Is it possible to buy a non-isolated wall transformer?

Thread Starter

murphey_tris

Joined Mar 30, 2013
2
I was wondering if it is possible to purchase a non-isolated wall-wart ac-ac step-down transformer. I have looked, but haven't been able to find one yet (aside from a variac haha)

Is every single ac-ac transformer out there going to use an isolated primary and secondary coil or is it possible to get a single-coil transformer?


Why do I need this? Well I don't.. I'm simply wondering if I'm paranoid to assume something like this COULD exist.
 

timescope

Joined Dec 14, 2011
298
Transformers that are used to convert 110v to 220v or 220v to 110v are usually auto-transformers with a single tapped winding that is not isolated.

Timescope
 

ErnieM

Joined Apr 24, 2011
8,140
Yes it could exist but I have not heard of any out there.

An "auto transformer" is nothing more then a taped inductor. Say you make an inductor taped at the center, so it had the inductance on either side. Now drive end to end with 110VAC: across the end to center you get 55 VAC (half).

Wall warts are made for the international market: if any market needs isolation (and most do) then they are all made that way for all markets.
 

Thread Starter

murphey_tris

Joined Mar 30, 2013
2
Are you aware of a market off-hand that doesn't require isolation?

Furthermore would there be any cost benefit for say a 12v ac output adapter to be made with a single coil vs two? If there wasn't one that seems like a pretty good indication nobody would ever want to make one.
 

GopherT

Joined Nov 23, 2012
8,012
Most switched mode power supplies (e. g. Computer power supplies) use a common neutral and are, therefore, not isolated. Many little wall warts are SMPS as well so, whole not just a transformer, I would answer your question, "yes, the do exist".
 

crutschow

Joined Mar 14, 2008
27,028
..........................
An "auto transformer" is nothing more then a taped inductor. Say you make an inductor taped at the center, so it had the inductance on either side. Now drive end to end with 110VAC: across the end to center you get 55 VAC (half).
.................
An autotransformer can also convert 110V to 220V by connecting 110V to the "tap", so it really is more than just a tapped inductor. It "transforms" 110V to 220V. Also, in this case, the input current would be twice the output current, again like a transformer.
 

KMoffett

Joined Dec 19, 2007
2,918
Most switched mode power supplies (e. g. Computer power supplies) use a common neutral and are, therefore, not isolated. Many little wall warts are SMPS as well so, whole not just a transformer, I would answer your question, "yes, the do exist".
I think you are very (dead) wrong there. Computer SMPS's are definitely isolated, as are SMPS wall warts. Think a bit about the safety issues inherent in connecting two non-isolated devices with one's power plug reversed.

Ken
 

crutschow

Joined Mar 14, 2008
27,028
Most switched mode power supplies (e. g. Computer power supplies) use a common neutral and are, therefore, not isolated. Many little wall warts are SMPS as well so, whole not just a transformer, I would answer your question, "yes, the do exist".
I agree with KMoffett. Virtually all line-power supplies, whether SMPS or linear are isolated. It's true that many switching regulators, such as typical buck and boost circuits that use only inductors in their design, are not isolated, but they normally operate from an isolated raw DC supply voltage. Line operated SMPS (e.g. computer power supplies) use inverter or flyback type configurations that have high frequency transformers with separate primary and secondary windings to isolate input from output.
 

GopherT

Joined Nov 23, 2012
8,012
It depends on the definition of isolated. If you check continuity between neutral input and DC common (black on an atx power supply), then they are isolated.

Interestingly, and always concerning, is that a continuity check of DC common to chassis ground will show a connection. Likewise, a check of chassis ground to AC ground will show a connection. Therefore, plugging the unit into a wall outlet will suddenly give you continuity from mains neutral to DC common (by current USA wiring standards).

Again, it depends on your definition of "isolated".
 
Top