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How do cheap power-supplies work?

I was briefly entertaining the idea of making my own power supplies for some electronics projects.  Everything I see involves the basic two step process of: 1)  Use a transformer between the 110V mains to bring the voltage down to the level you want, 2)  Use circuitry to smooth out the current from AC to filtered DC. 

However, when I went to price transformers, I quickly discovered that to buy a power transformer rated to hook into 110V AC, the transformer alone would always cost significantly more than the cost of buying a full ready-made power supply of similar output.

You can't buy a transformer rated for 110V for under $20-30, but you can find thousands of cheap wall-warts (transformer plus AD/DC conversion circuitry, plus connectors)  for <<<$10.  Or, if you look at higher wattage units, you find similarly rated (for volts and amps) power transformers cost $30-100+ dollars more than full power supplies.

How are these cheap power supplies made for the prices they are sold at?  What am I missing here?

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gmoon5 years ago
Probably several factors at play here. One is, of course, economies of scale.

But I suspect if you really "mine" down deep at mouser or digikey (not an easy task, if your interest is merely casual), you'll find inexpensive step-down transformers of the "wall wart" type. A quick check of mouser confirms this--there are lots of small PTs in the $3-$4 range.

Of course, the higher the current rating, the more expensive the transformer becomes. Copper doesn't come cheap anymore.

Looking then at high-current power supplies, are you comparing modern switching supplies with traditional transformer-driven designs? Almost all new supplies use sophisticated circuitry and small inductors to replace big, expensive inductors (transformers). I've seen some high-current, regulated switching DC PS units sold so cheaply it's amazing...
SvdSinner (author)  gmoon5 years ago
are you comparing modern switching supplies with traditional transformer-driven designs? Almost all new supplies use sophisticated circuitry and small inductors to replace big, expensive inductors (transformers). I've seen some high-current, regulated switching DC PS units sold so cheaply it's amazing...

This is what I'm seeing.  However, I've had no luck finding any real information on this "new" way to make power-supplies.  Everything I find only relates to the old way to do it. 

I'm really interested in learning how they make these supplies without big dollar transformers.
gmoon5 years ago
Can't reply to your last msg, for some reason...

Checkout the wiki on switching power supplies, it's a good primer.

Basically, the reactance of inductors rises with the frequency. Switching mode supplies do use 'em, but operate at high frequencies, so the inductors can be much smaller and cheaper.

They are a couple orders of magnitude more complex than an old-school transformer/rectifier/filter circuit. As Steve says, I wouldn't try to build one, at least one with significant current handling.
Mains powered switchers are not suitable for amateurs to design - and even pros generally specialise in it. Transformer cost depends on VA rating, and quality. I can buy a suitable transformer for a 12W PSU for less than 10 USD in the UK at least, though, like you, these days we just buy in PSUs.
caitlinsdad5 years ago
What you are looking at is the difference between wholesale and retail. It costs more for a company to try to sell one part to an individual than to offload a big lot to another company. The major portion of the cost in producing a transformer is probably the pure copper wire that is used for the windings. The greater the output of the transformer, the more metal is used. Depending on where that is sourced from, it will make things seem really cheap. The quality of the components/circuitry design/manufacturing facilities/labor all factor into the pricing so that we can get an imported product cheaper than domestic.