Re: power supply design
- From: "Tim Shoppa" <shoppa@xxxxxxxxxxxxxxxxx>
- Date: 5 Jan 2006 03:36:59 -0800
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?
Everyone else has already said "it doesn't matter as long as it's big
enough"
but it certainly can matter. As you make the capacitor bigger, the
conduction time of the rectifier diodes gets smaller and smaller, and
while average current stays the same, the peak current goes up.
As the peak current goes up, not only do you have to worry about the
peak current rating of the rectifier diodes, but also the I2R losses
in the transformer. What this means is that if you use a 500mA
transformer
but you're only conducting a few percent of the time, you have to
greatly
derate the current capacity of the transformer and you might only be
able
to get 100mA DC current (or less) out of your supply.
Actually the conduction angle doesn't drop as fast as you might think
because the resistance of the transformer windings (and also the ESR
of the capacitor comes in) limits peak current for you. And as the
transformer heats up, the winding resistance goes higher and prevents
thermal runaway.
But for a typical hobbyist lash-up where you are not running at even
20% of the rating of the transformer and everything is in a office or
bench-top environment you probably will never notice.
> 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?)
After you get to currents above a few hundred mA the dynamic resistance
of the zener makes it ineffective, and you have to go to a SCR type of
crowbar.
> what about other cheap ways to make the power supply more safe?
Safe as in protecting the load, or safe as in not catching fire? A
properly sized AC fuse is always the first link in stopping it from
catching fire! Make the capacitor too big and you won't be able to
put in a properly sized AC fuse (getting back to your first question!)
Tim.
.
- References:
- power supply design
- From: Abstract Dissonance
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