I'm working on a project which will require a 5Vdc signal from a 120VAC outlet. I know you can throw in a transformer, rectifier, and some other goods and achieve this, but I need something fairly small.

The ac to usb plugs come to mind, but I haven't found much on what is inside them.

Any suggestions on small parts to use or what is in the ac to usb plugs?

Thanks!

 

you didn't mention how much current you need but just get a 5V wall wart..not worth the time to reinvent the wheel its got basically the same components as the usb stuff
http://www.sparkfun.com/products/8269

 

Thanks for the quick reply.
Ideally, thats what I would want, but I have to have enclosed with the rest of the project. I'm trying to figure out a schematic of a wall wart.

As for current, < 2A.

 

Most phone chargers are already there. Typical specs, 5VDC at 1A.

 

I bought some name-brand 5V/2A power supplies at a surplus store for only $1.25 each. They were made for a modem. The store had thousands of them. The AC cord weighs more than the entire circuit.

 

That's why I'd like to know what is inside one of those. Does it use a current or voltage transformer, etc? I don't have a spare one to take apart and google is coming up with nothing so far.

Perhaps more info my project would help. I am building a circuit that will go inside a power outlet. So it needs to fit in the outlet box. Because of this, I won't actually have a receptacle to plug into.

 

The modern power supplies are switched-mode. They use a high frequency oscillator so that the transformer is tiny, lightweight and inexpensive. They use pulse-width-modulation with a varying duty-cycle to regulate the voltage.

 

Generally we don't discuss those circuits because they can be hazardous to beginners. It is a catch 22, if you are competent enough to work with them you don't have questions, if you have questions you aren't competent enough to work with them. These little units take shortcuts to do what they do.

If you want a variable power supply I've written an article on it a while back. I am a strong advocate of wall warts (a nick name for these kind of circuits), as they are much safer than the old way of doing things.

Basic Bench Top Power Supplies

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I understand what you mean with the catch 22. There is very little info on them online.

I'm a senior in electrical engineering and have built a variable power supply before. I was just curious if there were any schematics laying around for these devices before I started selecting parts for our senior design project. It's going to need a 5V rail before it's all said and done, so I'm trying to knock that out first.

 

If you must use the special custom-made high frequency transformer in a switching power supply to make your own switching power supply then you might as well use the original switching power supply.

 

i would just hard wire or put a barel plug on the project case then atach an adaptor. or hack an adaptor solder a cord up to it removing the prongs and put in the case. if using a transformer and a bridge rectifier you should include a filter capacitor also for pure filtered dc. smithc mode psu would probly be best bet. if you need to buy one i recomend looking on these sites. marlin p jones and osc inc. , electronc godmine (chany), sciplus.com. also all electronics looks good too.

 

thanks for the links, ill keep researching tonight and see what I can come up with.

 

Think really simple SMPS power supplies. The reason I think they are taboo is the lack of a transformer. This is one of several subject we do not discuss at this site, a transformer is considered an absolute must under all circumstances. A black box is different though.

If you want to discuss a transformerless switching mode power supply you will have to find another site. I can recommend one or two.

Part of the reason for this is the incredible numbers of beginners we get here, some of whom are preteens.

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Reading the other responses I hadn't considered the other possibilities, such as high frequency stuff. Still, I suspect the lack of a power transformer means there is a chance there is inadequate isolation for the AC mains.

 

Switching power supplies have a special high frequency transformer that has a ferrite core. They are safe.

Where do you find the special core to make a switching power supply?
Duh! From a switching power supply that you buy and take apart.

Duh! Don't take it apart, use the purchased switching power supply instead.

 

You're up against the classical "make vs. buy" dilemma. Even if you value your time at 50 cents per hour, it's not free. Wall warts are available all over the universe at giveaway prices. Buy one, and spend the time you didn't waste on building a supply to study up on something useful, or fun, or ....

 

Thanks for the advice. I think I found what I was looking for. Ordered some parts on digikey to test with this weekend.

I threw out 2A as a rough estimate, but turns out we really on need about 100mA or for the circuit. Not sure what I was thinking when I through out 2A. This is what I decided on.

BP5034D5

Any thoughts?

 

The Rohm power supply module is made for the 100VAC electricity in Japan.
Its input voltage range is 80VAC to 138VAC.

It is extremely dangerous because it has one of its output wires connected directly to one of the input electricity wires without any isolation.

 

There's not really another way that I can think off right now, because this will eventually live in the receptacle box. Do you see the dilemma? I'm always open to other options.

 

OK I have to through this in. If the OP has line voltage inside the device that they are working with, then they are already at some risk. I often find that I need 5VDC inside a 120VAC device. I don't want two plugs, the mains and a wall wart. Even if I make a 120VAC to 5VDC supply, I still have to connect it to 120VAC. What I've found to be the cheapest and safest is to gut a regulated, SMPS wall wart. I don't need the case or the plug. $0.39 to $1.99US at thrift stores...or free from you friends' boxes full of discarded cell phones.

http://www.electro-tech-online.com/general-electronics-chat/93884-not-transformerless-power-supply.html

Ken