# Series Voltage regulator.

Discussion in 'Electronics Homework Help' started by RaiRodrigues, Jun 25, 2018.

1. ### RaiRodrigues

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Jun 25, 2018
Hi guys, im a begginer in eletronics and i'm having difficulties with my first school project. If anyone coul help me please!
I have to find the values of the resistances in a series regulator with overcurrent protection. The circuit is this, if anyone could help me would really save me! i have to calculate values for differents V(in), so it would be best if anyone could point me the direction in a more generic way. Thanks!

2. ### kellys_eye

4,275
1,147
Jun 25, 2010
What voltage do you need to drop across R5 in order to make Tr3 Vbe conduct?

And since R5/TR3 is designed to limit the CURRENT flowing to the output you have to know what that limit is.

3. ### Harald KappModeratorModerator

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Nov 17, 2011
Unless you've been given information that doesn't show on the schematic, you'll have to make a few assumptions (or substitute realk components and use datasheet values). For example:
• Transistors' Vbe = 0.7 V ( a typical value)
• Transistors' current gain Hfe = 100 (again a typical value)
• Transistor's collector-emitter saturation voltage Vcesat - for a simplified model you could assume Vcesat = 0 V which is probably good enough for this project
• Minimum input voltage
• Maximum input voltage
• Maximum output current
• Target output voltage
Here are a few design goals:
• For calculating the regulator start by ignoring R5 and TR3 (current limiting circuit).
• R3 needs to be calculated such that for maximum output current there is enough base current through R3 into TR1 to allow that output current (which is equal to Ic(TR1), neglect current through the output voltage divider).
• R4 needs to be calculated such that for minimum input voltage there is still enough current through the zener diode D1 such that the zener diode operates well within the zener range (a typical value is e.g. 5 mA, stated as 'test current' in a zener diode's datasheet).
• For calculating R1, R2 and VR1 you'll have to make assumptions about the minimum and the maximum output voltage as well as for one of the resistors - typically I'd use the value of the potentiometer VR1 as a starting point. From there calculate R1 and R2 such that the desired range can be achieved.
To do that, you'll have to set up an equation relating the voltage at the base of TR2 to the output voltage. This is where the above mentioned Vbe and Vcesat come into play. Label all relevant voltages in the schematic with unique identifiers and use Kirchhoff's laws to set up the equation(s).

By the way: your instructor took this schematic from another website...

4. ### RaiRodrigues

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Jun 25, 2018
Thanks for the help guys, just asked my tutor and he said that the V(target) =12v , max current = 1A, the maximum and minimum input voltage are to be specified by me.

5. ### AnalogKid

2,502
719
Jun 10, 2015
Do you need to work out the design equations, or actual resistor values? For values, you need to start with the output voltage adjustment range and the max allowed output current.

Also, is Dz1 specified, or is it to be an element in the equations?

ak

6. ### RaiRodrigues

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Jun 25, 2018
Dz
Dz1 is not specified, i dont need actual resistor values, just the values that fit the equations for the desired V (out), and current

7. ### Harald KappModeratorModerator

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Nov 17, 2011
Work out the required Vz from the output voltage and the voltages across the transistors. Once you know Vz, use that to compute the other values. See my post #3.

8. ### WHONOES

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May 20, 2017
R4 is not needed. The information for setting the output voltage is on the diagram basically Vz (zener voltage) + Vbe (base emitter voltage) x the ratio of R1 and R2 (R1 / R2). VR1 is normally included to compensate for variation in the tolerance of the Zener voltage and to a lesser extent Vbe. It also allows for not being able to get the exact resistors you calculated.
As an aside, substituting R3 with a constant current and changing Tr3 from a Bjt to a Mosfet will greatly improve regulation.

9. ### AnalogKid

2,502
719
Jun 10, 2015
Disagree. By keeping the zener firmly biased into its operating region, it sharpens the regulator's response curve. Without it, the zener diode's "knee" adds to the TR2 Vbe knee to make for pretty soft regulation, increasing the load regulation error.

ak

Harald Kapp likes this.
10. ### Harald KappModeratorModerator

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Nov 17, 2011
I agree with AK.

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
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Jan 21, 2010
For a number of reasons, I would connect the base of TR2 to the top of R2 and connect the wiper of VR1 to one end of VR1.

However, I suspect that @RaiRodrigues may be supposed to use the circuit unchanged.

And do you also need to select appropriate transistors? And do you need to concern yourself with maximum dissipation and heatsinking (also relevant for the zener diode?

The amount you're allowed to ignore will determine how difficult the problem is.

Given that you say this is your first school project then I assume you can ignore almost everything.

I also assume that you've been given more information than you've given us, that you're expected to know more than you imply, or that this is a crap assignment. You're in high school, correct?