Or so I have been warned. I saw a picture of one like this and wondered why. Something to do with not having an isolation xformer. Some variac do, but big $$$$. Price restricted.
I had planned to take the output of 3V to a breadboard someway and didnt see any bare wires laying around with more than 3V. So, where is it going to bite me ? Is it that there is a potential for the full mains voltage to be present on the case ?
And thanks, I didn't buy one but I want to know why.

 

Is it that there is a potential for the full mains voltage to be present on the case ?

Yes. An autotransformer typically uses a single winding. If that winding fails, you can get the full input voltage applied to the load.

One of the moderators posted in one of your earlier threads about isolated autotransformers being available. Think it was @bertus, but I've never seen one.

Do yourself a favor and just go to a second hand store and buy an AC wall wart. 12VAC is a common voltage and is safe to work with.

Or, as I posted in your other thread, mount a typical transformer in a box (with a power switch), so line voltage won't be exposed.

 

Variacs are not isolated from earth. The variac pictured can put out 130V ac, referenced to earth ground. An isolation transformer is not referenced to earth making circuits safer to work on because you have to touch two nodes of the circuit to be shocked (you only need to touch one node of the circuit to be shocked when referenced to earth because you're usually also referenced to earth).

Failures of variacs happen also. And can kill you dead.

At 3Vac output, you're probably okay, but it still needs to be isolated to meet forum TOS.

Bottom line is that you need galvanic isolation to know that you're relatively safe - but still don't get both hands on a live high voltage surface.

 

Variacs are Very useful devices, but as pointed out, you need to be aware of the devices limitation as to Galvanic isolation.
I would not be without mine, but those that use these are generally aware of safety issues, if your power socket is wired correctly, when plugged in, the Variac output socket should have the common or neutral side retained and in common with the originating 15a outlet it is plugged into, but it pays to confirm this.
Max.

 

An open winding failure would be very rare, not using a proper plug is where problems crop up. User plugs in a 2 narrow blade AC line cord connecting to a low V rectifier. No problem .
Then Murphey rears his ugly head & OP touches circuit ground & a earth grounded case-- Ouch.

 

For all of the reasons already stated, don't use a variac for a low voltage step-down transformer.
Go to a second hand store and buy a wall-wart with the suitable low AC voltage.

 

For all of the reasons already stated, don't use a variac for a low voltage step-down transformer.
Go to a second hand store and buy a wall-wart with the suitable low AC voltage.

I never considered second hand stores for salvaged parts for projects. Seems like a good idea. Of course I have enough wall warts around to last a couple of life times of projects.

 

Ok, makes a lot of scary sense and I did buy a 9v AC, wall wart. The store where I bought it also had a variac with an isolation xformer and it was a large and heavy beast. The box was probably 1 1/4 feet to a side.
Which makes me wonder why it was so ginormous and my desktop dc power supply and the wall wart are so small.
Apparently they are safer than the variac. How can they build the safety into those little cases and not into a variac.
Every time I turn the page or ask a question, I generate an endless number of new questions.
Sheesh

 

How can they build the safety into those little cases and not into a variac.
Sheesh

Simple!
The Wall Wart SMPS, etc is galvanically isolated from the service power, the Variac Not, it would make it a lot costlier also.
Max.

 

The variac is larger because of the higher power capability.

 

I personally don't see the fear of working with AC. Guess it's because that's where I started with electrical things, AC. Or maybe you are still thinking about the Edison vs Tesla current war times? https://en.wikipedia.org/wiki/War_of_the_currents

Both can hurt you, but only if you do something stupid. You just need to play/work safe. If that wasn't the case we would all be living like the Amish.

 

I personally don't see the fear of working with AC. Guess it's because that's where I started with electrical things, AC. Or maybe you are still thinking about the Edison vs Tesla current war times? https://en.wikipedia.org/wiki/War_of_the_currents

Both can hurt you, but only if you do something stupid. You just need to play/work safe. If that wasn't the case we would all be living like the Amish.

Although there's a big difference between knowing what you're comfortable working near yourself vs knowing what to advise a noob to try. If I was face to face watching someone new, I could correct mistakes and keep them safe. Guiding them through the web I'm much less confident.

 

I don't think this has been mentioned yet. A variac is a power transformer with the coil exposed so a wiper arm can slide across it. It is a simple structure that does not hold mechanical tolerances well with use. 3 Vac is 2.5% of the input voltage, possibly below the adjustability limit for some models. Trying to get a stable 3 Vac out of a line powered variac is difficult, and going back to that exact spot for repeatability would be a challenge.

ak

 

My Variac arrived with the active (240V here in Oz) wired to the cold end! Definitely a no no!
I rewired the lead to fix it.
Variacs are an extremely useful device but not the thing to generate 3VAC to use directly. Putting aside the safety issues, just think what happens as you are working on your 3VAC powered project and accidentally bump the Variac knob. It WILL happen!
Use an isolation step down transformer please.

 

I had planned to take the output of 3V to a breadboard someway and didnt see any bare wires laying around with more than 3V.

I have to strongly agree with AnalogKid:

I don't think this has been mentioned yet. A variac is a power transformer with the coil exposed so a wiper arm can slide across it. It is a simple structure that does not hold mechanical tolerances well with use. 3 Vac is 2.5% of the input voltage, possibly below the adjustability limit for some models. Trying to get a stable 3 Vac out of a line powered variac is difficult, and going back to that exact spot for repeatability would be a challenge.

You would do well to just buy a simple little 6V CT (Center Tapped) transformer. If you just want for example 3.3 VDC buy a 5 volt wall wort and a 3.3 volt low dropout regulator that meets your current demand.

Ron

 

I have to strongly agree with AnalogKid

So do I. Go buckeyes.

 

So do I. Go buckeyes.

Absolutely. The match up reminds me of days gone by and the Rose Bowl games between OSU and USC.

Ron

 

Absolutely. The match up reminds me of days gone by and the Rose Bowl games between OSU and USC.

My dad was at the 68 Rose bowl, the one when we beat OJ for the title. 50 years later, if it weren't for the playoffs we'd be playing USC in the Rose Bowl, and I'd be there.

ak

 

My dad was at the 68 Rose bowl, the one when we beat OJ for the title. 50 years later, if it weren't for the playoffs we'd be playing USC in the Rose Bowl, and I'd be there.

ak

Exactly! Before they added all sorts of things to college ball. That's pretty cool with your dad. I never went to a bowl game but my parents lived in Worthington, north of campus and my dad would get OSU tickets. I made a few OSU home games.

Ron

 

Why the Variacs (or Autotransformer) is quite useful and it also very dangerous, because the output is connected to the Mains directly, you can see the internal structure from the links.