A variac is basically a variable auto-transformer that allows you to adjust the voltage between zero and the local mains supply voltage, or more if tapped mid-winding.
They find use in specialist applications where it's useful to ramp the mains voltage up slowly for regulation or testing. Some applications would be inverter, motor drive repair, soft start of vintage electronics and some specialist lighting applications where lights need to be dimmed without phase angle noise.
These units do not provide isolation from the mains supply, as they are just a single winding with an adjustable tap.
I've sometimes wondered how they can seamlessly jump from one winding to another without bridging loops and crating shorted transformer turns. Since I had my unit open for fitting a new digital meter, I thought I'd test that with a thermal imaging camera.
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They find use in specialist applications where it's useful to ramp the mains voltage up slowly for regulation or testing. Some applications would be inverter, motor drive repair, soft start of vintage electronics and some specialist lighting applications where lights need to be dimmed without phase angle noise.
These units do not provide isolation from the mains supply, as they are just a single winding with an adjustable tap.
I've sometimes wondered how they can seamlessly jump from one winding to another without bridging loops and crating shorted transformer turns. Since I had my unit open for fitting a new digital meter, I thought I'd test that with a thermal imaging camera.
If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
http://www.bigclive.com/coffee.htm
This also keeps the channel independent of YouTube's advertising algorithms allowing it to be a bit more dangerous and naughty.
Thank you BC. I'd been aware of this for a very long time but it's good to have it highlighted by you in your usual professional way. My 8 amp variac dissipates 30 watts on standby and the brush has an heatsink which gets quite warm on standby. When you think of it, there are only a few volts between each turn of the winding so the current flowing as the brush shorts them out is minimal on a miniature variac but it gets significant on a large variac with a correspondingly large brush hence the brush's heatsink on my 8 amp one.
I’d wondered about this myself, thanks for the video!
I have one that I put into a box with several switches: A dual-pole on-off switch that interrupts both sides of the line, just in case. A switch that selects whether the high side goes to the end of the variac or to a tap, allowing me to "boost" the voltage to somewhere around 130V, for some tests. And a DPDT center-off switch that in one position puts a light bolb in series with the output, in the other position connects directly, and in the "off" position allows for the connection of a meter to some binding posts so I can measure the current. There are also neon indicators to tell me when the power is on and when I've blown the fuse.
In a relatively recent acquisition of a pile of "stuff" I got another one. The rather distinctive knob that came with it is one that I've seen installed in premise wiring, as light dimmers. I had no idea that they were using variacs for that purpose.
In the 1970's I built a variable power supply using a variac rated at 10 Amps. It was used to run an electromagnet.
Because I was a laboratory technician I was able to get items from RS Components then. Since I retired they are no longer in a position to fulfill any orders from me.
I have a 2 kva variac .Not the greatest one ,but it works exactly the same way .My friend has one that was made in the 50s it uses a phenolic ring with silver contacts the wiper makes contact with .The wires are solderd from each winding to each contact .his is only 400va and wuite heavy and more robust in build .Cant remember the brand ,its a bluish green color .The scale is amazing and the voltages are spot on pending supply voltage .
Clive It was a typing error. My Variac is about 300v max. It`s 2,000 watts. I`m always getting watts mixed up with sieverts and cosmic strings !!!!
I've just got my 2 Kw Digital Variac from Amazon !!! It's a super device, but it's nearly as heavy as my Royal Enfield Motorbike !!!! I shall be using it with a Residual Circuit Device, for fixing power tools, motors and a bit of electronics !!!! Mine arrived in perfect condition. It only has 13 amp Ac output 0 to 240v which is just what I wanted !!!! But if there's a short it should trip the RCD unlike a Microwave Transformer which just kills you instead !!!! If the Brush sparks like a Bumper Car Pole in a Fairground, you might need to fit a new Carbon Brush !!!!!!
The no-load dissipation of your Variac seems high for its output power, possibly due to the shorted turns at the end connections.
My 1400 VA (10 A 140 V) Superior Electric unit draws 2 W and 2 VA with no load. The little 300 VA unit is 1 W and 2 VA unloaded. But none of my Variacs have shorted turns at the winding connections. Either the winding wire end is connected to a terminal board, or the tap connection is soldered to a single turn of the winding.
Mine which i think is 800VA has no shorted turns but wire comes out at one end and the other is bit looser wire on the side soldered on with insulator below it, mine allows going up to 310 ish volts
Hello Sir thanks for the video it was helpful but I have a direct question please. If I have a breaker at 15a or 20a and it trips out when I plug in the stove everytime could I use the Variac at a 120v plug and set it at 220v would the Variac take the load thus stop the breaker from tripping? Or is this not what this device is made for? Thanks in advance for any and all answers I sure hope you still respond to this channel!! Take care Sir and Peace. I ask because I have a landlord who is a Sherrif and he does not fix anything ever.
Not to insult another man's variac, but on higher quality models they lift one turn off the core, put kapton tape under it, scrape the insulation off that area, and solder that turn to the terminal wire.
Glue stick is your friend when printing PETG on PEI.
I also made the mistake of having an unsmoothed DC output on my variac. I didn't leave enough space for the capacitor and so now I always have to add a cap on the output if I'm using DC
Ahh, what a cute little Variac. I have one I call Big Bertha, over 50 years old with a rated current of 45A and still going strong. She is a beast!
Did you print the bezel in PETG? Taking chunks out of the glass is a common issue. Most peeps i know either put some glue or tape down when using PETG on a glass bed.
I have an FLSUN. Ran it into the ground this time last year printing faceshields. I'm going to put a better board in when I rebuild it.
edit Just realised this video is a few weeks old. Nvm.