Author Topic: Is building a power supply feasible for a beginner?  (Read 6720 times)

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Offline albert22

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Re: Is building a power supply feasible for a beginner?
« Reply #25 on: September 28, 2016, 12:51:09 am »
Building a power supply is a good starting project. And it can go as complicated as you want. It depends on your requirements, that is, what you intend to do with it. My advice is to start with a linear supply with a small transformer and aim for a 0-20V 1Amp or less. Dont be too afraid of connecting to mains, just wire the primary of the transformer thru a double pole switch and a fuse in series. ISOLATE everything so there is no possibility of any accidental contact, short or leak into the low voltage side. Double check everything BEFORE connecting to mains. And just work on the secondary side of the transformer.
Start with a simple linear regulator that you are able to understand and start experimenting.
 

Offline eventhorizon

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Re: Is building a power supply feasible for a beginner?
« Reply #26 on: September 28, 2016, 01:22:53 am »
I think we lost OP on this and never meant to hijack someones thread but hey I got some good information here.  I did notice that when adjusting the load the voltage out of the 317 would change and thought it was something in my wiring but it makes since on what Kleinstein said.  I am looking at the information on the lm723 and looks a little complicated at first but thats what makes it fun so I need to do a bit more reading to see how the voltage limit and the current limit works and get a better understanding.

 
 

Offline setq

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Re: Is building a power supply feasible for a beginner?
« Reply #27 on: September 28, 2016, 01:34:09 am »
LM317 drops out way early if you start to load it, if it gets hot, if it's Tuesday, if the planets are aligned in a certain way.

Voltage limit is a simple feedback loop i.e. it compares the output to a resistive divider attached to the built in voltage reference.

The current limit is set by Ilimit = 0.6 / Rs. So 100mA = 6 ohms.

If you need MORE POWAH then whack a 2n3055 on the output. It'll get rid of the additional voltage drop error automatically.

And that's it. All much more elegantly described in the datasheet.

LM723 is a power supply construction kit rather than a regulator IC. I use one to check meters against a reference as well, although I'll probably be shot for it.

However if you let all hell loose through it, it'll blow up. But it's cheaper than an LM317 so meh.
« Last Edit: September 28, 2016, 01:36:14 am by setq »
 

Offline mgysgthath

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Re: Is building a power supply feasible for a beginner?
« Reply #28 on: September 28, 2016, 03:08:31 am »
I think we lost OP on this and never meant to hijack someones thread but hey I got some good information here.  I did notice that when adjusting the load the voltage out of the 317 would change and thought it was something in my wiring but it makes since on what Kleinstein said.  I am looking at the information on the lm723 and looks a little complicated at first but thats what makes it fun so I need to do a bit more reading to see how the voltage limit and the current limit works and get a better understanding.

 

Oh I'm still here.  I just run a small computer store as a one man operation and I don't have much free time often.  As far as losing me, the advice is a bit daunting, feel pulled in ten directions.  However the general consensus seems to be yes I should try to build one or more, and to avoid 317s.  Fair enough.

Some further info and questions.. I don't have a particular use in mind for my power supply yet, but I know that I should have one and will find uses for it.  Personally I'd like more headroom than 1A, but as a first project I can find uses for =<1A.    I could use it for anything from a few milliwatt laser diode up to a larger device, say a 72W led light strip, or high powered LEDs or something.  I don't have a project in mind, I just don't want to limit myself.  I've seen some chinese up to 30V dc variable bench supplies under a hundred bucks.  And while I wouldn't ever think of it as a permanent solution or perhaps even a good one, I'd like to make something maybe along the lines of that if it's viable.

I have access to a great deal of PC switching power supplies, ranging in quality from junk to 88+ efficient or sometimes better.  Obviously these things can handle a great deal more amperage than 1A @ 12v, but sometimes you might want 24V or 30V etc, and from what people have said these are pretty 'dirty' power.  I also have a lot of laptop supplies around, but these are of course 18.5-19.5V generally, i even have a few bigger ones able to output say 6-8Amps.  But Really where to go from there I'm not sure.

I thank you all for the advice, it's given me a lot to read about.  I'll hopefully have some more informed questions before too long.
 

Offline tautech

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Re: Is building a power supply feasible for a beginner?
« Reply #29 on: September 28, 2016, 05:08:59 am »
Many of us can chuck together a 317 supply in minutes and despite comments to the negative for simple needs it's perfectly fine. Everything else is just more complex. Period.
But yes there are things you need be aware of, primarily dissipation.

If you need more current and still want to keep it simple the LM338 is another alternative. My first decent PSU uses one of these for a 5A supply. Again the datasheet tells you pretty much all you need to know.

Or you could go for something like these:
https://www.eevblog.com/forum/testgear/cheap-variable-switching-power-supply/

I've sold quite a few of these and they've been pretty good. Sure they are a SMPS and there'll always be some switching noise but appropriate local bulk capacitance can easily manage it.


Whatever you decide there'll be a day you wish for better and multiple outlets......
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Offline boffin

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Re: Is building a power supply feasible for a beginner?
« Reply #30 on: September 28, 2016, 05:31:07 am »
If you have a look at this post, you can see a little module I built into an old unregulated 12v supply I picked up for next to nothing at a ham swap meet.

https://www.eevblog.com/forum/chat/what-did-you-buy-today-post-your-latest-purchase!/msg980251/#msg980251

Fuse/Switch -> Transformer -> Rectifier -> 1000uF cap -> Module

The transformer was only 12v, so it only goes up to 12v on the output, but it created a nice small, simple finished product.  If I ever put significant current through it I might add an old computer fan (handily also 12v)


« Last Edit: September 28, 2016, 05:33:13 am by boffin »
 

Offline Kleinstein

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Re: Is building a power supply feasible for a beginner?
« Reply #31 on: September 28, 2016, 06:05:20 am »
A laptop supply could be a reasonable starting point. With a 18-19 V input you might get something like 15 to 17 V out after linear regulation. At this voltage a single power transistor could handle 2-3 amps. That laptop supplies are not that clean either and those without a PE connection might give capacitive leakage, that might be a problem in some applications.

The cheap Chinese supplies are generally not that bad in design, though a few are really bad and maybe even unsafe. But generally a 3 A supply is good only up to about 2 A. They just squeeze out too much from the circuits to make them cheap or better looking. So loaded to the max / worst case they tend to overheat and may break down early.
 

Offline eventhorizon

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Re: Is building a power supply feasible for a beginner?
« Reply #32 on: September 28, 2016, 06:18:04 am »
So I remember seeing a video where Dave shows that his laptop when connected to the power supply was mains earth grounded so im not sure if I want that in a power supply and being that various wall warts are cheap with different power options I could always go with say a 24v and check it for isolation.  My only thing is that until I am sure with myself and abilities I am not touching anything I make directly to the wall.  And im sure explaining why I am across the room with my beard on fire and the lights out to the wife (if I was still alive) would not be an easy task.
 

Online Gyro

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Re: Is building a power supply feasible for a beginner?
« Reply #33 on: September 28, 2016, 07:00:55 pm »
You'll probably find this thread helpful. Particularly the link in reply #5.  :)

https://www.eevblog.com/forum/projects/lm723-based-linear-power-supply/
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline george.b

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Re: Is building a power supply feasible for a beginner?
« Reply #34 on: September 29, 2016, 01:25:02 pm »
Learning is the important bit. Agreed.

I fixed the stickman above who dropped more than 12W off his LM317



Hahaha, that made me chuckle. Cheers!  ;D
 


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