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Linear Power Supply Help!!!!

ChanceAcidRapper

Apr 7, 2016
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Hello all,

I am currently in the midst of building a power supply for a school project, but I am stumped. The requirements- +12v -12v +5v +Var and -Var (7812, 7912, 7805, 317, and 337). Now when it comes to setting up each individual regulator, I got it; however, I cannot for the life of me figure out how to run all lines together on one PCB. I am slightly confused on what I can run off the main lines of the bridge rectifier to each individual regulator, on what lines I can share capacitors to lower parts count but maintain minimal noise, and so on and so forth. If anyone can shoot me some pointers it would be much appreciated! As I stated, I understand how to run each individual regulator (or pairs,) but cannot figure out how to run all 5 together on the same board!

Thanks you for any and all help!
 

Harald Kapp

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Nov 17, 2011
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The essence of your project is:
A power source (transformer, rectifier) with at least 1 positive and 1 negative raw output voltage. 5 separate regulators (3 pos, 2 neg.). They all need to share a common ground (0V).

The detailed setup depends mainly on the power source:
  • which output voltages and configurations are available from the transformer (e.g. do you have separate windings for the 5V output or are you going to take the 5V from the 12V rail)?
  • what current requirements do you have for the several outputs?
 

ChanceAcidRapper

Apr 7, 2016
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Apr 7, 2016
Messages
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The essence of your project is:
A power source (transformer, rectifier) with at least 1 positive and 1 negative raw output voltage. 5 separate regulators (3 pos, 2 neg.). They all need to share a common ground (0V).

The detailed setup depends mainly on the power source:
  • which output voltages and configurations are available from the transformer (e.g. do you have separate windings for the 5V output or are you going to take the 5V from the 12V rail)?
  • what current requirements do you have for the several outputs?
Thank you for the quick reply! Yes, a 115V transformer with a bridge rectifier with a positive and negative feed. They do need to share a ground. I do not have current requirements for the project. The test goes for unloaded and loaded for each output (both 12s, both Var, and the 5) along with ripple in mV (unaware of what the test load will be). I am still completely stumped.
 

Harald Kapp

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This is how you do it:
upload_2016-4-7_8-32-42.png

The variable regulator(s) are set up the same way.

Note that the 5V regulators will dissipate a lot of power as the voltage drop across the 7805/7905 is ~10V (power ~ 10V*Iout)!
 

ChanceAcidRapper

Apr 7, 2016
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This is how you do it:
View attachment 26135

The variable regulator(s) are set up the same way.

Note that the 5V regulators will dissipate a lot of power as the voltage drop across the 7805/7905 is ~10V (power ~ 10V*Iout)!
So essentially, I can tap off of the +15 for my +5, and since I do not need the 7905, I can come off of my rectifier lines with my 317 and 337 as well?
 

dorke

Jun 20, 2015
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So essentially, I can tap off of the +15 for my +5, and since I do not need the 7905, I can come off of my rectifier lines with my 317 and 337 as well?

You can as Harald showed you but the 7805 will disipate a lot of heat.
You should use the 317 and 337 in the same way as the others,
it has nothing to do with the use of the 7905.
 

ChanceAcidRapper

Apr 7, 2016
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You can as Harald showed you but the 7805 will disipate a lot of heat.
You should use the 317 and 337 in the same way as the others,
it has nothing to do with the use of the 7905.
As in, use the 317 and 337 as the 7812 and 7912 is used? Off of the bridge rectifier?
 

dorke

Jun 20, 2015
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Yes,
more accurately put: off the unregulated voltages.
 

Herschel Peeler

Feb 21, 2016
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Feb 21, 2016
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401
Hello all,

I am currently in the midst of building a power supply for a school project, but I am stumped. The requirements- +12v -12v +5v +Var and -Var (7812, 7912, 7805, 317, and 337). Now when it comes to setting up each individual regulator, I got it; however, I cannot for the life of me figure out how to run all lines together on one PCB. I am slightly confused on what I can run off the main lines of the bridge rectifier to each individual regulator, on what lines I can share capacitors to lower parts count but maintain minimal noise, and so on and so forth. If anyone can shoot me some pointers it would be much appreciated! As I stated, I understand how to run each individual regulator (or pairs,) but cannot figure out how to run all 5 together on the same board!

Thanks you for any and all help!

How much current do you need? The transformer and rectifier must be able to deliver these needs. LM317 and 337 can be added to the previous schematic easily.
Fused? Circuit breaker? Electronic fuse? Something is suggested especially for a classroom.
A single supply that does it all or various smaller supplies? The problem with one big one is if it dies all exercises are off. A lot of small cheap ones might be better.
What do the exercises actually call for? Low voltages at a fraction of an amp? "Wall warts" are pretty cheap.
For my use I have collected a bunch of adapters. I add a voltage regulator with current limiting.
Designs available if you are interested in this approach.
 
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