There should be a fuse on the output, otherwise the wiper will burn out when the output is shorted - primary fuse won't help because of transformer action!
Wow, I knew what a variac was but never really understood how it worked, especially the voltage boost. Thank you for helping me understand it and also the non-isolating aspect.
When I saw the thumbnail I thought it was some kind of mysterious camera. But that turned out to be completely wrong and the actual box with its variac was interesting. The little meter board is cool and amazingly accurate. 🙂👍 PS - When I was about 13 my parents were involved with doing stage plays. Backstage there was a board with about 20 switches and a large variac knob to dim the stage lights. There were also pieces of so called gel of all different colours which were cut into squares and put in the light holders to get different colours of lights. Although us kids were allowed backstage after the play we were under strict orders not to touch anything. We were allowed to keep small scraps of gel which are cool because they make everything look like weird colours.
I have too. I bought the main part at a flea market. I packed in a FR4 box. It's my friends who often use it. To fine tune their guitar tube preamplifier. I was amused to see the expressions on their funny faces while tuning their guitar amplifiers. I love to have many artist friends. They are kind and warm people.
Like most of you, as soon as I saw 133VAC WE KNEW that it had a Variac inside. IMSAI guy knew as soon as he felt the transformer windings thru turning the knob!
Those "weird shrouded male" connectors have been standard on computer servers for decades. The male end is IEC-320-C14. I think it's so that equipment can more easily have a "lock" mechanism that prevents accidental disconnect of the cord. Usually there is a little mechanism next to the socket that will secure around the cord, but this variac does not have such a mechanism. Some companies such as tripp-lite also sell them on computer extension cords. In that use case, the shroud protects from accidental electrocution of a user by covering a loose connector.
A 10 Amp Variac has been one of the more useful tools on my repair bench. I also use them around the house; one controls the speed of a vent fan and another converts whatever the utility's voltage de jour is to 110V for classic tube gear that doesn't really appreciate the 127V average we see where I live.
I got one that takes 120 in, and can give up to 250 out - very convenient for repairing 220V input (only) power supplies on my bench where I don't have 220V circuit
Hi, thanks for the video and the clear explanation. I think it is beter, though, to have the isolation tranformer first and the variac second on your bench. Quote from the internet: “Grounding issues aside, the isolation transformer first is the better way. If the variac is first, you will starve the magnetic field of the isolation transformer, causing poor voltage control, especially at lower voltages.”
They are useful test equipment for the lab bench. You can add a 150 Watt light bulb in series with the input, with a switch to short it out. When the shorting switch is closed you get normal operation of the Variac. When the switch is open, the bulb flows whatever current is being drawn, and at about 30 Watts which is the typical Device Under Test load, it glows dimly and you adjust the output voltage to the correct level. If the Device Under Test starts failing, then the bulb will either go out (DUT is open circuit), or glow brightly (DUT is short-circuiting), or start wavering (DUT is beginning to fail). You could also add a residual current trip between output and ground which senses whether even a small current is flowing between output and earth, and disconnects power if so. All-in-all a useful way to safely power up old equipment. I have one, and it sees plenty of use. ❤😊
My Variac is bigger at about 500 Watts and I test old vacuum tube equipment up to 60 Watts. For yours use perhaps a 100 Watt "dim bulb" safety load. Most mains appliances being tested draw only a few Watts... I have an analogue voltmeter at output that works between 0..300 Volts AC and an analogue ammeter in series with the output that measures up to 1 Amp AC. Apart from test whether equipment is safe to operate I also use my Variac to vary the "mains" voltage, to test how circuits behave if under or over volted.
Great gift you got for free there, and in an unusual case. Usually variacs are not just the component, but the product itself, already with its own case, knob and scale. The way how it was encased suggests it was installed as a component, on a panel of some larger machinery, and someone scraped it and made it a stand-alone Variac. One curiosity I have: Everybody (since my electronics school age) always explains the oddities of a Variac as the auto-transformer that it is, always emphasizing the fact that it's not isolated and the unsafeness that comes with it. But I always wondered, why there aren't isolated variacs? I mean... Their construction is quite simple, with a one layer winding, when there isn't any problem in constructing multi-layered toroidal transformers... Why nobody makes a two windings Variac transformer with an under layer being a primary and a top layer, an isolated secondary with the wiper... That's something I never understood.
