12 Expert

Nov 30, 2010
16,704
7,354
R4 depends on Vin minus Vz. What is left is the voltage across R4 and the current R4 allows must be within the current range of the Zener diode. You often meet with threat of excess power in the Zener because Vin is not stable. When that happens, you have to increase the zener from 1/4 watt to 1/2 watt or a whole watt.

or did you mean, "Give equations for every component without any voltages or current requirements given to work with?"

3. ### ghks Thread Starter New Member

Jun 27, 2015
3
0
Yeah, that's what I meant to ask, though I'm working with set values for Vin (35V), VL(18V), IL(900mA) and R4 (10KΩ). Already searched a few books and online articles, but they only provide equations for the components of the circuit without R4 added to it. Given it influences the Zener's current I'm assuming it will affect the dimensioning of all resistors in the circuit and T2 as well. Hope I'm not sounding lazy or anything, it's just that I'm having a hard time finding more info on this.

4. ### #12 Expert

Nov 30, 2010
16,704
7,354
R4 and Vz are almost completely isolated from affecting any other component values. They merely set a voltage reference to compare to the voltage divider, R1 and R2.

5. ### ghks Thread Starter New Member

Jun 27, 2015
3
0
Giving it another thought that seems to be the case after all, considering R4 has a fairly high resistance compared to the rest of circuit (R1, R2 and R3 should range between 500 and 1kΩ) and should have a minimal effect on the current divider. Thanks for your help!

6. ### #12 Expert

Nov 30, 2010
16,704
7,354
Meh. That's what we're here for.
It's a hobby between our day jobs.

7. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,977
1,136
R4=10k is in the ball-park if Vin ~35V. That gives a zener current of ~3mA (depending on Vz), which should establish a well-defined zener knee voltage.