How unsafe is an Autotransformer?

[Richard Ellis]

I'm on my way toward building my first tube amp & the most straightforward set-up would be using an autotransformer. Down here I can get a 220-110 at 300W for some $15 USD(60 Pesos) With a core of 5" by 5" by 2 1/2"....
Just how unsafe are these transformers & am I commiting the ultimate sin here by not using a "true" transformer....am I about to be censured?
Perhaps I could fuse it three ways to Sunday! Just to throw gas on the fire the grounds here are a hit & miss affair. Household wiring here is shoddy to say the least!

__________________________________________________ _____Rick.....

[infinia]

Gas on the fire is a good enough analogy

Perhaps I could fuse it three ways to Sunday! Just to throw gas on the fire the grounds here are a hit & miss affair.

If any part of that circuit shown above and its connected circuitry interfaces with the outside of a grounded enclosed metallic or doubley insulated case then it's considered a safety hazard in most industrialized countries.
I would recommend a proper step-down isolation XFMR.
In-fact any time I use an auto transformer it's paired with an oversized isolation XFMER.

EDIT>A Fuse or more than one especially is not a panacea for shocking hazard protection.

[Cassiel]

When I started building tube amps I once used an autotransformer. It gave me many shocks, but I only took it apart when the SOB blew up my expensive DAC. Not recommended.

Household wiring here is shoddy to say the least!

Same here.

[Mosquito]

Richard, maybe you can look for a 45/48V secondary and use a voltage doubler
(PSU designer says it can be done)

[Conrad Hoffman]

An autotransformer is about the same thing as connecting a wire directly from your circuit to the wall outlet. It has zero isolation. Don't do it. An autotransformer doesn't need quite as large a core (not sure why), so is often cheaper. Is there any part of "Don't do it" that needs additional clarification? Did I mention not to do this? We'd like you to keep posting and the internet connection from six feet under is really bad.

[Mosquito]

Lord of the Sparks! (the Tolkien lost manuscript)

[Eli Duttman]

Richard,

If you like life, you'll heed the other posters and acquire a proper power trafo. Depending on the B+ draw, either a Triad N-68X (50 VA) or a Triad N-77U (100 VA) will get the job done for you, SAFELY. Both of those isolation trafos have dual primaries, a "115" V. secondary, and they are inexpensive.

[Mosquito]

The best solution are the Triad xformers (I'm using these), but if You are by no means able to access to imported material, a couple of 220/12V cheap xformers, tied back to back, can do the trick. When feeded trough the step down autoformer ( this one rigorously outboard), You'll get half the socket voltage, and a safe chassis. I've just done this, this morning, to be sure that it works.

[richwalters]

Originally Posted by Richard Ellis

Just how unsafe are these transformers & am I commiting the ultimate sin here by not using a "true" transformer....am I about to be censured?

In my earlier days, anyone on my watch caught using an autotranny directly on the workbench without an isolating tranny where others also working was a punishable offence because it endangered others. Back to Tech evening classes on safety.

Simply put, engineering wise it isn't the done thing and contravenes a raft of IEEE regulations.. In many domestic European locations one often finds there isn't a neutral, but two live phases and earth. Don't blow ones chances.

richy

[kavermei]

Don't do it, a funeral costs more than an insulation transformer!