Author Topic: My Simple bench power supply  (Read 13066 times)

mariush

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Re: My Simple bench power supply
« Reply #15 on: August 21, 2013, 04:02:50 PM »
One minor observation I would like to add.

I only now noticed that you've added a switch to the output.  That's OK but now you can have the following scenario:

Set psu to 12v
Turn off power supply from output switch
Set psu to 5v
Turn on power supply from output switch.

When you turn off the power supply from the output switch, for a few ms, you'll still have 12v... if the 10uF cap. doesn't discharge through the D2 diode.. I'm not sure.
By itself, the capacitor would probably self discharge within minutes, if not less than a minute.
Still, it wouldn't be a bad idea to put a high value resistor in parallel with it to have it discharge within a few of seconds... try for example with a 4.7k-10k resistor... put the multimeter on the output, flip the switch to power off and see how fast the voltage goes down. If the voltage goes down too fast or you notice the output voltage wobble during operation, increase the resistor value.

I would say if you're worried about the charge on the large 3300uF capacitor, you should put a discharge resistor on that as well - but if you don't plan to touch the bottom of the pcb then just leave it be.  Same values, around 4-10k should be high enough to slowly discharge.





ttyz

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Re: My Simple bench power supply
« Reply #16 on: August 22, 2013, 12:57:57 PM »
mariush, Thanks for mentioning this, I will check if this will be an issue when I get all parts I need.
Cheers,
Evgeny

ttyz

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Re: My Simple bench power supply
« Reply #17 on: August 29, 2013, 02:29:18 PM »
Hello again,

I am a bit stuck now with the transformer selection:

There is an ordinary one (don't know how this type is called) that is rated for 2A.


And a toroidal that is rated for 1.5A.


Both of them have double 12V output.

I assume the toroidal should be better. But this leads me to some current control circuit, because the regulator might be overloaded with even 12V input voltage and 1.5A maximum current.
Cheers,
Evgeny

mariush

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Re: My Simple bench power supply
« Reply #18 on: August 29, 2013, 08:19:50 PM »
Use whichever you want.
If you think you're going to need more than 1A of current, use the first type.

The toroidal is better mostly if you use it in audio applications. They're also preferred for other purposes, including adjustable linear power supplies, but for a simple power supply like yours, it won't make a difference. 
The main benefit is smaller size and the fact that magnetic field is contained better inside the transformer which helps with some things.

see http://www.raftabtronics.com/TECHNOLOGY/ElectromagneticBasics/ToroidalTransformerBasics/tabid/112/Default.aspx for more detail...

dr_p

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Re: My Simple bench power supply
« Reply #19 on: August 30, 2013, 03:35:34 AM »
Hello again,

I am a bit stuck now with the transformer selection:
...
But this leads me to some current control circuit, because the regulator might be overloaded with even 12V input voltage and 1.5A maximum current.

LM78XX and LM317 both have Internal Thermal Overload Protection and Internal Short-Circuit Current Limiting. So basically it's a lot harder to kill them. However, the project that you are powering will possibly be damaged if something in that project goes wrong and tries to pull 1.5A out of your power supply. Having a adjustable current limit can save your precious electronics if used properly.

But it's a bit more complex for a beginner, so I say go for a simple, non limited supply at first. You are on a learning curve, so you'll inevitably make some other mistakes. You can then use your experience from this build and other projects to build a better power supply, or maybe buy one ready built.

IMHO, it's better to have a DIY cheap ass LM317 power supply than no power supply because you're trying to build a better one but it doesn't work.

Any type of 12V transformer will do, go for the cheaper, smaller one.

Also, as a side note, I think it's worth noting that you will not always get the full 1.5A from the LM317.
The LM317 Datasheet says on page 3:



So this means you typically get 2.2A (minimum 1.5A) when input-output voltage differential is less than 15V and the maximum power dissipation is not reached and at 25 degrees C. That's not gonna happen too often...
From my experience, using a small heatsink, you get about 200-300mA at 1.25V, around 1A at 5V and 2A at 12V. So just be aware that this will most likely happen.

ttyz

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Re: My Simple bench power supply
« Reply #20 on: August 30, 2013, 05:15:44 AM »
Thank you marius, dr_p

I think then I will use the toroidal one, as it seems to be smaller.

