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Linear power supply advice
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RogerThat:
Hi,

So, the winter is cold and I need something to play with which also heats me: Building a linear power supply  :-/O

This is my first venture into mains powered electronics so starting with something simple. I've been building electronics (up to 24v) for some years so I'm aware about the dangers playing with this stuff.

What I want to build is:
Mains powered linear power supply with fixed 5v and at least 4A. As high efficiency as possible with linear and ultra low noise. Cost comes low on the list....

I found this schematics which looks simple enough:
http://www.circuitstoday.com/regulated-dc-power-supply

I have a few questions:

How to dimension the capacitors C1 and C3?
Suggestions for diods in the bridge? I'm thinking the one with the lowest Vf, am I missing something?
What voltage should the transformer output give to be able to have a 5v output?(How do I calculate)

Thanks for any advice and education.


 
Kleinstein:
The circuit shown is about 1960s  technology. It is neither high efficiency nor very accurate or stable. Depending on the zener, with lots of luck it may be low noise, but with typical parts it can also be quite noisy.

For low noise linear regulation is much easier, but this limits the efficiency. So high efficiency and lowest noise are kind of conflicting requirements.

A beginners project should avoid going for high power, high current, as there is more magic smoke to escape, but not much more to learn. So the first test should usually better be low power like 0.2 or 0.5 A.  Scaling up to higher current can than be some once the small version works.

P.S. was there any winter this year ?
not1xor1:

--- Quote from: RogerThat on January 22, 2020, 04:52:02 pm ---I found this schematics which looks simple enough:
http://www.circuitstoday.com/regulated-dc-power-supply

--- End quote ---

forget it
-1) 2N3055 has really low gain. I'm unsure since I never used 2N3054, but I suspect that for 4A you'd probably need to add another BJT
-2) that circuit provides really poor line and load regulation, ripple rejection and NO current limit

As a beginner you'd better start with a lower power circuit. A LM317 based variable voltage PSU (e.g. 1.25-15V) would be easier to build and more useful.
RogerThat:
Thanks for the feedback. I actually live in southern France now but my parents tells me the winter in Sweden is mild this year. Down here we had a proper snow storm and right now it's hovering around 0, which is cold for being here.

Regarding the schematic I was not thinking of using the exact same components but just the layout. I found it nice and simple with just a zener as regulator. Is there a known good regulator design or do most designs use an off-the-shelf LDO(directly or controlling a transistor)?

But before the regulator I need to have a DC which is steadily above the output voltage. This is really the main confusion for me now. Say I want 6vDC (minimum) for feeding my regulator. How do I arrive there?

In my head I can't use a transformer which peaks at 6v beacuse my output will then be too low after rectifying and filtering. How high does it need to be(the peak)? Is there a rule of thumb for this?

What I gather I should use a Toroid transformer which have the closest output voltage(after rectifying and filtering) to what I actually want to use. Let the transformer do the main part of the voltage lowering and then let the regulator just fin-tune it down to the correct voltage.

Btw, 20w power supply can hardly be 'high power'? ;). I'm aiming for 5v 4A because it will also be useful in my lab when it's finished. I will put two USB ports on it (2A each) and a normal bana jack output. I've built a switched 12v 10w USB charger, so this part I know well.





andy3055:
Use something like this:

http://www.ti.com/lit/ds/symlink/lm338.pdf

You can size the transformer from the data given for the voltage differential.
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