SparkFun Forums

[kabram]

It used to be that AC (110V, 60hz) to DC meant a big heavy transformer. But I see products such as http://amzn.com/B0038HYPZS Does anyone know the circuit that they use? I'd like to incorporate something like that directly in what I'm trying to build. Any help appreciated.

[KeithB]

They are switching power supplies which greatly reduce the size of any transformer/inductor due to high switching frequency.

However, for hobbiest projects you can't beat the mains isolation of a standard iron core transformer. If you are not careful, a switcher can expose you directly to the AC line.

[Joeisi]

Saw this on HaD

[AndyC_772]

However, for hobbiest projects you can't beat the mains isolation of a standard iron core transformer. If you are not careful, a switcher can expose you directly to the AC line.

Eh? It's nothing to do with the size of the transformer or the core material, the isolation from the mains is provided by the insulation between the windings. The risk is just the temptation to cut costs on the flyback just because they're inherently smaller and cheaper to begin with.

[KeithB]

If he just looks for a typical switching power supply, it provides no isolation input to output, right?

[Joeisi]

No, not really. So builder beware. Wall voltage doesn't agree with microcontrollers much.

[AndyC_772]

Any power supply designed to plug into the mains is required to provide isolation between primary and secondary circuits (ie. between the mains supply and any low voltage electronics that it's designed to power). It's one of the most fundamental requirements of EN 60950 (and its international equivalents).

There are detailed requirements specified in the standard for what constitutes "isolation" in this context, including physical separation distances, dielectric strength requirements for insulators and so on. It's a much more stringent requirement than mere functional insulation, which may be what you get with very cheap, low quality units. These are the dangerous ones; a supply with weak insulation and/or inadequate spacing is liable to become dangerous in the event of a fault.

Here in Europe every piece of electronic equipment is required to carry the CE mark, which indicates compliance with various directives that govern electrical safety and other issues - though, of course, not every manufacturer is honest or diligent in using the mark correctly.

Avoid the very cheapest, no-name supplies and you should be fine. A product with a legitimate CE or UL mark will have been designed to meet the standards, and that means robust isolation from input to output.

Do note that some supplies have the 0V output connected to the Earth pin, and some don't. Either is absolutely fine. Some applications require this connection and others don't - it doesn't necessarily indicate a safety issue. On units without this earth connection you may even see a high ac voltage superimposed on the output if you connect it to a scope, but the source impedance of this will be extremely high.

[Joeisi]

But the point I was trying to make, is that if a common person were to design one of these and mess up in some way, then you may not have enough isolation.

[emf]

If you are interested in how you'd design your own off-line switching power supply, I learned a lot on Power Inetgrations' web site. They are one company that makes chips that are the brains (and sometimes muscle) of those power supplies. They have a lot of reference designs you can look at -- you enter in what input voltage, output voltage, and wattage you need, they give you a list of designs using their chips, and the designs include lists of the external parts you need, schematics, PCB layouts, and sometimes a bit of narration too.

One thing you'll find is the transformers are going to be the hardest thing to find. For commercial supplies, they'll be custom wound, and you'll have a really hard time finding an off-the-shelf transformer as small as the commercial supplies have.

If you just want to have a regular power cord going in to your widget without wasting a lot of space, you can buy pre-packaged power supplies that you can just drop on to your PCB. For example, this one has similar specs. It's probably twice the size of the iphone adapter you linked, but easily half the size of what I'd be able to make on my own. And it's UL listed.