Re: power supply design
- From: "Abstract Dissonance" <Abstract.Dissonance.hotmail.com>
- Date: Thu, 5 Jan 2006 05:04:03 -0600
"Bob Monsen" <rcsurname@xxxxxxxxxxx> wrote in message
news:pan.2006.01.05.05.59.05.345822@xxxxxxxxxxxxxx
> On Wed, 04 Jan 2006 19:12:46 -0600, Abstract Dissonance wrote:
>
>> I'm going to build a power supply for my projects and I was wondering a
>> few
>> things. One thing is simply does it matter if the filter cap is to large?
>> Most power supply circuits I've seen use around 2200 to 4700uF's and I
>> was
>> wonder that it couldn't hurt if it was larger as it should "filter" it
>> even
>> better? Obviously if its to large it would take longer to charge but
>> after
>> that it should work fine? I'm not talking a drastic increase in the
>> capacitance but maybe 2-10 times as much?
>>
>
> The point of the filter cap is to prevent the supply from drooping below
> the point where the regulator can no longer regulate. Now, with a typical
> diode bridge scheme after a transformer, what happens is that the cap gets
> charged up to the peak voltage - 2x the diode drop twice a cycle. So, if
> you want a maximum of I amps, and the peak is Vp, and the minimum voltage
> you can have before your regulator drops out is Vmin, then an
> approximation is given by
>
ok, so I would need to figure out what the regulator can do then get a cap
that can handle it and maybe just double it just to be sure?
> I = C * (Vp - Vmin - 2*Vf) * 2 * AC_frequency.
>
> So,
>
> C = I / (Vp - Vmin - 1.4) / 2 / AC_frequency
>
> As an example, if you want a 10 amp max supply, and your transformer
> gives you 21.2V peak voltage, and you want a regulated 12V, using a
> regulator with a 3V dropout, then the smallest C you can use is 10 / 4.8 /
> 2 / 60 = 17.36mF
>
> You can do the math for your situation.
>
ok.
> Using a cap that is bigger than you need is not really an issue, except
> that it'll store more energy (make sure you use a bleed resistor of like
> 100k to keep it from storing power over long periods). Also, the cost of
> caps goes up quicker than the rating.
>
yeah but wouldn't this bleed resistor be wasting power? Maybe just have a
method that when the power switch is shut down it would discharge the cap to
ground through a resistor? or is there some other saftey reason not to have
such a large cap... say if my fuse is before the cap there is a short then
that fuse won't do much good?
>> Another thing is about regulation and overvoltage protection. Can anyone
>> recoment some sites that discuss then for use in power supplies? I've
>> seen several schematics of basic power supplies and it seems pretty
>> simple but I'd like to know a little more about it just to be sure.
>> (i.e., is simply a zener diode good enough for the overvoltage
>> protection?) what about other cheap ways to make the power supply more
>> safe?
>
> A zener isn't really adequate unless you have a tiny supply, because if
> your supply can provide a large amount of current, then the power
> dissipated by the zener is Vz * I. If I is 10, for example, and Vz is 12,
> then you need a zener that can dissipate 120W until the fuse blows. If it
> fails before your fuse blows, your circuit is toast.
>
I found a schematic that talks about this... he uses a thyresistor or
something to prevent this sorta thing. I'll have to look into it more.
> A better way is to use what is called a 'crowbar' circuit, which consists
> of a device called an SCR, and a zener reference. When the zener conducts,
> the SCR turns on, and it CAN handle the current until the fuse blows.
> There are other schemes as well, search for "crowbar" and "SCR".
>
yeah, I think that was the circuit he used for this.
> If your regulator is meant to be adjustable, then you need to figure out
> another way to limit current. Often, regulators do this for you, using a
> scheme called 'foldback' current limiting, which just means that if you
> short the output, it won't give you the full current you've designed, but
> will 'fold back' the amount of current to a far smaller value.
>
yeah, that would be nice too. I'll have to look into it.
> As far as regulators go, you can get one that can handle up to an amp at
> radio shack. That isn't too bad, actually. It is named an LM317, and can
> regulate voltages from about 1.5V up to 34V, using a few resistors. There
> are also fixed regulators, like the 7805, which regulates 5V with no
> external components (well, maybe some capacitors to prevent oscillations;
> look at the package).
I've seen this in a lot of circuits I've seen. I'll probably end up using it
once I figure out everything I will end up doing.
> If you are looking for a general hobby supply, however, I'd buy one. you
> can get them that have adjustable voltage, and also adjustable current,
> which is often useful. They often have a display of the voltage and
> current, and various other features. I have one of these:
>
> http://store.yahoo.com/webtronics/00addcreposu.html
>
> I like it, but are various issues with it; the pots they use are fairly
> cheesy, and tend to be hard to adjust. Also, they don't have 'kelvin'
> leads, meaning that the regulation happens at the supply, and not at your
> circuit. Better supplies will allow you to have 4 wires, two for the
> current, and two for the regulation. Not doing this causes issues with
> higher current applications, because resistance in your wires drop some
> voltage.
>
> You can also occasionally pick up nice bench supplies on ebay.
>
yeah, I was going to buy when then they seemed sorta expensive. I'm not sure
why they are so expensive though and thought that I might as well just build
one as I've always kinda wanted to do this anyways and it would be a good
learning experience(and then I could customize it even more like put a
signal generator on it too and have some other stuff).
The main issue I kinda have is that I've seen a computer power supply that
has all kinds of crap in it and it kinda scares me cause I figure that all
that stuff is needed to make it safe... I don't want to make a power supply
thats going to kill me or ruin my circuits. But I'd rather build one if I
can make it 90% of one that I could buy for 50$.
Basicaly I want something like +15 -15 variable down to +1 and - 1 (maybe in
steps of like 15, 12, 9, 5, 3, 2, 1 or something like that).
And its not so much that I can't build one its just I don't know if I can
build a safe one and I haven't found anything about making a good and safe
power supply(all the technical details).
> --
> Regards,
> Bob Monsen
>
> "I am turned into a sort of machine for observing facts & grinding out
> conclusions"
> -- Charles Darwin
Thanks,
AD
.
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