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I'm trying to design an ac to dc conveter (230V ac to 12V dc). Haven't decided on the current yet. It's likely to be around 2A. I want the circuit to be compact and hence I think it is best to follow what's found in ordinary wall adapters. I haven't noticed wall adapter circuits much, but as far as I can remember, it is not an smps. I don't have a broken one lying around to take a look. Can someone help me with it?

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  • \$\begingroup\$ It is really much easier to buy AC-DC converters or use a supply ("charger") from an old laptop. Almost all laptop supplies are around 20V. So if you design your thing so that it works from, say 18-25V Vin, and puts out 12V, you can run it from any old laptop supply. You could also find a 12V AC-DC converter if you don't want to use a laptop supply. \$\endgroup\$ – mkeith Jul 24 '16 at 16:49
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They're nearly all SMPS (switched mode power supply) nowadays. Advantage is efficiency, weight and cost - not necessarily in that order.

12 V is a standard voltage for many computer peripherals - some monitors, routers, etc. - so you should find a surplus one easily. SMPS building is not for the beginner as debugging is dangerous as the switching circuits are at mains potential.

Another alternative is the wide variety of SMPS modules readily available from all the major suppliers.

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Figure 1. SMPS block layout. Source: Wikipedia Switched mode power supply.

If you are interested in learning how they operate I suggest that you open some up (with power off, of course), try and find the switcher chip, get its part number and search for the datasheet. Very often you will find that the SMPS manufacturer has used the application note design with small modifications rather than risk designing from scratch. SMPS board layouts can be important as high frequencies are involved and if the manufacturer has a suggested layout this too will be adopted.

Remember the high voltage capacitors will be charged to \$ \sqrt {2} V_{MAINS} \$ and will take some time to discharge.

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    \$\begingroup\$ Isolation of high voltage from low voltage is also a major concern. And why I personally would buy rather than design one, unless I was specifically trying to learn how to design AC-DC converters. \$\endgroup\$ – mkeith Jul 24 '16 at 16:51
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Older wall-wart supplies were simple transformer-rectifier-filter capacitor, often without any voltage regulator.

Modern wall-warts are often regulated switch-mode supplies. (much lighter than the old transformer-based supplies)

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