Do I need any kind of protection for the transformer as it is rated for 1.5A? Can I overload it somehow?
« Last Edit: August 30, 2013, 08:42:36 AM by ttyz »
Cheers,
Evgeny

dr_p

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Re: My Simple bench power supply
« Reply #21 on: August 30, 2013, 06:57:10 AM »
To be safe, I would add a 3A fuse right after the secondary of the transformer and a 1A fuse on the mains primary. Some people only fuse the primary, considering that a short-circuit on the secondary would increase primary current, thus pop the fuse. I don't know... I'm not trusting that, so I prefer to add a fuse on each winding of the transformer.

ttyz

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Re: My Simple bench power supply
« Reply #22 on: August 30, 2013, 08:45:19 AM »
Yes, it can never be over protected I think. Especially for such a beginner like me.

Just wanted to ask if it is ok to use self-ressetable fuses or better to use normal ones?
Cheers,
Evgeny

ttyz

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Re: My Simple bench power supply
« Reply #23 on: October 31, 2013, 02:37:06 PM »
Hello everybody,
 
I have been collecting all the bits and pieces for my project, and ran into a problem, while searching for LM7812 (Did not expect this to be a problem, but you never know when russian stores get you  :) ), but I've found an alternative LM2576, and I really liked it, especially the adjustable version, it seems to be much better than LM317 in terms of efficiency and heat dissipation, so I might even do not need this complicated windings switching circuit and have 30V/3A power supply, so for me it sounds like a win  :)
 
The only problem I now have is an inductor. It seems impossible to get the right inductance and current rating at once. I noticed that it seems not very hard to make one, there are plenty of online calculators, but I did not find any good tutorial on this. So if somebody knows a good one,  I would really appreciate the help. I understand the theory, but I need some practical information. The only other option is to get it on eBay, but then it take up to 2 month to get it here, and I might be quite late for the competition.
 
After this I will still need a decent transformer. Guy in the store ensured me that they will have some good toroidal ones in two weeks, so I really hope on that :)
Cheers,
Evgeny

dr_p

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Re: My Simple bench power supply
« Reply #24 on: November 01, 2013, 10:33:59 AM »
You must be joking! Surely you can source a 7812 somewhere in Russia. Actually, you must have like 10-20 shops that deliver stuff(tme.eu, ru.farnell.com etc). And they have inductors too. I understand that paying 20$ to ship a 0.30$ regulator is hard, but bulk up the order and get a lot of the stuff you need. Or you can ask the guy at the shop to order some for you. Then 3-4-5 days later you go get them.

Switch mode is great, but maybe it's easier to build a linear one, first. I mean there are a little more complicated. You try to run away from switching the windings and then you choose a switch-mode regulator that you know nothing about, lack the inductor and want to buid it yourself...
It may work and it will be awesome if it does, but I strongly doubt that you can pull it off, since you well... lack the experience.

IMHO, it's better to have a lesser perfect PSU, but a working one. I would advise beginers to build a simple 15V @1A, linear, maybe current limited PSU. When you're done, you not only have a PSU, you have experience in PCB making, fiting parts inside a box, making a front panel, making knobs not stick out too much as to show the nut holding them. Little thing like that, that will serve you very well in the future.

NOW, this is for the contest, so maybe you're better off in sticking to the hard route, give all your best and hope for success, but consider that you might bite more than you can chew.

ttyz

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Re: My Simple bench power supply
« Reply #25 on: November 01, 2013, 01:30:40 PM »
Not joking, I was shoked myself. I suppose there are places where I can get those, but I don't know much yet :) Thanks for the links, shipping does cost quite a lot but they promise a 8-day delivery, so I should give it a try sometime. I asked at the shop and they said that they can get any part on demand in 4-7 weeks if I order 10 or more of each. This does not sound like a great deal to me :)

I thoght that switch mode it might be not as easy as it seemed :) It was not really an attempt to build a petter PSU but to build any PSU without having to wait month doing nothing :) But I guess you are right.

Thank you for the advice.
Cheers,
Evgeny

mariush

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Re: My Simple bench power supply
« Reply #26 on: November 01, 2013, 02:55:51 PM »
Go to the store and ask for an ADJUSTABLE linear regulator, whatever they have. You only need two resistors to set the output voltage on most such linear regulators.

If you still have no luck... PM me with your address and I'll mail you something on Monday which should get to you within a week... .
I don't know if i have 7812 but I should have an adjustable LM317 (1-2A) or at least a LM1117 (0.8-1A max).  Maybe not LM, but 1117 made by other company (it's a generic part like a 7805 or 7812)


Switching regulators are more complex and don't have the good thing about linear regulators, low ripple and simplicity.

ttyz

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Re: My Simple bench power supply
« Reply #27 on: November 01, 2013, 03:00:45 PM »
mariush That's so stupid of me, I already have a spare LM317 so I can use it as a 12V or 5V reference, did not realize that. Thank you very much, I think that solves all my problems :)
Cheers,
Evgeny