Wall isolation transformer

[Tage]

Just wondering if anyone uses isolation transformers when testing amps that come in for repair. I was wondering because I believe I should be using one before my variac when I bring up new builds or amps that have not been powered up for some time.

My variac does have a fuse, do I absolutely need isolation before it? Or is this only added security?

[Enzo]

Fuses protect things from excess current. Isolation transformers protect you from a direct connection to the mains. Those are totally different things. A variac provides zero isolation. An isolation transforme provides zero current protection. (well unless it has a fuse in it)

I use an iso for specific things. Switchmode power suplies, and video monitors. OH, and probably some occasional odd circumstance. For an amplifier, unles it has a switching power supply, or SMPS, it has a power tansformer. ANy transformer with separate primary and secondary windings (and that eliminates a variac right there) by its nature provides isolation from the mains. SO to mee an iso is redundant. yes, the mains wiring in the amp is hot to the mains, but I have no need to ground my scope to it. Remember, and iso puts the same 120 or 240 on your work as a direct connection, it just doesnlt do it the same way. But if you grab it, you are going to get 120v regardless.

[Tage]

Ok good, that is what i figured. I cant say I've never got a jolt from the mains wiring in an amp, but I'm always very careful to insulate those connections on my own builds. So i suppose when using my variac, I should simply continue to be very careful... duh.

I had just read that a iso should be used in conjunction with a variac, but I figured I should check in here before dishing out bucks to get/make one.

[TomCarlos]

Tage and Enzo... Thanks for the post and reply. I found an interesting video on YouTube. ToddFun.com: Isolation Transformer - Tripp Lite IS1000 - YouTube.

Years ago, I had a Heathkit VTVM and used it to measure voltages on a power supply I built. Sparks flew everywhere. At the time, the instructor didn't have a clue as to what happened. Now I know!!

[TomCarlos]

Just a follow up Q.... that video link I posted above mentions connecting the scope to the ISO. But if that is the case, wouldn't you connect ALL other test equipment to the ISO as well?

The better option is to connect the item under test to the ISO?

The video mentions disconnecting the ground wire from the ISO secondary to the duplex outlets. I need to watch that again and read the comments.

[Enzo]

If all you use is a scope, you could use an iso on the, but that doesn't make me feel safe. For example, in a typical SMPS, the primary side control circuits use V- for a common, and that is about -165vDC. The control circuits use that as a "ground". So if I iso my scope and connect the scope "ground" clip to that -165vDC, that means the frame of my scope is now sitting at -165v with respect to the earth ground in my shop. So I have to be careful how I touch it, and not let anything else touch it, like the grounded cases of other test gear.

No I wouldn't want all my gear on the iso. if I iso my scope, I might still want my signal generator grounded because I am not connecting them to the same places.

Guys tell me they put insulation under the scope so it doesn't ground itself to the shelf, and so on, but I am not convinced.

I always iso the unit under test when I iso.

The iso provides mains voltage and current as a direct connection to the mains would, but it is not directly connected. Earthing is a separate issue.

[TomCarlos]

Thanks Enzo... that makes sense. I just watch another video where the device being tested is on the ISO.

[TomCarlos]

Adding a footnote and additional video...

The video link I posted before explains the problem of connecting the ground to the secondary side of the Transformer. In that video, we see the connections for a Tripp Lite ISO 1000 (Hospital Grade).

The Tripp Lite ISO 500 and ISO 800 have a similar issue. The secondary side of the transformer have a direction between ground and Neutral. These transformers also have caps across the Positive and Neutral terminals of the Duplex Outputs. I also see an MOV across one of the devices.

For an additional reference video on the topic - https://www.youtube.com/watch?v=nRfXG3EqFXc

And another video - https://www.youtube.com/watch?v=XBsQ3sZ45Fk