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Variable Power supply circuit

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Nickhole

Sep 17, 2019
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Hello. May I ask if theres something wrong in this circuit. I already tried to rebuild it on breadboard 3times but got the same result.
Problem is on the postive cylce fixed voltage only. And in negative cycle. It varies but only a little. Thanks for the answers. First time building power supply.
 

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Harald Kapp

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if theres something wrong in this circuit.
Lots. Where is this circuit from?
The potentiometers R3 and R4 are useless as teh wiper is short circuited to the upper end of the potentiometer. What are thes pots supposed to do?
What are Q2 (and Q4 respectively) supposed to do? Looks like they should work as current limiters (Q2 turning on when the current through R1 is approx. 6 mA), but turning these transistors on does not draw current away from the base of Q1 (or Q3 respectively), thus the output voltage will not be decreased. Besides: 100 Ohm is way too much for a current sense resistor: it will dissipate power unnecessarily.

Problem is on the postive cylce fixed voltage only.
No wonder as R3 is without function, see above.
And in negative cycle. It varies but only a little.
Same.
First time building power supply.
Why not use a proven design as your first step?
 

Nickhole

Sep 17, 2019
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Lots. Where is this circuit from?
The potentiometers R3 and R4 are useless as teh wiper is short circuited to the upper end of the potentiometer. What are thes pots supposed to do?
What are Q2 (and Q4 respectively) supposed to do? Looks like they should work as current limiters (Q2 turning on when the current through R1 is approx. 6 mA), but turning these transistors on does not draw current away from the base of Q1 (or Q3 respectively), thus the output voltage will not be decreased. Besides: 100 Ohm is way too much for a current sense resistor: it will dissipate power unnecessarily.


No wonder as R3 is without function, see above.

Same.

Why not use a proven design as your first step?
Haha. Its a school project. We are not supposed to use regulators. Ive looked on variable power supply circuits on net without regulators but I havent found one. So i tried somthing like that and as you said its garbage. Hahaha. Well thanks for the answer. Appriciate you.
 

Harald Kapp

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Ive looked on variable power supply circuits on net without regulators but I havent found one.
Look for "linear regulator zener diode". Show us which circuit you select.
Tell us your parameters (input voltage range, output voltage range, current limit etc.) so we can check whether your selection matches these requirements (or can be made to match them).
 

Audioguru

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Why didn't your teacher teach you how to design it properly like this:
 

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Harald Kapp

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Please note I moved this to the homework section. So any help should be along our guidelines for homework, please.
 

Nickhole

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Look for "linear regulator zener diode". Show us which circuit you select.
Tell us your parameters (input voltage range, output voltage range, current limit etc.) so we can check whether your selection matches these requirements (or can be made to match them).
These are the requirements sir.

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Moderators note : adjusted the image and showed inline
 

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Harald Kapp

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You can easily find transistorized, zener diode based regulator circuits. Mostly for positive voltages, but it will be easy to create one for negative voltages using the same techniques.
I suggest you proceed in steps:
  • Create an unregulated input voltage for your regulator. You'll need more than +- 30 V as some voltaeg will be dropped within the regulator. You've done that part quite o.k. in your schematic diagram, but the filter capacitors are a bit on the small side. Look up which filter capacitor size is required for 4 A output current.
  • Create the positive adjustable regulator part. If you want to, start with the regulator only (without current limiter), then add the current limiting circuit later. Or use a circuit with current limit from the start.
  • Create the negative regulator part. This will involve mainly replacing NPN transistors by PNP transistors (and vice versa if required) and inverting polarities of polarized components (diodes, electrolytic capacitors...)
  • Calculate the required heat sink: the regulator will dissipate approx. 30 V × 4 A = 130 W for each polarity when set to a low output voltage! This will require a BIG heatsink - and powerful transistors. One for each polarity.
  • Develop the trigger circuit to turn the output voltage on (item #3 from the requirements list) and the LED indicator.
  • Build the circuit and a project box to house it.
If you have doubts, present your design steps and ask your questions here in this thread.
Note that it is our policy not to give complete solutions to homework. Rather we will guide you in finding your own solution by pointing out errors and hinting you into the right direction. This may not be what you expect from this forum, but we sincerely think that this method offers you more insight.
 

Nickhole

Sep 17, 2019
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You can easily find transistorized, zener diode based regulator circuits. Mostly for positive voltages, but it will be easy to create one for negative voltages using the same techniques.
I suggest you proceed in steps:
  • Create an unregulated input voltage for your regulator. You'll need more than +- 30 V as some voltaeg will be dropped within the regulator. You've done that part quite o.k. in your schematic diagram, but the filter capacitors are a bit on the small side. Look up which filter capacitor size is required for 4 A output current.
  • Create the positive adjustable regulator part. If you want to, start with the regulator only (without current limiter), then add the current limiting circuit later. Or use a circuit with current limit from the start.
  • Create the negative regulator part. This will involve mainly replacing NPN transistors by PNP transistors (and vice versa if required) and inverting polarities of polarized components (diodes, electrolytic capacitors...)
  • Calculate the required heat sink: the regulator will dissipate approx. 30 V × 4 A = 130 W for each polarity when set to a low output voltage! This will require a BIG heatsink - and powerful transistors. One for each polarity.
  • Develop the trigger circuit to turn the output voltage on (item #3 from the requirements list) and the LED indicator.
  • Build the circuit and a project box to house it.
If you have doubts, present your design steps and ask your questions here in this thread.
Note that it is our policy not to give complete solutions to homework. Rather we will guide you in finding your own solution by pointing out errors and hinting you into the right direction. This may not be what you expect from this forum, but we sincerely think that this method offers you more insight.
Thank you for this sir.
 

davenn

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old thread posted in my different people


thread closed before more necro-posting occurs
 
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