AC to DC converter

[daviddoria]

I see lots and lots of stuff in a computer power supply. i can't imagine it is that complicated to convert ac to dc. anyone have a simple diagram of this? i bet i'll have to buy a couple of transformers too wont I?

pls let me know

thanks
david

 

[Sebi]

If You need only few amper and will use a heavy 50Hz trafo it`s simply.
Write here what voltage and amperes You need, and i draw for You a sematic.

 

[kinjalgp]

There are ways to convert AC into DC and each method has its own disadvantages and disadvantages. A computer requires very high quality ripple free DC source having high current capacity and should also be compact enough which can only be obtained using a Swith Mode Power Supply (SMPS).
If you try to use transformer, rectifier and all that stuff in place of SMPS, just assume how big a 25A transformer would be? And also don't forget about the ripple and noise stuff associated.

So now which one would you choose?

 

[bogdanfirst]

you are very pesimist. look at what you wrote, K, only disadvantages....

 

[Gene]

Starting at the bottom, a single diode will, in theory, convert AC to DC. Half wave, ripple, noise...in fact, I'm not sure what you could do with this but it is (somewhat) DC.

Next, you might want to change from a diode to a bridge (or four doides). Now you have a full wave with ripple. This is starting to look like pure DC.

OK. So add a hefty electrolytic capacitor to filter ripple across the output and, yep, you got DC. This is traditional.

For the smallest package and the smoothest DC, computer manufacturers use a switching power supply (an entirely different philosophy). Yes - there are lots of components in there but they are necessary for the intended purpose - lots of clean DC power in a small box.

 

[Dean Huster]

The comparisons.

The traditional power supply (some call them "linear" supplies) are the old-time type and the most common. You'll usually see them in audio equipment and nearly any "wall-wart" battery eliminator. They consist basically of a power transformer to convert the line voltage up or down (or both) to suit the need, a rectifier, a filter and, optionally, a voltage regulator. The advantages of these types of supplies are simplicity, low maintanance, high reliability and low radiated RF. The disadvantages are that they're heavier, more costly for high-current versions, less efficient and take up more space.

The switch mode power supply (SMPS or switcher) is used in most computers and modern televisions. Most high-end oscilloscopes made by Tektronix since 1972 use switchers and low or high end scopes made by them since 1990 use switchers. They consist of a rectifier/filter, an "oscillator" (the switcher) that operates at a frequency of 10KHz to 1MHz), power transformer, rectifier and filter. Because of the high switching frequency, a much smaller power transformer is required since higher frequencies don't need the large cores needed by the 60Hz "linear" supplies. Also, filter components are smaller in value because the RC values for 10KHz and up are smaller than those for 60Hz. Because of the smaller components and the high switching frequency, the advantages are smaller size, lighter weight and much higher efficiency, sometimes as high as 95% compared to 60% or so for a "linear" supply. Switchers can be made with precision-wound transformers and sampling of output voltage so that it can precisely regulate the output voltage. Theyu don't often require voltage regulators at the output. This comes with some disadvantages, though. A switcher has all kinds of components sinces it's a major circuit in itself. With that is the attendant reliability problems associated with all those complex parts. Also, a switcher produces a lot of radio frequency noise that must be filtered out so that it doesn't interfere with radio and television reception. It's for this reason that you won't see switchers in sensitive radio receivers such as ham radio or short wave listening receivers.

Dean

 

[kinjalgp]

you are very pesimist. look at what you wrote, K, only disadvantages....

You being an optimist can you list out some advantages of conventional diodes and transformer based power supply over SMPS?
I know only one that is simplicity of construction.

 

[mechie]

Advantages of linear power supplies...

En garde !!!

Easier to fault find
(usually) safer to build
Availability of components is much better (especially wound components) and therefore ...
Usually cheaper (for the sort of things I do anyway!)
Easier to design
Easier to modify
Less RF noise

As an aside, most people seem to assume that all SMPSUs start with rectifying the incoming mains then chopping the result up at some kHz to drive a physically small transformer (primary switching), there are important examples out there of a conventional mains transformer doing the step-down and then an SMPSU doing the regulation (secondary switching). These can still have advantages in that a single secondary winding (cheap transformer) will power a secondary SMPSU which, in turn, generates multiple DC power rails.

Even if you lump the latter together and call them all DC-DC converters they are still swiched-mode power supplies!