DIY Isolation transformer box

[queennikki1972]

I was thinking of building a DIY Isolation transformer rig for 120vac input , and Im sure I need a transformer, switch, fuse, power jack, and ac receptacle. I was just wondering how much current is considered safe as far as the size transformer used in a project like this? I could not find exactly the type of drawing or diagram for the project.

The project would be mostly for small light fixture type applications or printers , faxes etc that usually call for 12-24vac input. Nothing high powered.

Nikki

[Wor3d]

Not 100% sure what you are trying to achieve

Isolation transformer is usually a 1:1 (e.g. 120VAC in, 120VAC out), and is used for safety purposes , or advanced measuring.

You also say that you need 12-24VAC output, so I guess you have other transformer? Or you want to use autotransformer (transformer with a rotary know, which is used to "dial in" the required voltage).

The current / power requirements will be driven mainly what are you trying to do - printers/lights -> find out how much they consume and buy transformer which will handle that kind of power. Also all the other components - connectors, cables, fuses..., altho they are not that hard to source, even for big currents, opposed to transformer - its price will increase fast, when you will go higher in power

[queennikki1972]

What Im trying to accomplish is to be able to power a project that has ac input dc output, but be able to safely measure the ac side without lethal current. I had considered a variac but i watch videos that said this was not a safe way to do that.

[Wor3d]

Yes, variac (autotransformer) is not (usually) isolated.

So that we are on the same page, you want a device that has 120VAC in and "safe" 120VAC out.

Some of it is covered in a older EEVBLOG video


Also, by isolating the output, you are not limiting yourself to high currents if you touch both leads. It just isolates the Line voltage from the ground (!!) so you can touch device's "high voltage" with one hand, and you will not get shocked to the ground (through the foot to the floor, etc.). This is also if you wanna measure floating (non-earth referenced) voltages.

You will still get shocked when you touch 120VAC behind the isolation transformer.

[dcbrown73]

Sounds to me like you want a power supply that also supports AC.

It might be good to learn on a PSU kit that supports AC first.

Maybe the Elenco AC/DC Triple Output Power Supply Kit

While it's not the best, I learned quite a bit during the build.  I also learned ways to improve it on it's design.

[ledtester]

This is a variable AC supply which includes a non-isolated output of 0-140V and an isolated output of 0-30V AC:

https://youtu.be/DGo88XWzFrQ

I was just wondering how much current is considered safe as far as the size transformer used in a project like this?

All of the components (transformers, variacs) come with VA and current ratings. For transformers VA is approximately proportional to weight - ie. the higher VA rating it has the heavier it is.

[queennikki1972]

PERFECT!

[MarkF]

[queennikki1972]

MarkF I read several places that you should not float your scope, rather you should float the circuit instead?

[MarkF]

MarkF I read several places that you should not float your scope, rather you should float the circuit instead?

That is correct.  You could bring the chassis of your scope live if you float it.
Never float your scope or any other test equipment for that matter.
Float your Device_Under_Test (DUT) instead.
Also, watch where you connect the probe GND clips, when using more than one probe, not to short anything (Remember, they are all connected together.).

And be especially careful if you're debugging a 'switching power supply' (even with it floated).


Note:   You can emulate a differential probe by using two regular probes (without their GND) and using the subtraction math function.

[Mr. Scram]

The fact that oscilloscope grounds are tied together and to ground cannot be stressed enough. A typical oscilloscope differs from a multimeter in that regard and this difference may not be immediately obvious. You can use active probes to create isolated inputs and there also are oscilloscopes with isolated inputs but that's generally a specially listed feature.

[Zero999]

MarkF I read several places that you should not float your scope, rather you should float the circuit instead?

That is correct.  You could bring the chassis of your scope live if you float it.
Never float your scope or any other test equipment for that matter.
Float your Device_Under_Test (DUT) instead.
Also, watch where you connect the probe GND clips, when using more than one probe, not to short anything (Remember, they are all connected together.).

And be especially careful if you're debugging a 'switching power supply' (even with it floated).

MarkF I read several places that you should not float your scope, rather you should float the circuit instead?

Yes and it's generally recommended to connect the oscilloscope's chassis to earth, even if it's being run off a battery (as in the Owon SDS7102-V), although it's not always practical, as if one is not using the mains, then there's little chance they'll have access to an earth connection. The exception being a scope meter, which has a fully insulated chassis.