240VAC -> 110VAC voltage step down transformers

[niko]

Hi Dave and fellow EEVBlog followers,

I have always been weary of purchasing and/or using a step down transformer and I think it would be cool to have a few of the cheap and mid range step down transformers reviewed. Your thoughts on which ones are cheap but safe to buy/etc would be great to hear and would put my mind and several others at ease to know if safe practices are used in these.

For example, I've bought the "brick style" in the past but am now thinking if it's worth buying the bench style units and a review/benchmark of some of these cheap units on eBay would be awesome.

Supposed 50W brick style unit I'd bought to transform 220v to 110v for a 17W device and yet it failed to power the device on beyond standby: http://dx.com/p/50w-220v-to-110v-power-transformer-2228#.Uvgcy7RryRQ

Thanks again, hope to see a video on these one day!

[mariush]

That thing you linked to on dx.com is not using a transformer to generate the 110v.

It works on the same principles as a light dimmer, probably using a triac or something similar to chop the incoming AC to the levels a 110v output would be.

Scroll down under the stupidity at the top of the page (advising to put 190 diodes in series to drop voltage) on this page and you'll have a circuit that might be similar to what's inside your converter:

http://homemadecircuitsandschematics.blogspot.ro/2012/08/making-220v-to-110v-transformerless.html
http://homemadecircuitsandschematics.blogspot.ro/2012/03/how-to-make-220v-to-110v-dc-converter.html

[niko]

That makes a lot of sense, is there any danger in doing it this way? What is the proper way to do it?

[mariush]

When you say "step down transformers" most people would think you refer to something that provides some degree of isolation.
Classic transformers and most switching power supplies offer isolation making a circuit safer for humans.

Now of course, that product never claims to provide isolation and never claims to "transform" the voltage, at least from what I can deduct from pictures.. ignoring the title which is written by stupid dx.com employees.

Classical transformers are big and heavy, even for some measly 50VA the adapter would be probably twice the size of that converter and about 1-2 Kg in weight.
For high wattage, I imagine other solutions would be based on designs similar to how inverters work, or how an UPC works.

This converter just screws with the AC wave, could be gaps where the triac or whatever it used turns off or on, the parts used may change characteristics as they heat up from the current, who knows.