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Not a transformerless power supply!

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Well-Known Member
I see many posts of people wanting to drop AC line voltage to 5VDC to run some circuit..."without using a transformer". I think they're trying to save either money or space. Though I have done that, I find that in the long run I don't really saved either...and I've added the risk of a non-isolated supply. It was usually to add a small low voltage source inside a devise that already had line-voltage internally and didn't want the add a cheap, external wall-wart plus internal regulator circuit.

When cell phone manufacturers started using switch-mode power supplies in their chargers I found the perfect source for small, isolated, wide range AC voltage, relatively high output current power supplies.

OK this is sort of an "instructable". ;) I can find a variety of SMPS chargers at places like Goodwill for $0.69 USD. You can find them by noting how light they are for the labeled current output, compared to the weight of standard wall-warts with line frequency transformers with a similar current output.

1. Cut off the output cord at the case.

2. Use a bandsaw, Dremel with a saw blade, or hand scroll saw to cut through the case along the glue line. Cut just enough to break through on three sides. Be very careful not cut any deeper, as you may cut into the circuit board.

3. Use a flat blade screwdriver to pry the case halves open.

4. Remove your small, step-down, isolated SMPS.

There are many variations on the cases, complexity, and internal connections to the line plug contacts, but the same technique has always worked for me.

I have even found some that used standard controller ICs, and have been able to tweak the output sensing voltage divider to get a different regulated output voltages.

The down side...there is always one... you need to be creative on how to safely mount the PCB. ;)



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Or leave it in its case and use it as the manufacturer intended.

Beware: not all of these SMPs wallwarts are regulated, many aren't.
If you are looking for space savings, the case doesn't add anything. Well, maybe another layer of safety. So far, I haven't found one that wasn't regulated. The newer ones, that are meant for things like lithium chargers probably will not work. But, I'm picking up old chargers.

No solution works for everyone. ;)

Here's a picture of the PCB of an unregulated SMPs.

It's just a simple transistor oscillator, no fancy regulating ICs, just two transistors, a few of diodes, resistor and capacitors. There's no feed back from the secondary which is on a separate board joined to the transformer.

The secondary side consists of a Schottky diode, a 120µF capacitor and a 3k bleeder resistor.

I haven't completely reverse engineered the schematic but I could easily do so if I wanted to.

The case was the same size as a 13A plug you wouldn't gain anything by removing it. I just cut it out of its case for educational purposes.


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I like this idea, I'm using a similar supply in an enhanced alarm clock for my wife. There's not much space in the housing so I have extracted the board from the original plug-in unit too.
I say keep it in it's case so that the UL, CSA, etc rating is preserved. It'll make negotiating with the insurance company easier after your house burns down.
This is not a technique that's intended for inclusion in commercial products. Well maybe something I bought and decided to modify. ;) Once a hobbiest starts building anything involving line voltages you pretty much negate any compliance with UL, CSA, you are not going to go through the certification process anyway. It's the same as making a PCB that has 120VAC connected, and installing it in an enclosure. Safety precautions should be incorporated, but that goes for any design.

Very good advice from KMoffett. I think it is absolutely absurd to use a transformerless power supply.
If the power leads are reversed, the project has a potential of over 300v AC (for 240v mains) and making sure a project is totally enclosed is a very difficult thing to do.
Transformerless power supplies are not a regulated supply and are really quite worthless. They don't offer any advantage over a SMPS and these can be bought for a few dollars as mobile phone chargers in some of the "$2 shops."
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