When working on old radios or mains power stuff I use a 500VA isolation - 500VA variac - 60W dim bulb tester. If its looking good I'll drop the dim bulb tester. Gives me a better chance of saving my skin and the device if things go wrong.
I usually put the isolation transformer on the input side of the variac - I'm not sure if it makes any difference. You can also get panel meters that measure voltage on 2 channels - the input (which also powers the meter) and the output. That allows the panel meter to show the output voltage down to 0. They're less than 10 on the usual big rock candy mountain site, but the trick is finding the two channel ones
Which sounds like the far saner thing to do. That way the isolation transformer always works at mains voltage which it was designed to do - as opposed to try to work at some low voltage AC you get on the output which it definitely was NOT designed to do. Transformers are NOT "just input some AC here, any AC" magic boxes, even if they can be misused that way.
That configuration is also known as an auto-transformer. They're often used for higher powered voltage conversions as you only need one coil of wire rather than two. Though, you do see regular transformers with one end commoned with each other - you will find this with the power transformers leading to your house. The phase and neutral go into the transformer, and out comes your house voltage. But then the the neutrals are bonded to ground and tied together. And yes, both are called auto-transformers, which are not isolated. (Isolation may not be the goal as without isolation certain protective devices do not work - RCDs/GFI breakers for example do not work in isolated circuits).
Hi, a nice little regulation transformer, my transformer has a diameter of almost 20cm and a coil height of 10cm. Personally, I prefer to connect the isolation transformer and then only the Variac, so I get two independent connection options - direct output from the isolation transformer and the other regulated. Nice day 🙂 Tom
Hi Thats nice of her --Variacs are expensive here in the UK , even the smaller ones . Those output mains sockets were used a lot on older PCs for power out to the VGA monitor --so that the screen was turned off by the computers on/off switch . Alan
Hmm, I was thinking of putting an isolation transformer before the variac, but am I thinking it wrong? I have a 230V/2*230V/1*24V 2400 VA transformer and a 230/0-260V 4A variable transformer waiting to be installed to a bench.
I have a 2.5 kW isolation transformer for all things on my bench, including the Variac. I believe that provides the best protection unless you get really stupid and let the current go through your body somehow.
Be careful with isolation transformers you find used or new , some have the incoming neutral connected to the output side winding.This defeats its isolation from earth ground we need because neutral and ground are bounded in the main power panel in your home.
Hey Imsai guy, thanks for exploring this on video. For a DIY box, that looks pretty well constructed, but you made me ponder my own set-up. My isolation transformer is BEFORE the variac. That seems OK to me but am I missing something? Seems like it would be the same to me, but I'm just a hobbyist and make plenty of dumb mistakes! Thanks again for the content - jrh
Interesting (to me) that the internal plates do not appear to be earth connected, although the external screws and plate surrounding the knob are relatively small surface areas.
Hmm, I always thought the distinction between a Variac and a autotransformer (or variable transformer) was that the Variac was electrically isolated between the "primary" and "secondary" windings and that the autotransformer used a single winding functioning as both the primary and secondary with windings above the top of the primary to give you the higher voltage. Isn't this thing an autotransformer?
I have a 1.5VA Statco autotransformer and, comparing it to my other Chinese made autotransformer, it is much smaller and lighter with 1/3 more output. I assume the Statco has a superior core material and the Chinese version is only a crude facsimile.
@@jim9930 that gave me an idea for testing the frequency response of transformers I'd plan to use as output transformers in tube amps. First check if the core is up to the job - then modify the windings...
Don't urn it on - take it apart! Well, that's what I do with unknown vintage electronics. This is modern enough but could always be dodgily built, if it's some DIY thing, but a look inside proved it was not the case. Very nice build quality here. The variac itself is so teeny tiny. Cute!
Seems to be up in the air in the comments: which is better, isolation xformer b4 or after variac ? Pros and cons of each ? Somewhere out there is a poor little HP device that has given up it's KNOB !😭
Confusing isolation transformer setup, output voltage is not what you set from variac. Transformers are not very good with input voltage they are not calculated for. Better is to put 1 to 1 isolation transformer first and then variac, especially when variac can output more than input is.
I'd say "it depends on the power ratings of both". The first stage will be loaded with the second stage's losses on top of the proper load, that's a thing to consider if power rating is limited.
With the Variac at the output you get the low output impedance at low voltages. At al low voltage of for instance 10V you can avoid to go through a 120V isolation transformer.
