Running a variac from a small transformer

[Rontone]

I have just hooked up the variac, to test a little mystery FX pedal power unit, turned out to be a 12v, so its useful for a few things

Then I thought, run the variac after it to test an OT transformer I scrapped last night, so 12v across the variac's winding

It gave me a more accurate way of dialling in 0.500v on the OT's secondary to work out turns ratio, I wired up the normal way, line-voltage not over-voltage,

Is this safe, I've never even looked at chaining transformers, obviously if you wired the variac up wrong or in series/rheostat for example you could get quite a voltage out the other end!

[I have seen a video on youtube where someone tested their new variac with a small 9v or 12v PSU and wired it up wrong, about 500v came out the other side, woohoo!]

[sluckey]

My variac stays connected to the wall outlet. I have a 6.3VAC test transformer that I just plug into my variac and connect the 6.3v secondary to the OT secondary. Adjust the variac to give 1VAC across the OT secondary. Then use the same meter to measure the primary voltage. Since the secondary is set to 1V the math is easy. The voltage measured on the primary is the same as the voltage/turns ratio. For example, 30VAC means the turns ration is 30.

[PRR]

...I've never even looked at chaining transformers...

The stuff in your wall is the end of a LOOOOOOONG chain of transformers.

I have a tranny at the street. Another one where my street hits the road. There's a big one at the former hydroelectric dam, and probably one in between. The dam only runs when the fish are swarming (it seems), power really comes from 30 miles at the big river where they have been transitioning from hydro to coal to gas? The generator makes 4KV to ease insulation stress, the long lines run at 25KV or 100KV to reduce conductor size, and there may be switchgear and bussing at intermediate voltages, all mated with transformers. So at least 5 and probably 7 or 9?

However all this stuff was evolved by smart people and set as industry standard. And the industry runs stuff _AT_ the nameplate nominal voltage and current. Running 6V into a 120V Variac may be safe for the Variac, but could severely strain a little train transformer (the idle current of a large Variac may exceed the max current of the toy transformer).

As Slucky says you can adjust the INput to the small transformer, get the same effect, safer. 

[Rontone]

Running 6V into a 120V Variac may be safe for the Variac, but could severely strain a little train transformer (the idle current of a large Variac may exceed the max current of the toy transformer).

As Slucky says you can adjust the INput to the small transformer, get the same effect, safer.

That's what I was wondering, if I had a mystery OT to test, the unknown winding ratio could be large,

The first small 12V TX in the chain, lowered in voltage by the variac, into the OT on test, could end up with a large current induced on the end of the chain,

Sluckey's method must be safer as it uses less voltage on the OT secondary to then measure the primary,

My method was badly thought out, trying to put 1V on the secondary 'through' the primary, and only being able to run 12V through the small transformer at the start of the chain, you can only put up to 12V on the OT primary which may not get the desired 1V on the secondary to make the maths easier,

Plus I'm still a bit scared of this variac, [As PRR mentioned the street mains TX's, years ago at work, I have seen someone take a HUGE shock from a power substation transformer, not nice, blisters raised up inches, hair gone, clothes on one side gone... amazingly still walking around with wisps of smoke, it was very nasty...]

Just dont want to slip with the dial and run higher AC voltage to something not rated for it, although the windings on the primary are hit with 300v of DC in some valve amps, I was unsure what hundred of AC volts could do to a little transformer

I'm going to improve this variac soon, into a testbed, current limiter bulb, RCD on the outputs etc

[sluckey]

I would not worry about it. Nothing dangerous about connecting a 6.3v PT to a variac. I have a power cord on the primary of my 6.3v PT that safely plugs into the variac. the PT secondary has gator clip leads attached and the gators have rubber boots on them. Just clip the gators onto the secondary of the test OT, adjust the variac for 1VAC into the OT secondary and measure the primary voltage. I doubt you'll ever see over 50VAC. The variac dial will be set really low, probably around 20.

The whole procedure is almost idiot proof and not even close to being as dangerous as your substation transformer example. Hopefully we'll never read about you in the Darwin Awards.   

[Rontone]

The whole procedure is almost idiot proof and not even close to being as dangerous as your substation transformer example. Hopefully we'll never read about you in the Darwin Awards.   

Yes, that was quite an incident, it looked literally like the Pink Floyd - Wish You Were Here album cover!

You'll more likely see my cat on the Darwin Awards, for some reason he's attracted to electronic work, he likes to sniff the variac and amp chassis's so have to make sure he's locked away when I'm testing anything,

He makes a good grounding device when your handling transistors though and dont have a wrist strap, bit of cat food spread on the radiator pipes, hold the tail firmly with one hand, other hand free to work

I have seen your topic https://el34world.com/Forum/index.php?topic=25028.msg306555#msg306555 on adding a digital meter to a variac, that is something I need to do

The hardest part is finding an enclosure to house the beast, I went and got a huge 15 amp quality one as it was such a bargain [£40 compared to RRP of £600-700!] the guy had not tested it and didn't really know what it was, turned out to be clean and very little use/wear, so I got me a good one,

Picked up some nice analogue meters, 15A and a 300V range that are panel mount, a quality RCD wall socket for the output, and a quick-connect test block next to it, also fused,

Its just getting that enclosure, I have seen car/truck battery chargers that look good, large heavy duty metal box with a solid base, some go cheap