Feedback on a 3 outputs Regulated Power Supply Schematic and PCB Design

Thread Starter

sbusso

Joined Mar 6, 2019
11
Hi,

I m self learning electronics and this is my first PCB design, done with Kicad. I'd like to get feedbacks and comments from experienced people about the schematic and PCB design. This is a basic regulated power supply for a eurorack format (+5v +/- 12V). The power is coming from a 24VAC CT transformer. I could have found plenty of existing designs, but my goal here is to learn.

Thank you!
Stephane
 

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danadak

Joined Mar 10, 2018
3,570
You show no output cap on the 7805, is that intentional.

Also if your loads are large switching loads you might consider bulk caps,
polymer tants preferred, there as well. At least put the pads down to take
them on PCB.


Regards, Dana.
 

Picbuster

Joined Dec 2, 2013
982
Difficult to say however;
a: I do not understand why you limit the +12V by sucking +5V from it.
b: the capacitors (elco) at input are defined by the used current and total resistance from the circuit.
The ripple is a RC function related to frequency (bridge 2 x Freq)
I assume that you try to draw many amp's
but also a point is your inrush current @ 3 x 4700uF
c: d9/d10 are not really needed.
d:Can you layers handle that current ?
eq(2.5Amp's over 20 mm length need at leased 1mm track width @0,035 thickness

Picbuster
 

Thread Starter

sbusso

Joined Mar 6, 2019
11
Thanks for your feedbacks

@danadak missing cap on 5V output is not intentional and I need to add one. And yes I just saw I need to put the pads on the other side.

@Picbuster for a. I though it was easier to take it from regulated 12V, would it be better to take it before ? from input ? b. I will need to research more on this point, I would need (much) less than 1 amp. c. I took 7812/7912 datasheet recommandation, I will look at that too. d. I think so but yet I will check that point .

Good points to give some more homework :)
 

Picbuster

Joined Dec 2, 2013
982
Thanks for your feedbacks

@danadak missing cap on 5V output is not intentional and I need to add one. And yes I just saw I need to put the pads on the other side.

@Picbuster for a. I though it was easier to take it from regulated 12V, would it be better to take it before ? from input ? b. I will need to research more on this point, I would need (much) less than 1 amp. c. I took 7812/7912 datasheet recommandation, I will look at that too. d. I think so but yet I will check that point .

Good points to give some more homework :)
Use this simple calculator http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html
This will allow you to calculate inrush current.
Picbuster
 

dl324

Joined Mar 30, 2015
8,849
I'd like to get feedbacks and comments from experienced people about the schematic
The fixed loads you have on the regulators may be too low for them to regulate.

The part numbers you're using aren't standard (L7812). Expecting to see something like LM7812, 7812, LM78L12, or 78L12. The first two are 1A regulators, the last two are 100mA.

Since you have a center tapped transformer, you don't need a bridge rectifier and you'll get back one diode drop after the rectifier.

Dropout voltage is around 2V, so likely have more filter cap than you need. If you have 4700uF, you don't need the 220uF.
 

Thread Starter

sbusso

Joined Mar 6, 2019
11
thanks @dl324 your feedback

What fixed load are you referring to ?

The part number is the one I ve got from kicad and on the board I use the L7812CV, should I use L7812CV on schematic ?

How should I do without bridge, only 1 diode in each direction ?

Ok for the cap, I though it was required by the L7812CV.
 

dl324

Joined Mar 30, 2015
8,849
According to the TI datasheet, the regulator isn't guaranteed to regulate with less than a 5mA load:
upload_2019-3-10_9-35-41.png
The 7812/7912 have around 10mA, but the 5V regulator has no load.
The part number is the one I ve got from kicad and on the board I use the L7812CV, should I use L7812CV on schematic ?
I'd use 7812 or LM7812. The schematic editor might let you change it to a standard part number.
How should I do without bridge, only 1 diode in each direction ?
Scratch what I said; wasn't thinking clearly yet. It only applies to a single polarity supply.

Ok for the cap, I though it was required by the L7812CV.
A filter cap is required, but it's value depends on the transformer secondary voltage, the type of rectification, and the load.
 

Thread Starter

sbusso

Joined Mar 6, 2019
11
ok I understand better the different points.

So for the load on 7805 i should add a LED or if there is no load the IC just shutdown ?

I'd use 7812 or LM7812. The schematic editor might let you change it to a standard part number.
ok, I will change it.

A filter cap is required, but it's value depends on the transformer secondary voltage, the type of rectification, and the load.
I definitely need to read more on the capacitor role and behavior in the case of a power supply. I though the first big capacitors were there to smooth and act as reservoir, and the input capacitor to help in the frequency behavior of the IC output.
 

dl324

Joined Mar 30, 2015
8,849
So for the load on 7805 i should add a LED or if there is no load the IC just shutdown ?
Worse. It likely won't regulate.

There are design documents that give formulas for capacitor size, but some of it is common sense. If you were using an 18V secondary, you'd have more head room and could tolerate more ripple.
 

Thread Starter

sbusso

Joined Mar 6, 2019
11
There are design documents that give formulas for capacitor size, but some of it is common sense. If you were using an 18V secondary, you'd have more head room and could tolerate more ripple.
I guess everything is question of balance. More secondary input means more heat to dissipate, I will add a LED to it. Thanks
 

Thread Starter

sbusso

Joined Mar 6, 2019
11
So here is what I get now for capacitors, giving a load of 300mA:
- secondary peak is about 12V*sqrt(2) = 16.9V
- diodes drops about 1.4V
- IC drops another 2.5V

So i get about 1.168 V left for a 10ms half-period at 300mA, it is about 2568uF needed. are 2x2200uF / rails enough or should I ensure the required capacity per capacitor in case one is failing and then use 2x3300uF ?
 
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