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Simple 5V power supplys use a transformer and a diode bridge followed by a linear regulator like in the picture. You get these for about/under 1$ from china. I wanted to integrate such a circuit into an PCB, but I am not able to find an transformer as cheap as these 5 supplys (I don't want to solder on one of these crap PCBs). All transformers I found cost over 1$. So I wanted to ask if this transformer like component is really called transformer since I'm not an native english speaker and don't find anything relevant.

EDIT: cause in the comments high currents are often mention I wanted to say that I only need some mA for low Power µController. Also corrected the wrong circuit description (phone charger to simple power supply)

enter image description here

Fritz
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    "Most cellphone chargers use a transformer and a diode bridge followed by a linear regulator" - No they don't. They use switch-mode power supplies. – brhans Mar 29 '16 at 11:50
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    The drawing shows a transformer. There is an error in the circuit: the centre-tap of the transformer is connected to ground (GND) and so is the 5 V common. D1 and D3 will fail. Not worth $1. – Transistor Mar 29 '16 at 11:53
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    "You get these chargers and PCBs with circuits like these for about/under 1$ from china." No you can't, if the circuit was like the "classic" supply from your schematic, it could **never** be made for such a price. That is why they are switched-mode fly-back converters like brhans says. – Bimpelrekkie Mar 29 '16 at 11:55
  • @FakeMoustache They certainly can be made for that price, in quantity. – Spehro Pefhany Mar 29 '16 at 12:32
  • @SpehroPefhany I think you misunderstood, what I meant was: You could **not** make them for $1 **if** you used the **classical** design as in the schematic. – Bimpelrekkie Mar 29 '16 at 12:36
  • @FakeMoustache Agreed, not at 2A, but at 200mA-300mA, maybe 500mA, it's possible. A few watt transformer is about 35-45 cents. – Spehro Pefhany Mar 29 '16 at 12:48
  • @SpehroPefhany, but where to get these low current transformers? Didn't hat luck at aliexpress or mouser – Fritz Mar 29 '16 at 13:06
  • @Fritz Order at least $10K worth in one shipment, specify INCOE terms "FOR" and "no approvals required". – Spehro Pefhany Mar 29 '16 at 14:03

2 Answers2

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Yes, it's a transformer. No, that's not the right circuit. See How do the tiny AC->USB power supplies work? for details.

Basically, the transformer is run at much higher frequency than the 50 or 60 Hz power line. That allows the transformer to be much smaller, lighter, and cheaper for the same power capability.

Community
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Olin Lathrop
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The loathsome and dangerous pieces of c**p that you can buy for $1 from China are always going to be cheaper than what you can assemble from safe, reliable parts purchased through distribution chains in small quantities.

I've taken a couple samples apart- they were designed by people without any appreciation of value of life- clearances of a mm or so between the mains and output and conductive debris rattling around inside. No doubt the transformers are also made without proper pedigreed materials (for approvals, all the safety critical material specifications and sources have to be submitted to the safety agency, inspections take place, etc.)

If you want to make the circuit you show- 50/60Hz transformer and linear regulator, it will probably cost around $5-10 in smallish quantity and will be capable of maybe 300mA not 2A. If you need 2A, a switching supply is going to be the most practical- but it will cost a bit more, have many more components and will be a bit harder to design (see Power Integrations website for one-chip devices) and a lot harder to get safety and EMI approved.

If you want to avoid this, power your device from a proper approved external adapter from a major manufacturer, or suck it up and spend the money to get something safe internally. Few may care if your circuit doesn't work quite right, but a lot of attention will be drawn to you if it kills someone.

Spehro Pefhany
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